A Simplified Electronic Thermoregulator WARREN E. GILSON and HAROLD A. WOOSTER University of Wisconsin Medical School, Madison, Wisconsin
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N RECENT years several articles on thermoregulators have been published, describing the use of vacuum tubes for the control of relays, and thus indirectly, of heating elements (1-6). When relays are used in this manner, it is necessary to renew the contact points quite frequently, especially if the electronic circuit does not have a time delay feature to prevent chattering caused by slight vibration producing intermittent closing of the mercury contacts. This difficulty is obviated by the use of a thyratron, eliminating the relay entirely, as well as providing a quieter, more reliable, and generally more elegant control. The advent of small thyratrons (the 2A4G) and their wide use in pinball machines and other amusement devices has brought their price to a reasonable level. Large thyratrons have been used in commercial practice for many years for the convenient and efficient control of power. Inasmuch as a small water bath requires only 10 to 15 watts of electrical energy for thermostasis at the conventional temperatures, it seemed desirable to construct a thyratron control, regulated by a mercury-toluene thermoregulator. The grid current of a thyratron is a fraction of one milliampere, much less than that required for satisfactory operation of a relay. The prime advantage of this is the greatly lessened current at the mercury contact, and the consequently retarded formation of oxide at this point. We have used such a thyratron control for over three months, with entirely satisfactory service. The accuracy of contro :is dependent on the thermal characteristics of the system used. In our case the regulation was within plus or minus 0.05"C. A thyratron is a gas-filled tube, usually a triode, which is non-conducting when the grid is sufficiently negative. As the grid becomes less negative the tube "breaks down," or "fues," ionization occurs, and platefilament current flows. This is indicated by a blue glow between the plate and the cathode. The grid then loses control of the plate current, which can he stopped only by momentarily removing the plate voltage. The voltage drop in a thyratron is approximately 15 volts, and is almost independent of the plate current. For further information on the 2A4G consult the "Sylvauia Tube Manual." Figure 1A shows the simplest form of our circuit, as fust constructed. I t is relatively inexpensive, the parts costing about $6. The circuits which have been described for such control of laboratory water baths generally use either batteries to bias the thyratron (7), or use the thyratron to control a relay. Because of the small amount of energy needed in this application, no relay is necessary. The use of batteries where
is available is ordinarily undesirable. For these reasons it seemed logical to apply phase coutrol (as used in a much more complicated form in welding circuits), thus making it possible to use a small power transformer of the replacement type for control voltage as well as heating cnrrent. The circuit operates as follows: if the mercury contacts are open, the grid is a t cathode potential and plate current will flow during the half of the cycle in which the plate is positive with respect to the cathode. When the plate cnrrent passing through the heating coil has warmed the bath to the required temperature, the mercury column will rise to close the contacts. This brings A.C.
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Z O O 0 OHMS 50 W A T T S
110 U A.C.
the grid to a negative potential during the part of the cycle when the plate is positive, and thus prevents conduction. This occurs because the o ~ ~ o s iends t e of the
high-voltage winding are 180 degrees out of phase with respect to the center tap. Figure 1B illustrates a more complex circuit in which the operation is safeguarded against tube failure. One of the tubes will operate in preference to the other until i t fails. The other tube will then assume its function, thus indicating the necessity of replacing the first tube. This should be quite infrequent, as the tubes are guaranteed for 90 days of continuous use. In some cases R1 must be adjusted on an empirical basis when the circuit is first placed in operation. This may be done by trial of various fixed resistors, as in lA, or by the use of a variable resistor, as in IB. For protection of the contacts it is desirable to use the highest resistance which will give satisfactory operation. The switch in the plate circuit is for the purpose of protecting the cathode of the tube. This switch should not be closed until several seconds after the filament voltage is applied. The heating element is a conventional 2000-ohm, 50-watt bleeder. It is supported in a brass cylinder by glass spacers a t each end. This cylinder is tightly stoppered and partially immersed in the water bath. The voltage applied to the mercury contacts is not dangerous because it is only a small part of the second-
ary voltage, and is further rendered innocuous by the high resistance in series with it. The transformer should be able to furnish a t least 60 milliamperes, but should not have a voltage higher than 300 volts on each side of the center-tap. If larger amounts of power are necessary, a larger thyratron may be used, or a relay may be put in the plate circuit in place of the heating element. Understanding of the circuit is not necessary for successful construction and operation, as i t is merely necessary to wire the numbered connections on the octal socket to the points indicated. If d i c u l t y arises, a radio service man acquainted with pinball machines or a physicist working with Geiger counters should be able to find the trouble readily. When they are again available the 2050 should be used in preference to the 2A4G, because of lesser grid current and greater sturdiness. LITERATURE CITED
HAWELInd. Eng. Chem., Anal. Ed., 11, 222-3 (1939). RUDYAND FUOASSI, ;bid., 12, 757 (1940). SERFASS, ibid., 1 3 , 2 6 2 3 (1941). Ibid.. 13, 352-3 (1941). WADDLE AND SAEMAN, ibid., 12,225 (1940). REDFERN, i b i d . , 14, 64 (1942). DANIELS,MATHEWS, AND WILLIAMS, "Experimental phyrical chemistry," 3rd ed., McGraw Hill Book Company, Inc., New York, 1941, p. 416.
