INVESTIGATION OF REMOTE CONTROL PROBLEM FINAL REPORT U. S. - 132

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03172A000200020007-2
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 54
• Document Creation Date: December 22, 2016
• Document Release Date: February 6, 2012
• Sequence Number: 7
• Publication Date: November 6, 1957
• Content Type: REPORT

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INVESTIGATION OF REMOTE CONTROL PROBLEM
By
25X1
NOVEMBER 6, 1957
This document contains information affecting
the national defense of the United States
within the meaning of the espionage laws,
title 18, U. S. C. , secs. 793 and 794, the
transmission or revelation of which in any
manner to an unauthorized person is prohi-
bited by law.
DOE OATS :tctl BY _JO ORIG COMP apt TYPE -_______
ORIG CLASS ?Rats REV GLASS C-
JUST ~1- IuXT RSY U RUTMt Ith 1w-2
REPORT NO. CE57-0348.
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._ CONFIDENTIAL
This Document Cons sts Cf.. ~f.......Page.5............
This Is Copy...../......... ..Of.... S'..... Copy, Series........
INVESTIGATION OF REMOTE CONTROL PROBLEM
FINAL REPORT
U. S. 132
TASK III
Nov. 6, 1957
Author
Countersigned
REPORT NO. CE57-03)48
CONFIDENTIAL
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The feasibility of using a 720 cps carrier voltage for remotely
operating a small, highly reliable accessory switch was established.
The choice of 720 cps as the operating frequency is not necessarily
optimum. Of the switches developed, the thyratron with L-C tuning
is the most promising. A thermistor bridge was developed as part
of the switch to provide transient immune, unambiguous, self synchron-
izing operation in response to time-duration coded control signals.
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Abstract
Introduction
Operational Requirements
Choice of Operating Mode
Choice of Operating Frequency
4
Transmitter
Functional Diagram
Transmitter Limitations
720 cps Voltmeter 12
Photograph 13.
Schematic of 720 cps Filter 14
Laboratory Area Propagation Survey 15
0
16
Power Distribution Schematic,
Power Distribution Schematic, 5th Floor,
17
Bench Power Circuit Survey Data 19
Auxiliary Power Circuit Survey Data 21
Residential Area Propagation Survey 22
Residential Area Survey Data 23
Photograph of Transmitter 24
Analysis of Surveys 25
Residential Area Power Distribution Schematic 26
TG-1 Load Switch 32
Thermistor Bridge 34
Schematic 36
Mercury Accessory Switch 38
Schematic 39
Photograph of Mercury Switch Sensitive Relay 40
Photograph 4
Thyratron Accessory Switch 41
Schematic 42
Photograph 4
Resonant Relay Accessory Switch 46
Schematic 48
Conclusions 50
Recommendations for Future Work 51
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INTRODUCTION
The purpose of this s t u d y w a s to establish the f e a s i b i l i t y o
a working model of, a small sise, highly reliable remote con.
The requirement imposed on the remote control system and its cos l
4,77
As follows i
1. A very high degree of reliabiliti- must be.attained.
The switch must be of very small physical size and
The switch must have a high operating sensitivity co."Go
25X1
25X1
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L. The switch must be immune to false operation caused by transients on
the power line.
CHOICE OF OPERATING MODE
A study was made of various possible operating modes. Most were not
acceptable because they failed to meet one or more of the operational requirements.
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Carrier current operation is considered the only practical means of pro-
pagating the signal from the transmitter to the switch while conforming to the
operational requirements. It utilizes an existing circuit already common to
both the transmitter and switch. It is relatively secure, requiring simple
equipment without which unauthorized operation cannot be accomplished. A
reasonable transmissiontefficiency can be attained.
CHOICE OF OPERATING FREQUENCY
Carrier current is used successfully for many commercial applications,
principally by utilities for communicating and signaling over long distances via o
power lines. Frequencies in the range of 50-250 KC are most common for this
purpose. Lower frequencies, down through the audio region are also used for
control, over shorter distances, of consumer functions such as off peak load
water heater control. These systems are operated by superimposing a small
magnitude audio frequency voltage on the power frequency and are referred to
as low frequency carrier, or ripple, control systems.
The off peak load water heater control system, for example, uses a carrier
frequency of 720 cps. This frequency has been found to propagate efficiently
in the direction of the power flow through a power system and through distri-
bution transformers. It was selected as a good comprpmise among several con-
flicting requirements. It is essential to avoid using odd harmonics of the
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power line frequency in the interest of reliable operation, since the power
frequency has present in it appreciable magnitudes of odd harmonic voltage
which may interfere with the operation of the control system.
A lower frequency control signal will propagate through a power trans-
former with less loss than a higher frequency, but the lower the control
frequency, the more difficult it becomes to separate it from the power fre-
quency. This occurs because resonant circuits are used for the control fre-
quency and it becomes more difficult to obtain high enough "Q" in reasonable
size inductors to obtain adequate selectivity. A high control. frequency is
more severely attenuated by capacitive effects along the power line but less
energy is absorbed in transformers shunted across the line because of their
higher impedance at the higher frequency.
Higher audio frequencies are avoided in a large control system because of
their tendency toward creating telephone inductive interference in paralleling
telephone lines. This interference maximizes in the 1000-3000 cps region..
This is not an important consideration where low frequency carrier is operated
over short distances.
In view of the above considerations, along with the availability of adequate
performance data on 720 cps systems in the literature, it was decided to conduct
the feasibility study at this frequency. It must be emphasized, however, that
720 cps is not to be considered the optimum frequency for this application. A
rigorous analysis of low voltage power system characteristics, supported by
extensive field testing, would be necessary to determine the optimum control fre-
quency. Such a detailed investigation was considered beyond the scope of this
short feasibility study.
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A means of impressing the 720 cps control signal onto the power line
field surveys.
