Liquid Temperature Controller. This solid-state controller can maintain a liquid bath to within a few .... thermistor R4 and a 10,000-ohm rheostat is substituted for ...
Liquid Temperature Controller
This solid-state controller can maintain a liquid bath to within a few hundredths of a degree for a 24-hour period. By William D Scott / University of Washington
THE economical, sensitive, solid-state, proportional temperature controller to be described has a stability of ±0.l°C per month. With reasonable consideration in the placement of heater and thermistor sensor and adequate stirring, the ternperature of a water or oil bath can be controlled to within a few hundredths of a degree for a 24- hour period. Control can be varied from nearly proportional type of control to ordinary on-off control by changes in the sensitivity of the control circuit. The circuit shown works quite well between 60°C and 200°C with some loss in sensitivity at the higher
temperatures. With proper choice of thermister and other bridge components, almost any temperature range can be obtained. With appropriate circuit modifications, other devices such as positive temperature coefficient thermistors, tungsten wire elements, light-sensitive resistors, and even solar cells could probably be used to control an unlimited number of variables. The schematic is shown in Fig. 1. Temperature preset pot R3 has its dial marked in temperature in degrees after the controller is calibrated against an accurate thermometer. As the temperature of the bath rises, the re-
Fig. 1. The schematic and parts list for the proportional temperature controller.” Rl--2200 ohm, 1/2 w. res. (approx.. see text) C2--See text R2--500 ohm, 1 w. res. C3--0.1 µf, 600 v. paper capacitor R3--100.000 ohm, 10-turn pot” C4--.005 µf, 600 v. paper capacitor R4--100.000 ohm thermistor (Fenwall GA55PR) L1--See text F1--See text R5, R7--10,000 ohm 1/2 w. res. (matched ± 1%) PLI--NE-51 neon pilot light R6--3000-10,000 ohm, 1/2 w. res. (see text) D1-1N536 silicon diode R8--15,000 ohm, 1/2 w. res. D2--18 v., 1-watt zener diode ( 1N1526) R9--5000 ohm, 1/2 w. res. D3-1N91 diode R10--2200 ohm, 1/2 w. res. D4-200 p.i.v., 0.1 amp diode (1N1695) R11--170 ohm, 1/2 w. res. D5-Thyrector voltage suppressor ( G-E 6RS 20SP4B4) Rl2--20 ohm, 1/2 w. res. D6--Silicon controlled rectifier (G-E C22B) R13--2000 ohm. 5 w. res. Ql, Q2--2N2712 transistor R14- -100,000 ohm, 1/2 w. res. Q3--2N2616 unijunction transistor” C1--Approx. 0.1 µf (see text)
sistance of thermistor sensor R4 decreases and current through the thermister increases, drawing off some of the base current from Q1. This drastically reduces the emitter current of Ql, and to maintain a constant common emitter voltage, this necessitates an equivalent increase in the emitter current of Q2, re sulting in an increase in current through R7. This increase in current results in a decrease in the charging voltage of capacitor C3, thus the firing voltage of unijunction transistor Q3 is delayed, silicon controlled rectifier D6 fires in the the positive half cycle, and a smaller r.m.s voltage reaches the heater load. The temperature of the bath therefore decreases and the resistance of the thermistor increases. This increases the power being supplied to the heater therefore the bath temperature goes up. The system will then cycle about the pre-set (by R3) operating temperature. The heater rating is a of the SCR conducting
function current.
