PB 151393

NBS

^ecknical ^lote 34

RESISTANCE DIODE BRIDGE CIRCUIT

FOR TEMPERATURE CONTROL

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS

technical ^Ylote

34

OCTOBER 1959

RESISTANCE DIODE BRIDGE CIRCUIT

FOR TEMPERATURE CONTROL

Lawrence H. Bennett and Van M. Johnson

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Price 50 cents

Resistance Diode Bridge Circuit for Temperature Control

Lawrence H. Bennett and Van M. Johnson

The conventional ac bridge gives irregular performance

including loss of temperature control when the temperature

error exceeds a certain critical value. The present note

describes a simple method of achieving stable temperature

control over a large range of temperatures.

2/ A common type of temperature controller *

' uses a platinum resistance thermometer as one arm of an ac resistance bridge. The output

of the bridge is amplified and fed to a thyratron, which in turn

controls a relay. One of the shortcomings of this instrument is loss

of temperature control when the temperature error exceeds a certain

critioal value. When the bridge is unbalanced beyond this critical

value, the temperature controller continues to supply heat to an oven

when the temperature is already too high. Large temperature errors

which give rise to this type of faulty performance may occur during

initial warm-up of the oven, or during a change of operating temperature, or in the event of an open circuit in the platinum resistor.

The last circumstance can be most serious, since it frequently occurs

when there is no one present to monitor the performance of the oven.

The faulty performance of the controller is due to the failure of the

thyratron to properly recognize the phase of a large error signal and

due to distortion and phase shift in overloaded amplifier stages. It

is not practical to increase the critical value of the temperature error

substantially by appropriate biasing of the thyratron because of the
-2-

accompanying severe reduction in sensivity and stability of the

circuit. Satisfactory performance of a thermistor temperature controller from 20 to 300°C was achieved by symmetrical limitation of

the signal at each amplifier stage with the help of silicon diodes

1/ and by use of an average responding phase detector. The present

note describes a simpler circuit which provides stable control over a

very large range of temperature.

The circuit shown in Fig. 1 has been tested in control of an

oven from room temperature to 900°C without loss of temperature

control. The bridge is driven by an ac voltage. The use of the two

diodes in the bridge provides an error signal which is pulsating dc,

the polarity being determined by the direction of unbalance. When

the error signal is of the proper polarity and magnitude at the output

of the bridge, the thyratron will fire and heat will be supplied to

the oven.

If the oven is at a high temperature and it is desired to control

at a much lower temperature, the rheostat may be adjusted to the lower

control point without first cooling the oven. The error signal from

the resistance diode bridge will be of positive polarity and after a

phase inversion in the amplifier stage, will not be able to fire the

thyratron. The limitation of the thyratron as a phase detector for

high signals is avoided in this case, as well as the effect of any

distortion or phase shift of the error signal by the amplifier.

The characteristic behavior of the thyratron provides some pro2/ portioning control when the error signal is small. Further pro-
-3-

portioning has been obtained by placing the resistance thermometer in

close proximity to the heater coils and isolating the specimen region

of the oven from the heater coils by means of a large cylindrical metal

tube. With this arrangement, a control of ±1 C has been achieved.

Control to within a smaller temperature interval should be possible

2/ with an improved furnace design.-7

Very stable operation of the controller was noted using semiconductor power diodes in the bridge circuit. The forward resistance

of either diode is not critical as long as it is small compared with

the 100 ohm resistor in the same arm of the bridge. Similarly, any

mismatch between the pair of diodes is not critical. The balance point

of the bridge is, of course, unaffected by the nonlinearity of the

resistance of the diodes. Another arrangement of the bridge which is

essentially equivalent to that shown in Fig. 1 places the diodes in

opposite rather than adjacent arms of the bridge.

The authors wish to thank Dr. J. R. Manning for calling their

attention to this temperature controller and for several informative

discussions relating to its construction and improvement. In addition,

the authors are indebted to Dr. Forest K. Harris for valuable comments

on the manuscript.

References

1. J. M. Wilson, Electronics 22, 84 (December 1950)

.

2. C. T. Tomizuka and D. Zimmerman, Rev. Sci. Instr. .2Q, 4-0 (1959).

3. P. R. Malmberg and C. G. Matland, Rev. Sci. Instr. 2£, 136 (1956).
!i
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KnilrrirL H. Mufllrr. SrvrrUry NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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