TRANSMITTER
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As the work progressed and field measurements were made, it became irk-
ingly evident that 720 cps was not the optimm frequency for this applicat .
This will be discussed in detail later in the section on the analysis
required to determine the 720 cps characteristics of the line and to aid in the'
development of a suitable switch. A transmitter was assembled from a'taihable
laboratory components as an expedient. No attempt was made to refine the.;t1
mitter beyond its elementary function of exciting the line.
Figure (1) is a functional diagram of the breadboard traanswAtter. ::'
oscillator was used as the 720 cps signal. source, which was standardized asst
the 60 cps line frequency by means of the oscilloreope. s The signkl voltage vax
wide range of source impedances.
the line. This matching transformer had multiple windings, majda sailler :a
transformer, T1, to match the output of the amplifier to the lower impedan
then fed into the 165 watt audio amplifier which had available output impede
of 8, 16 and 32 ohms. Preliminary measurements of the 720 cps impedances of the
pater. lines investigated indicated a range of impedance from 1/3 to 1 1/4 O1i
This low, impedance necessitated the use of an additional impedance matching
amount of 60 cps current would flow through the secondary. This would . ? be.
cause the source impedance at the secondary of the transformer io extrsmelyy last,
being the source impedance of the amplifier ? 10% of Impedance of thee Output con-
If the secondary of T1 were connected directly to the line a dens ve
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nection in use - divided by the impedance ratio of the matching transformer.
Therefore some means of blocking the 60 cps current is required without unduly
impeding the signal current.
A reactor, L2, resonated to 720 cps by C1 forms a low impedance trap which
will pass the signal current readily. Much of the 60 cps current is blocked
by the reactance of Cl. The value of C1 is a function of the reactive component
of the line impedance, which is also tuned along with the reactance of L2 and the
leakage reactance of the matching transformer. A typical value of Cl is 54 MFD
in the setup used. The 60 cps reactance of 54 MFD is approximately 4.5 ohms,
which would pass 25 amps when connected to a 115 volt line. Consequently some
means of further reducing the current is required.
The reactor, L1, tuned to 60 cps by C1 forms a parallel resonant circuit
having high impedance at 60 cps. The resonant impedance is a function of the
"Q" of L1, being Q times the 60 cps reactance of L1. The 64 MH reactor used
has?a reactance of approximately 25 ohms and a Q of about 5 at 60 cps, making the
resonated impedance some 125 ohms. A 60 cps current of slightly under 1 amp
results when the network is connected to a 115V line. The addition of L1 across
C1 has a negligible effect on the performance of the 720 cps series tuned trap.
The 25 ohms inductive reactance across the capacitive reactance of C1 is. tuned out
at 720 cps by a slight increase in the value of Cl.
In order to have efficient coupling of the signal into the line the values of
L1 and C1 must be carefully determined. The series resonated impedance of the
combination of L1 and C1 should be reasonably small compared to the impedance of
the power line to allow the highest signal current into the line for a given VA
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input to the coupling network. Ideally L2 should have low inductance and high
Q, since the resonated impedance of the series tuned combination is the reactance
of L2 divided by its Q. The reactor used had a reactance of 23 ohms at 720 cps
and a Q of 20, making the resonated impedance about 1 ohm. This is not an
optimum value since it is 3 times larger than the lowest impedance line measured
and results in a substantial reduction in signal current from that which could
be obtained with a lower impedance coupling network.
Using a low value of inductance for L2, although advantageous for reducing
losses in the coupling network, has the disadvantage of requiring a very large
value for Cl* Since this capacitor is subjected to the line voltage, and being
part of a resonant circuit, a high quality paper capacitor is required. This
results in a large, heavy capacitor. In a final model of the transmitter, a
compromise would probably have to be. made between efficiency of coupling and size
and weight. The transmitter output coupling network is an area which requires
further development in future work.
TRANSMITTER LIMITATIONS
The transmitter has a definite ceiling on its output if it is to fulfill the
requirement of operating from any available 60 cps outlet. Branch power circuits
found in residences and some commercial buildings are most commonly fused for 15
amps, using #14 conductors. 20 amp circuits are less popular, being used prin-
cipally for residential kitchens as required by the Underwriter's code. Because
of the common use of the 15 amp branch circuit, the r'.m.s. sum of the 60 cps cur-
rent to operate the transmitter' and the signal current supplied to the line should
not exceed 15 amps to avoid fuse blowing difficulties.
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If the fuse cabinet supplying the transmitter operating area is accessible
the 15 amp restriction can be removed by changing the appropriate fuse to one of
a higher rating. This must be done with care, since the normal overload protec-
tion for that circuit will be affected. Normally the transmitter will apply the
signal to the line in short pulses of 15 seconds duration or less. A 15 amp fuse
of the slow-blowing varietay substituted for the ordinary fuse would enable the use
of signal currents greater than 15 amps because of its slow response. For ex-
ample, a 15 amp slow-blow fuse will pass 30 amps for 22 seconds before blowing.
The second transmitter limitation is that imposed upon its size, which may
indirectly limit its power. The breadboard ;transmitter used was by no means
portable but only because it was assembled of immediately available components.
With careful design and perhaps transistorized circuitry in a final model it
seems likely that the current restriction described above will be reached before
the transmitter size becomes unreasonably large. Transmitter design was con-
sidered beyond the scope of this feasibility investigation and therefore is an
area requiring future development.
720 cps VOLTMETER
A voltmeter capable of reading only the 720 cps control signal in the pre-
sence of the 60 cps power voltage was required to conduct the propagation survey.
A tuned 720 band pass filter was constructed for use with a Hewlett-Packard Model
400-C vacuum tube voltmeter. The filter is shown attached. to the voltmeter in
Photograph (1).,
Figure (2) shows the schematic of the filter. Cl and L1 form a series
resonant circuit which is tuned to 720 cps by adjusting the inductance of Li.