The value of R6 is quite critical and depends upon the absolute magnitude of the zener voltage, the exact value of R5, R7, R8, R9, R11 and the properties of Q1, Q2, and Q3. Resistances R5 and R7 should be matched to within 1%. All other resistors are 10% tolerance, 1/2 watt except where noted. Wirewound resistors could be used in the bridge although they are not necessary. R11 is the temperature compensating resistor for unijunction transistor Q3. Theoretically R11 could be adjusted to compensate the circuit perfectly for fluctuations in the temperature of the room. However, the unijunction transistor introduces little thermal drift in comparison to Q1 and Q2, H2 limits the current flow through the thermistor R4, and D1 limits the inverse voltage to Q2 should R3 (the temperature control preset pot) be set at a low value. Ordinary paper capacitors are used. The diodes need not be the equivalent of the diodes listed as long as they have the proper voltage and current ratings, Diode D1 should be sili con but inexpensive diodes, a zener diode, and even an inexpensive SCR will suffice. The value of R6 is found as follows: After the circuit components are in place, a carbon
resistor of about 5000 ohms is substituted for thermistor R4 and a 10,000-ohm rheostat is substituted for R6. The heater, or a substitute for it, must be in its place or the SCR will not fire owing to lack of current — a 100 watt light bulb makes a good substitute. The rheostat is set at 10,000 ohms and the circuit is connected to the a.c. supply. Temperature preset control R3 is adjusted until the base voltages of the two transistors are nearly equal. (The heater should now be on.) The value of R6 is then lowered until the heater just turns completely off. One of the leads to R10 is unsoldered (with the a.c. supply disconnected) and the collector voltage of Q2 is measured using a vacuum-tube voltmeter. This is the required r.m.s. firing voltage of Q3. R6 and R3 are now adjusted until the collector voltages of both transistors are equal to the required firing voltage. R3 should not require much adjustment. If it does, it may be necessary to replace one of the transistors (Q1 or Q2) to obtain a better match. R3 adjusts the relative magnitude of the two collector voltages and R6 adjusts the absolute magnitude of the two collector voltages. Finally, R6 is replaced by a fixed resistor (or resistors) equivalent to the value of the rheostat, and R10 is reconnected. Capacitor C3 is listed as 0.1 µf. However, it can have any value from 0.02 µf. up to nearly 1 µf. In the former case, the control is nearly on-off; in the latter, the heater voltage varies over a large range. With larger value capacitors, the charging current is so great that it is an appreciable part of the collector current of Q2, Thus, some method of compensation is necessary. This is the purpose of R1 and C1. This network compensates by drawing an appropriate charging current from the collector of Q1 during the first cycle of unijunction Q3, Once Q3 has fired, there is no need for compensation. With the smaller value of C3, the charging current is so small that it is greatly influenced by transients in the a.c. supply. These effects can be partially eliminated by bypassing the a.c. portion of the wave by capacitor C2 which should have approximately twice the capacitance of C3. Inductor L1 in series with the SCR consists of roughly 65 turns of #16 enameled wire wrapped on a 1/4” ferrite rod, In conjunction with C4 it helps to cut down the
radio-frequency noise generated by the SCR. L1 also should help to delay the surge of current created if the heater leads should be shorted out. It could delay the current long enough for the fuse to blow and save the SCR. For this reason, the fuse should have as low a current and voltage rating as possible. If the load has appreciable inductance, a diode should be installed in parallel with it to eliminate high-voltage surges. Thyrector diode D5 should also be used to eliminate voltage transients. The thermistor specified is a 100,000-ohm nominal resist ance, glass-coated type. Ordinary disc, washer, and rod thermistors can be used but with decreased stability. In se lecting a thermistor, care should be taken that the operating point is well below the maximum safe r.m.s. voltage of the thermistor.
Positive temperature-coefficient thermistors (such as the Westinghouse Type 83401) can also be used to increase the sensitivity of this unit. H o w e v e r , PT C t h e r m i s t or s h av e a narrow tempera ture range and require inversion of the Q1-Q2 bridge. Care should be taken in modifying the bridge that the reverse voltage rating (5 v.) of the base-emitter junctions is not exceeded, Another silicon shunting diode in parallel with D1 but with opposite polarity provides one way of limiting the reverse voltages, If full-wave control is desired, the “RC Diode Slaving Circuit” (page 61 of the G-E “SCR Manual”) is ideal. This circuit has the advantages of half-wave control over full-wave power. An easier method of increasing the power control is to use a larger capacity SCR.
Method of mounting the thermistor in a protecting metal probe.
This version of the temperature controller was built in a small metal box having several ventilation holes drilled in it. Temperature controlling potentiometer has vernier dial calibrated in degrees C, The thermistor probe is ready to be affixed to tank of liquid.
