J. S. Ratcliffe Newcastle University College New southWales, Australia
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A Thermistor Temperature Controller with Increased ~ e n ~ i t i ~ i t y
change of 0.005' C.' The accompanying circuit diagram is They are useful particularly an inexpensive and simple application of a temperaturefor remote operation due to their high resistance. A use involving a 1000 ft underwater cable has been sensitive resistance (thermistor) to the control of de~crihed.~ temperature. Their use for this and similar purposes In use, the variable potentiometer can be calibrated is, of course, not new. However, the present apor left non-indicating. The latter is easiest. I n this plication is extremely dependable, having been used for case the instrument is successively switched to a higher several years without any attention. Its novelty lies or lower range position until brought within the desired in the "band spreading" of the normal bias network to temperature range as indicated by some external indiincrease the angular sensitivity of the setting of a cator, such as, say, a '/lo degree thermometer. Manual particular temperature by a factor of 5. This is acre-setting is used until the desired temperature is atcomplished by the use of a 2 X 5 "wafer" switch which tained. If it is desired to have an indication of the apis used to select a 5000-ohm area of the total field of proximate setting then calibration scales around the 25,000 ohms in the bias network. There is then an arc 270' arc of the potentiometer movement will he necesof 270° available for setting purposes by means of the sary. The calibration is almost linear for small power variable ,5000-ohm potentiometer. dissipations in the thermistor. The 6X5GT is required only to supply the dc bias It was found best to use commercial 10yo quality for the 2D21 Thyratron, "raw ac" being used on its resistors in the "bandspread" bias arrangement. This anode. The use of a 6 volt battery would allow disallowed a slight overlap a t each end of the scale, pensing with both the 6X5GT and the transformer, eliminating the necessity of switching to another connecting the anode directly to the mains through a range, without effectively altering the bias supply suitable fuse, if this is permitted. The 4 S condenser voltage. across the relay reduces relay "chatter," and the anode resistance limits the current which the thyratron can draw. The thermistor used was a type .. F. This is a 3/r-in. diameter glass tube, 3 inches long, for the introduction ' BECKER,J. A,, GREEN,C. B., AND PEARSON, G . L., Electrical of the two connecting wires, having a pin head, bead E ~65,711 ~ . (1946). element fused into the glass tip a t one end. They are DEETER, E. L., Eleet~onics,21 (S),180 (1948). available in the range -80 to 300" C. The unit used in the present instrument had a reloAMP REUY - ., sistance variation of 100,000 ohms at 20' C to 100 ohms a t 300' C. The range of bias voltage selected for use in the bridge circuit gave a working area between 25-125' C. The relay was used for switching 10 amps at 240 volts ac and by the use of "changeover" contacts, the cessation of heating commenced a cooling cycle and vice versa. The thermistor used had an instantaneous response to tem- Z X 5 SW perature variation and had negligible heat capacity. This is attained by reducing to a minimum the power dissipated in the thermistor itself. Thermistors have been used for indicating a temperature Circuit d i a g r a m for temperature sontrot~er.
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Volume 39, Number 12, December 1962
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637