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PHOTOGRAPH 1
720 cps VOLTMETER
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To
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A 720. cps 'voltage, Q times the voltage applied to the filter, appears at the June-
tion of Cl and L1. The Boil used had a resonated Q of 8 so that ,& voltage
magnification of Tresulted. To keep the voltmeter direct reading, and to presert+s
the Q, the 1400-C.-.is connected across In through the isolating resistor, R1. R2
and R3 in series form a shunt across the 10 meg input impedance of the 400-0 and are
used for calibrating the combination of the filter and 1100-C so that the ter
reads directly in 720 cps volts. The Q of 8 results in adequate suppression of
the 60 cps voltage - 572 db, or a ratio of 1/770. All 720 cps voltages measured
during the investigation were made with this filter.
LABORATORY AREA PROPAGATION SURVEY
In order to determine how effectively the 720 cps signal could be propagated
over a power distribution system 3 propagation surveys were made, two in a labora-
tory area and one in a residential area. In all three surveys the transmitter
was set up at a selected point and the 720 cps voltage measured at other points on
25X1
the system by means of the above 720 cps voltmeter.
Figure (3)
Power is brought into the
yard adjacent to the building at 13.8 Kv. A transformer bank reduces this to
550V for distribution in the building via busses which run vertically in the
north end of the building. Transformers located on each of the floors reduce
the 550V to lower values for distribution to the loads on that floor.
? The power distribution on the 5th, floor is shown in Figure (11) Only the
two circuits used in the survey are shown, although others of different voltages
are available. The bench power circuit is used to supply lls/230V to the
benches in the laboratory area. The transformers for this circuit are located
about midway on the floor in a small room devoted to electrieal.equipment
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shows in simplified form the power distribution in
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supplying the floor. The 550V to, and the 115/230V from the transformers are
carried in open wiring attached to the ceiling. Drops to the individual benches
are made with wiring enclosed in conduit. This circuit has the peculiarity of
using two independent single phase transformers having their secondaries in series
to supply the 115/230V, and their primaries paralleled to accommodate the 550V
feed. This results in relatively poor coupling between one 115V line to the
other, since the secondaries are not magnetically coupled. Instead, coupling
is through the first transformer to the primary of the second, and then through
the second to the other side of the line. The effect of this is noted in the
discussion of the results of the survey.
The transformer for the auxiliary power circuit is located in the northeast
corner of the floor and is connected directly to the S50V risers. The 115J230V
is also carried on wiring attached to the ceiling, and is supplied to each of
three distribution panels. These panels are about equally spaced along the
length of the hall and each serves about 1/3 of the office area. The load on
this circuit is relatively light, consisting mainly of electric typewriters,
office machines and intercom amplifiers.
The result of the survey along the bench power circuit is shown in Figure
a floor plan of the 5th floor. It indicates the points at which the signal
voltage was measured, and also the magnitude at that point. The signal was fed
to the circuit at the indicated point, with the transmitter connected across one
115V line to neutral. It is evident that the signal voltage decreases rapidly
toward the distribution transformer from the feed point. Beyond the transformer
the signal remains fairly uniform. In the direction from the feed point away
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uT
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from the transformer the signal also remains reasonably uniform.
The signal measurements fall into two groups - one between 3.2 to 1.6 volts,
and another of 0.7 to 0.14 volts. The higher group represents the signal along
the 115V to neutral side of the line to which the transmitter is connected. The
lower voltages are those appearing on the unexcited side of the line. The use
of the two transformers to supply the bench power circuit results in inefficient
transfer of the signal from the excited side to the unexcited side. The more
normal use of a center tapped transformer would increase the signal in the un-
excited side by the autotransformer action of the center tapped secondary.
The result of the signal survey along the auxiliary power circuit is shown
in Figure (6). In this case the transmitter was connected to an outlet fed from
the distribution panel nearest to the transformer. More uniform signal coverage
was obtained on the auxiliary power than on the bench power circuit. Again the
signals fall into, the high and low groups, indicative of the side of the line to
which the voltmeter was connected. Voltages on the excited side range from 3.2
to 2.0 volts while the unexcited side is more uniform in the range of-1.3 to 1.1
volts.
It was noted during the bench power circuit survey that practically no signal
was transferred into the auxiliary power circuit. It was suspected that the
transmitter power was simply inadequate to drive the unusually stiff 550V feeders
found in the lab, and that this was an unusual situation. Later measurements
made on a residential distribution system confirmed the difficulty in transferring
the signal from the secondary of one transformer to the secondary of a second
transformer, both of which are connected to a common feeder. This problem is
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treated in the section on the analysis of the surveys.
RESIDENTIAL AREA PROPAGATION SURVEY
The transmitter was set up in a suburban residential area to determine the
propagation along a utility distribution system. The area contains typical 2
and 3 bedroom frame dwellings in the $14,000 - $20,000 price range. The street
involved is 2 blocks long, dead end, running perpendicular to a main road. The
survey was made in the end block, farthest from the main road.
Power is carried into the area along the main road by a'4150V 3 phase feeder.
A line to neutral tap of 2300 volts is carried down the street to supply two 25KVA
distribution transformers, one serving each block. Their secondaries are not
connected together, each transformer supplying only the dwellings in that block.
The lines are carried overhead with ordinary utility practice, the primary wires
on crossarms and the secondary wires on racks and spools. No information on
wire sizes or transformer characteristics was available.
The relative locations of the dwellings and the utility equipment are shown
in Figure (7). The transmitter was set up at point A and Photograph (2) shows
the setup. Two sets of signal voltage measurements were made at discretely
selected residences and the results noted on Figure (7). One set of measurements
were made with the transmitter connected line to neutral, giving two values at
each of the indicated residences. The higher value is obtained across the excited
side of the line, the lower value obtained across the opposite side of the line.
A check was also made with the transmitter connected across the 230V line-
to-line wires to determine if this method of excitation would increase the signal.
This 230V connection excites both sides of the line simultaneously, resulting in
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Ci vc IMPEDANCE
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TRANSMITTER SETUP
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equal signal voltage on each side of the line at a given point. A transmitter
current of 7.5 amps with the 230 connection produced the same 1.2 volt signal at
the end of the line as did a 12.5 amp transmitter current using the 115 volt
connection.
With both the 115 and 230 volt transmitter connections there was no trace
of the signal at the'secondary of the second distribution transformer serving the
opposite end of the street.
The normal loads in the power line had only a slight effect on the signal.
There was essentially no difference in signal voltage at the end of the line
during the high loading periods. Applying an 11 KW load at the transmitter loca-
tion caused a reduction in signal voltage measured at the transmitter of 7% on the
excited side of the line, but raised that of the. unexcited side by 5%.
ANALYSIS OF SURVErS
The residential area distribution system surveyed is shown schematically in
Figure 8. The diagram will also apply generally for the bench power and auxiliary,
power circuits except for the relative locations of the transformer and transmitter.
The voltages indicated are those measured with the 230 volt transmitter connection,
which was selected for analysis because of the resulting balanced signal conditions
on the system. The same general concepts will apply to the system with the 115
volt transmitter connection.
The surveys suggested that the distribution transformer. is the doom$nating
influence in determining the signal propagation over a power system. The particular
residential transformer in the area surveyed is rated 25.KVA at 230 volts so that
its full load current will be 110 amps. According to the literature 1 such a
SECRET
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-27-
transformer, when energized from a stiff source, may be expected to have
1.5% IR, 1.3% 1X and 2.0% 1Z regulation. These figures are the percentages that
the rated output voltage will drop at full load current as a result of the resis-
tance, reactance and impedance, respectively, of the transformer at 60 cps. These
constants may be approximated at 720 cps by applying the ratio of the frequencies.
They then become 1.51do IR (the resistance being essentially independent of fre-
quency), 15.6% IX and 16.3% IZ.
Since the IZ is 16.3%, the secondary full load voltage would drop 16.3%
or 37.5V, because of the internal impedance of the transformer if it were energized
by a stiff 720 cps source. The impedance can then be determined by dividing the
full load voltage drop, 37.5 volts, by the full load current of 110 amps. This
indicates that the transformer has an internal source impedance of 0.34 ohm at
720 cps. This is also the impedance of the transformer as seen by the transmitter
since the primary is shunted by a low. impedance feeder. The transmitter must
drive the 0.34 ohm internal impedance of the transformer to develop the 720 cps
signal voltage across the secondary.
The transformer.is fully loaded when the connected load has an impedance of
2 ohms. Allowing for a 100% overload_on the transformer during peak load periods,
the load impedance would drop to 1 ohm. This is still relatively large compared
to the 0.34 ohm transformer secondary impedance. It is evident, therefore, that
the transformer is the dominating factor in determining the line conditions at
720 cps, with the 60 cps loads having only a second order effect even under peak load.
If-the effect of the connected 60 cps loads are neglected - which it appears
reasonable to do at least for light load conditions - more of the properties of
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-28-
the system can be determined. Although negligible at 60 cps, the impedance of
the distribution lines and service entrance cannot be neglected at 720 cps. As
noted on Figure 8, there was a signal voltage drop of 2 volts in the service
entrance and watt-hour meter between the transmitter.and the first pole, repre-
senting an impedance of 0.27 ohmr. Between the first and second poles a voltage
drop of 2.L volts indicates an impedance of 0.35 ohm. A similar impedance between
the second and third poles results in another 2.4 volt drop in signal. Of the
9.4 volts generated at the transmitter only 2.6 survives to appear across the
transformer secondary as a result of the 720 cps impedance of the line.
There is little loss of signal beyond the transformer, since the 60 cps load
impedances are relatively high compared to the 720 cps impedance of the connect-
ing line. This explains the more uniform signal coverage found on the auxiliary
power circuit, where all the outlets are on the transmitter side of the transformer.
The bench power circuit exhibited a similar high signal loss between the trans-
mitter and transformer for the same reason. More uniform coverage was found
beyond the transformer and, in the opposite direction, beyond the transmitter.
The line impedance measured at the transmitter for the 220 volt connection
was 1.27 ohms, as determined by dividing the transmitting voltage by the current,
The impedance at the same point determined from the voltage measurements indicated
on Figure 8 and the calculated transformer impedance is 1.31 ohms.
The conditions for the more normal 115 volt transmitter connection are very
similar to those discussed above. The principal difference is the unbalance of
signal voltage appearing on the two sides of the line. The signal is transferred
to the unexcited side of the line by autotransformer action in the distribution
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-29-
transformer, and by the voltage drop which occurs in the common impedance of the
neutral in the region between the transformer and the transmitter. This voltage
adds to the autotransformed voltage, so that the voltage on the unexcited side
increases toward the transmitter. Beyond the transformer the voltage on the
unexcited line is the autotransformed voltage less the line impedance drop. This
drop is small however, because the shunt impedance of the 60 cps loads is high
compared to the 720 cps impedance of the line. The drop along the excited side
of the line beyond the transformer is also small for the same reason.
It was found that with both the 115V and 230V transmitter connections there
was no evidence of the signal being present on the secondary of the second dis-
tribution transformer on the street. The literature on the use of low frequency
carrier control systems indicates that the control signal: can be propagated with
ease in the same direction: as the power flow in a system. The surveys made
indicated that it is difficult to propagate the signal in the direction opposite
to the power flow, or "upstream," with any reasonable size transmitter.
It was found that the 25 KVA transformer had a secondary impedance of 0.34 ohm.
When this impedance is-referred to the primary by multiplying it by the square of
the primary to secondary voltage ratio, 100, it is found that the primary impedance
is 34 ohms. This becomes the driving impedance with which the transformer excites
the primary feed line when its secondary is driven by the transmitter. No informa-
tion was available on the source impedance of the primary feeder, but it can be
assumed to be low. The 60 cps primary impedance of the transformer under peak
loading is 100 ohms. The two transformers on the feeder represent a load of
50 ohms. To maintain a reasonable 60 cps voltage regulation of say 5%, the feeder
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source impedance would be 2 ohms. This 2 ohms is the load impedance that
must be driven by the 34 ohms source impedance at 720 cps. It becomes apparent
that it is difficult to transfer 720 cps power into the feeder because of the
large impedance mismatch involved.
The power delivered to. the secondary of the distribution transformer was only
a small part of that delivered into the line by the transmitter. Because of the
impedance drop in the line only 2.6V of the 9.bV at the transmitter appeared at
the transformer. On a 230V basis, the voltage appearing on the primary feeder
would be 2/31 of the 2.6V across the secondary, or only 0.15V. Allowing for a
20% loss, which the literature indicates is typical when propagating downstream,
only about 0.12 volts would be delivered to the secondary of the second trans-
former on the street. Only about half of this voltage would appear across each
side of the secondary.
The 720 cps voltmeter used had a selectivity such that when an input of 115V
60 cps was applied a reading of 0.15V resulted, Therefore the minimum 720 cps
voltage that could be read would have to be somewhat above this value. Since
the voltmeter was connected line to neutral when checking on the secondary of the
second transformer, the anticipated 0,12volts could not be confirmed. It seems
reasonable to'conclude, however, that such a small signal would be inadequate for
control for this application.
Several other conclusions can be drawn frown the above discussion. Perhaps
the most significant is that the exploratory frequency of 720 cps is not necessarily
the optimum for the application. It is apparent that a lower frequency would im-
prove the propagation of the signal along the 60 cps system by reducing the
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-31-
reactance of the distribution lines and transformers. The use of a lower,
frequency may enable the propagation of a useful signal through two'trans-
formers within the size limitations of the transmitter. Additional tests must
be performed to evaluate the: possible benefits of a lower frequency.
Conversely, if there is no urgent requirement for propagating through two
transformers, a higher frequency could be used and would suffer less attenua-
tion caused by the shunting reactance of the transformer. A smaller trans-
mitter would then be required, but would be useful only on the secondary load
circuits of a given transformer. A higher frequency would make possible the
use of higher Q reactors of smaller size with correspondingly higher voltage
magnification, producing additional stable sensitivity in the switch. This
factor would contribute toward reducing the size of the transmitter required.
It is advantageous to install the transmitter and the switch along the line
on the same side of the transformer, and preferably with the transmitter between
the switch and the transformer. This arrangement produces the highest signal
at the switch because the shunt loading of the transformer is reduced by the
reactance of the line between the transmitter and the transformer. The signal
voltage along the line in the direction away from the transmitter and transformer
then suffers only slight attenuation and remains more uniform over a greater dis-
tance. This situation is illustrated by the survey along the auxiliary power
circuit.
The most adverse situation encountered in the surveys was that of the bench
power circuit. This circuit is`unusual in the use of two separate transformers
having their secondaries connected in series to supply the ll5/230V. As a result
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-32-
there is almost complete absence of autotransformer action for the signal. The
secondaries are coupled only through the paralleled primaries, and as discussed
previously, the resulting transfer of signal is very small. The unenergized
side of the line received its share of the signal principally through the common
impedance of the neutral wire. Even under these poor conditions a reasonably
adequate signal was found over the operating area.
Although certain configurations of transmitter, transformer and switch may
have propagation advantages, the design goal for the system must be to secure
satisfactory performance under the most adverse condition. It would be a serious
handicap to the application if each of the components had to be placed in a pre-
scribed relation with respect to the transformer.
TG-l LOAD 91ITCH
A 720 cps off peak load water heater switch, was obtained to 25X1
evaluate its operation and determine its usefulness for the application. The
load switch consists of two main components - a 720 cps sensitive relay, and a
power relay with a time delay. The sensitive relay consists of a magnetically
actuated mercury switch, an actuating coil on a magnetic core, and a tuning
capacitor to resonate the coil to 720 cps. The mercury switch and core is shown
in Photograph(3A.) The series combination of the coil and capacitor is connected
across the line and the capacitor limits the 60 cps current flowing in the coil
to a sufficiently low value which will not actuate the mercury switch. When a
720 cps signal of about 1 volt appears on the line, a high current flows in the
coil because of its low series resonated impedance and the mercury switch closes.
Closure of the. mercury switch applies power to a thermostatic element which
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-33-
deflects as it heats. This deflection is used, through a mechanical latch-
ing toggle action, to tilt a power mercury switch. The toggle action is
employed to maintain the power mercury switch in its tilted attitude after
power is removed from the bimetal,, and, in combination with the bimetal, to ob-
tain a positive sequential action. The means of obtaining this sequential action
in the TG-1 switch, while interesting in itself, is of no direct value to the
application because of its relatively large size.' However, its operational
functions were deemed to have great merit.
Because of the use of the time delay introduced by the thermostatic bimetal
the TG-l is totally immune to arr short transient disturbance on the line. A
control signal of from 3 to 12 seconds duration is required to turn the switch on.
During this time the power mercury switch is tilted to its on position by the
deflection of the bimetal and then restrained in the on position by the latching
action of the toggle. Upon removal of the control signal the bimetal is de-
energized and returns to its normal undeflected position in about 1 minute.
A control signal of from 15 to 36 seconds duration is required to turn the
switch off. During this time the bimetal is energized and, because of the longer
time, deflects further than it did during the on period. This increased deflec-
tion then releases the toggle and the power mercury switch is returned to its
off position. A cooling period of about 3 minutes is required after removal of
the control signal before the bimetal is ready to repeat its'action.
This action results in three features desirable for the application. The
time constant of the bimetal immunizes the switch to disturbing transients.- The
selective time response of the bimetal and toggle produce unambiguous operation
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as a function of the control signal, duration. A positive, self synchronizing
action is obtained from the unit.
THERMISTOR BRIDGE
The large size of the bimetal-toggle-mercury switch of the TG 1 load switch
prevented their adaptation for the application. Their performance however, was
so attractive that an investigation was made to determine a means of obtaining
similar action by electrical means. Ordinary thermal delay relays were ruled
out because of their fragility and poor reliability. The investigation quickly
narrowed down to thermistors because of their small size and high reliability.
Thermistors are temperature sensitive resisters having a high negative
temperature coefficient of.resistance. As their temperature increases, the
resistance decreases. By varying the composition during manufacture and by the
use of various shapes, wide control of their properties is possible. The
,gharacteristics are stable and uniform over wide operating ranges.
The negative resistance property of a thermistor may be employed to generate
stable timing circuits. When a voltage is,applied to a thermistor and a resistor
in series, a current will flow which is determined by the impressed. voltage and
total circuit resistance. If the voltage is high enough, some heat will be
generated in the thermistor. This, will lower its resistance and more current
will flow. This effect is cumulative, the additional heating producing more cur-
rent. The process continues until the thermistor reaches the maximum temperature
possible for the amount of power available'in the circuit. Equilibrium is then
established.
The thermistor has a finite mass, so that it takes time to be heated to its
maximum value. The time required is a function of,the mass, the applied voltage,
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-35
and the value of series resistance. By a suitable choice among these parameters
it is possible to produce a wide variety of delay times.
A thermistor bridge circuit was developed in which these parameters were
adjusted to duplicate the performance of the TG-l load switch. The bridge was
developed using empirical methods and, while it functions well, it is not neces-
sarily an optimum design. It was developed to establish the feasibility of using
thermistors for the purpose. The bridge involves so many dependent variables
that the use of an analog computer would be advantageous in optimizing the design.
Having established the feasibility, the optimization of the bridge was considered
beyond the scope of this preliminary work.
The circuit of the thermistor bridge is shown in Figure (9). Two legs of
the bridge contain resistors only., R2 and R3. The remaining two legs are made
up either of a resistor and two thermistors, or of three thermistors. A rec-
tifier energizes the bridge with unfiltered rectified AC, required because of
the use of a DC relay at the output of the bridge. The bridge output is a
differential current which flows through the relay coil.
A polarized D-C relay with'magnetic latching is used. This relay has two
stable positions. Coil current'of one polarity only can cause the armature to
transfer from a given fixed position to the other. Opposite polarity is then re-
quired to return the armature to the original position. The function of the relay
is analagous to the toggle of the TG-1 switch.
At the moment the bridge is energized the current flowing in each branch of
the bridge is determined by the rectifier output voltage, 104V D.C. for 115V input.
and the cold resistance of the thermistors, about 1000 ohms each. This initial
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25X1
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-37-
current heats the thermistors and their resistance decreases, increasing the
current. However, since there are 2 thermistors in side A and 3 in side B, the
current is higher initially in side A and increases at a faster rate than side B.
These currents flow through R2 and R3 and divide between them so that the difference
current flows through the relay coil. When the current in branch A is greater,
the net current flow through the relay coil causes the relay armature to close in
one direction.
As the thermistors continue to heat, the current in side B becomes sufficiently
large so that the net current through the relay is reversed. This causes the relay
armature to transfer to the opposite position. The thermistor resistance decreases
to about 20 ohms when they reach equilibrium. The final current is determined by
the resistors in each leg of the bridge. About 20 watts are dissipated in the
bridge components at equilibrium.
About 2 seconds are required after the bridge is first energized to actuate
the relay to the position corresponding to turning on the load. At this time the
excitation may be removed and the relay will remain in its on condition because of
the magnetic latching feature. If the bridge remains energized, the relay will
transfer to the opposite, of off, position in about 10 seconds. The excitation
may then be removed, leaving the relay in the off position.
The self synchronizing feature of the TG-l load switch is duplicated in the
thermistor bridge. If the position of the relay armature is unknooh, applying
excitation to the bridge for the 10 second period will always cause the relay to
go to the off position. If the relay was in the on position previously, it will
simply transfer to the off position in the normal manner. If the relay was off
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_38_
previously, it will first turn on after 2 seconds, remain on momentarily, and
then transfer to the off position. Thus the relay can always be brought to the
desired position even though its previous position is unknown.
An interval is required between operating cycles to allow the thermistors
to return to ambient temperature. About 5 minutes are required after the off
cycle before the relay can be turned on again. The relay can however be turned
off immediately, since this phase of the operation does not require that the
thermistors cool. The bridge operating time will be influenced slightly by
variation in ambient temperature: The power input to the bridge was deliber-
ately made relatively high to produce a thermistor equilibrium temperature
adequately above the highest anticipated ambient temperature.to improve the
stability.
MERCURY ACCESSORY SNITCH
A breadboard model of the accessory switch was assembled using the frequency
selective relay components taken from the TG-l load switch. The use of the
magnetically actuated mercury switch appeared to have the advantage of simplicity
and potentially reliable operation. The disadvantages are the relative fragility
and position sensitivity of the glass enclosed mercury switch.
Figure 10 shows the schematic of the switch. The coil of the magnetic
assembly, I,l, shown in Photograph (3A), is resonated to 720 cps by Cl. The im-
pedance of C1 is 1800 ohms at 60 cps, so that the resulting 60 cps current in the
coil is not large enough to actuate the magnetic mercury switch S. However,
at 720 cps the resonant impedance is 11 ohms, so that only 1 volt is required to
cause the mercury switch to close. It was found that when the switch was
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S
PHOTOGRAPH 3
MERCURY SWITCH SENSITIVE RELAY
0R ~}f.RPli{ ~9i
PHOTOGRAPH 4
MERCURY ACCESSORY SWITCH
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packaged in an aluminum box as shown in Photograph 1.,, the Q of the coil was
reduced from 16.5. to 12 because of the effect of the aluminum of the box near
the coil. As a result, the packaged switch required 1.3 volts at 720 cps to
close the mercury switch. When the mercury switch closes, the 115V,60 cps is
applied to the rectifiers which energize the thermistor bridge to functions as
described previously.
The magnetic structure as used in the TO-1 switch is too large to allow
packaging in a compact assembly. An attempt was made to reduce its size by
employing a more efficient magnetic circuit wing a small "C? core. Several
trials,. produced the smaller assembly shown in Photograph 3B. While the re-
suiting sensitivity was not quite equal to the original, it did serve to indicate
the feasibility of reducing the size. Additional development could have Pro-
duced a smaller assembly having at least equal, or perhaps even greater sensitivity
but it became increasingly apparent that the mercury switch approach was far from
ideal for the application.
During the work with the mercury switches their fragility became more obvious.
Two were broken during handling while constructing the packaged accessory switch.
Although first appearing attractive because of their simplicity, the lack of
ruggedness and their position sensitivity, prevented the mercury switches from
meeting the requirements of this application.
THYRATRON ACCESSORY SWITCH
The thyratron accessory switch uses a cold cathode thyratron in the circuit
shown in Figure 11. The thyratron is connected in series with a relay coil,
RY1, and an inductor, L1, across, the 115V 60 cps line. A fraction of the 115 volts,
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-43'-
determined by the reactance divider Cl and C2, is applied to the trigger electrode
to bias the thyratron to slightly less than the breakdown bias.
A capacitor, C4, resonates the inductor to 720 cps. When a 720 cps voltage
appears on the line, "Q" times this voltage is developed across the inductor. This
voltage appears between the cathode and trigger of the thyratron and adds to the
bias already on the trigger. The thyratron then breaks down and continues to con-
duct while the 720 cps voltage is present.
Current to the thyratron passes through the relay coil, causing its contacts to
close and apply power to the rectifier and thermistor bridge. The bridge then
performs as described previously to turn the accessory on or off. Since the
thyratron conducts only on alternate half cycles of the 60 cps, the capacitor C3
is required to maintain current in the relay coil. This capacitor is charged dur-
ing the conducting half cycles and supplies the current necessary to hold the relay
closed during the non-conducting half cycles. The resistor Rb limits the charging
current of C3.
A reactance voltage divider, Cl and C2, was used to avoid the stand-by power
dissipation that would result from the use of resistors. The stand-by line cur-
rent. of the receiver is entirely reactive so that the switch consumes no true
power. The 60 cps stand-by line impedance of the thyratron receiver is 20,000 ohms
capacitive, causing a 0.006 amp current at essentially zero power factor. The line
burden is 0.7 volt-amps reactive. The absence of arty true power dissipation makes
the presence of the device impossible to detect by watt-hour meter observations.
The switch cannot be detected by a resistance measurement across the line since DC
is blocked by the capacitive divider and by Cb. The capacitive input of the
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switch would add little to the existing capacity of the wiring across which it is
installed, making difficult the detection by capacity measurements. A high degree
of security exists in the thyratron switch.
The type 395-A thyratron was selected because of its small size, high sensi-
tivity and ruggedness. It is a JAN approved type and has a 1000G shock and
vibration rating. Its small dimensions are well. adapted to miniaturization of
the switch.
A laboratory sample thyratron switch was constructed and is shown-in Photograph
5. While made conveniently small, no attempt was made to miniaturize it. The
volume required could be reduced substantially, perhaps by 50%, by judicious use
of miniaturized components.
This switch has a sensitivity of 0.8 volts, determined by the bias applied
to the thyratron and by the Q of L1. The bias of about 50 volts was selected as
being well below the lowest trigger breakdown voltage of the 6 samples available
for test. This bias could probably be increased to yield greater sensitivity but
more information is required on the statistical variation of the trigger voltage
for a larger quantity of thyratrons. A higher Q for Ll would result in greater
sensitivity, but at the expense of larger physical size. The reactor used was
a 1" dia x 1/2" high quality toroid having a resonated Q of 20, yielding a signal
magnification of the same value. Greater magnification can be obtained with the
higher Q of a larger reactor.
The thyratron switch was tested on both the lab power systems and the residential
system. Good operation was attained at the maximum available distance of 2501 in
the laboratory power circuit and at the maximum distance available, 11501, on the
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PHOTOGRAPH 5
THYRATRON ACCESSORY SWITCH
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-146
secondary of the residential circuit. An interference check was made on the
switch by operating it from the same outlet to which was connected, in turn, an
electric shaver and a kitchen mixer. Both of these are notorius interference
generators, the ones used producing snow on a TV set during the test. The switch
functioned normally in the proximity of this interference .
Although the thyratron switch uses more components than the mercury switch, the
reliability can be made very high by the use of top quality components. The 395-A
thyratron is rated as a reliable type and has found widespread use in telephone
service where reliability is important.
RESONANT RELAY ACCESSORY SWITCH
A sample of a Frahm resonant reed relay was evaluated as the sensitive fre-
quency selective element. The relay consists of a mechanically resonant reed
placed in the field of an electromagnet. When the electromagnet is energized
with an A-C voltage of a frequency at which the reed is resonant, the reed is
set to vibrating. Contacts actuated by the reed are then closed intermittently
by the vibration of the reed. The rate of contact closure is equal to the excit-
ing frequency. '
The resonant reed relay has two principle advantages - high sensitivity and
sharp selectivity. The reed is excited at its resonance and very little driving
power is required. The mechanical Q of the reed is very high, resulting in a
very narrow frequency range over which the reed will respond. Both sensitivity
and selectivity are stable over a wide temperature range.
The disadvantage of the relay is the low duty intermittent contact produced.
The contact dwell time is 5j or less, and the maximum peak contact loading is
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.447-
only 200 Mw. As a result it is necessary to employ an auxiliary relay or
thyratron to obtain continuity.
The Frahm RR-10 resonant relay was originated for airborne operation and
is MIL approved. It is small -'1/2" dia x 2" long - and rugged, meeting 100 g
vibration and acceleration requirements.. Those used were of g standard varibty
having a resonant frequency of 716 cps and a coil impedance of 16 ohms. A
0.08V signal on the coil caused the contacts to close intermittently.
The difficulty encountered with the resonant relay was in coupling the low
coil impedance to the power line. A large coupling capacitor between the coil
and line is required to preserve the high sensitivity- of the relay. This large
capacitor causes an excessive 60 cps current through the coil, resulting in over-
heating of the coil. A compromise was arrived at in which a 720 cps series re-
sonated trap was connected in series with,the relay coil across the line, as shown
in Figure 12.
The series trap has a resonated impedance 65 ohms and'reduced the effective
sensitivity of the combination to 0.3V. The 0.15 capacitor has a reactance of
20,000 ohms at 60 cps, effectively blocking the 60 cps current from the relay
coil. It has since been determined that there is available another relay similar
in all respects to the RR-10 except that the coil impedance is 1000 ohms. This
coil could be connected directly to the line through a 0.25 MFEf capacitor to give
a 0.16V sensitivity.
The resonant relay switch was not carried beyond the simple breadboard stage
because of the difficulty of coupling the coil of the available sample to the line.
The circuit used for the breadboard is shown in Figure 12. A 395-A ''ration
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was used to convert the intermittent contact of the resonant relay to a sustain-
ed contact to actuate the thermistor bridge. The momentary closure of the
.resonant relay contacts applies the 115 volts to the trigger of the thyratron,
causing it to conduct on alternate half cycles. As in the thyratron switch,
the plate relay contacts close and are maintained closed by C2. The plate relay
applies excitation to the thermistor bridge. The resistor Rl limits current
through the trigger during the initial breakdown of the thyratron.
The use of the resonant relay offers some promise for the application. The
1000 ohm coil version would yield a switch having high sensitivity with greater
selectivity than attainable with a simple series resonant circuit. ? The use of
the resonant relay eliminates 2 capacitors and the inductor as used in the thyra-
tron switch.
The use of an additional relay could tend to reduce the reliability. Frahm
claims a minimum life of 2 x 108 contact closure cycles. The contacts are not
likely to wear out in this application but could be more subject to a catastrophic
failure.
The advantages of the resonant relay approach appears sufficiently attractive
to warrant further investigation to more thoroughly evaluate its possibilities.
It would be potentially more useful at a lower frequency where it is difficult
to obtain reasonably high Q in an inductor for use in a tuned circuit. The
inherently higher Q of the resonant reed should result in a sensitivity. greater
than could be achieved with an L-C tuned circuit of reasonable size. The greater
selectivity of the resonant relay would permit. the use of control frequencies not
far removed from the power line frequency.
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CONCLUSIONS
The feasibility of using a 720 cps carrier voltage for remotely operating
a small highly reliable accessory switch was established. The choice of 720 cps
as the operating frequency is not necessarily optimum. It was found to be im-
practical to propagate a useful control signal through two distribution transformers
at 720 cps. The use of a lower control frequency may produce more efficient
propagation through two transformers. The use of 720 cps and a transmitter of
reasonable power limits the use of the control to those circuits energized from
a common transformer secondary. A further study is required to determine the
optimum frequency.
Of the three types of switches developed, the thyratron with L-C tuning is
the most promising. The mercury switch is impractical because of its fragility
and position sensitivity. The resonant relay switch may have an advantage with
lower frequency. control signals. Both the thyratron and resonant relay!switches
could be made suitably small and would meet the requirements of the application.
The reliability of the thyratron switch can be made adequately high, with that of
the resonant reed relay closely approaching it.
A thermistor bridge was developed as part of the switch to provide transient
immune, unambiguous self-synchronizing operation in response to time-duration
coded control signals. As a result the condition of the switch can be relied
upon without the need of repeat-back information.
A reasonably secure mode of operation is attained. The switch consumes no
power in its standby condition and cannot be detected by watt-hour meter obser-
vations. Detection of the switch by electrical measurements is diffic1t be-
cause of the high impedance it presents when connected across the line.
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0
RECO10,12NDATIONS FOR FUTURE WORK
The following are areas in which additional future work is indicated to
complete the development of the system and its components:
1. OPERATING FREQUENCY:
Further study and tests are necessary to determine-the optimum
frequency. Effort should first be directed toward a lower fre-
quency in an attempt to obtain propagation through two transformers.
If this is demonstrated as impractical, a higher frequency should be
explored as a means of reducing propagation losses, reducing trans-
mitter size and power, and increasing switch sensitivity.
2. THEH4ISTOR BRIDGE
The design of the bridge should be evaluated, preferably with an
analog computer simulation, and optimized. The effects of ambient
temperature on the bridge operation should also be evaluated.'
3. TRANSMITTER
An optimum transmitter should be developed, the design of which is
contingent upon the results of item 1 above. Transmitter coupling
networks must also be evaluated.
S14ITCH
The switch should be redesigned with a view toward full miniaturtza-
tion. The usefulness of the resonant relay should be evaluated if
the optimum frequency is low. Tests should be made to establish the
effective reliability level of the switch.
ISECREJI
N TIAL
3
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This material contains information affecting
the national defense of the United States
within the meaning of the espionage laws,
title 18, U.S.C. , secs. 793 and 794, the
transmission or revelation of which in any
manner to an unauthorized person is prohi-
bited by law.
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