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Build this universal temperature controller Ever wanted to control the temperature of liquid in a tank or perhaps the air in an enclosure? This low cost controller will do just that. It will hold the temperature at any setting from near freezing to near the boiling point of water. By BRANCO JUSTIC & JEFF MONEGAL When you think about it, there are quite a few applications for a universal temperature controller. For example, you might want to control the temperature in a tropical fish tank or the temperature of etchant when making printed circuit boards. Perhaps you are a keen "home brewer" and want to make the best beer. Again, this unit will closely control the temperature of your " brew" . On the farm , it could hold the temperature of a small hatchery at a set level. We're sure readers will come up with a lot of other uses. To ensure complete safety, the 60 SILICON CHIP circuit works on low AC voltages (anywhere between 20 and 40VAC), with 32VAC shown as the nominal input on the circuit. The controller is housed in a neat black plastic case measuring 185 x 125 x 50mm. It has rounded corners and clips together. On the front panel, it has a single heater control and two LEDs, one to indicate that the desired temperature has been reached and the other to indicate· that the heater element is actually on - ie, it shows the heater element cycling on and off. There is also a 6.5mm jack socket to connect a temperature sensor which is a negative temperature coefficient (NTC) thermistor. This is placed in the medium (liquid or air) to be controlled. We'll talk about this later. On the back panel, there are two pairs of banana plug sockets. One pair is for the 32VAC input while the other is for the leads to the heating element. The required temperature setting, as determined by the front panel knob, will be controlled to within ± 1 °C, once the "Temp Correct' ' LED indicator comes on. Circuit details This temperature controller has been designed by Oatley Electronics. The circuit is perhaps a little unusual (and even overdesigned) in some aspects but uses cheap and readily available components. In fact, the full kit for this project is so cheap there would be little point in trying to buy the individual parts separately, even if you have some of them on hand. Now let's look at the circuit details. Power for the circuit comes from a 32V transformer (not supplied) which is fed to diode Dl and the 470µF capacitor Cl to produce about 45V DC. This is then fed to a regulator circuit consisting of transistor Ql, diode D2, zener diode ZD1 and .associated resistors and capacitors. ZD1 is a 15V zener which provides a constant voltage at the base of Q1, a BD679 Darlington power transistor. Ql functions as an emitter follower, producing a regulated voltage of close to 13.6V at its emitter. The 100µF capacitors C3 and C4 provide extra filtering for the DC supply to the base of Ql. D2 also helps to improve the filtering but it really is gilding the lily. Op amp IC1 is the heart of the circuit. It is connected as a high gain (x 10,000) inverting amplifier but really functions as a comparator. It compares the voltage across A negative temperature coefficient (NTC) thermistor is used as the temperature sensor. These feeds back information to the controller so that temperatures can be kept within ± 1 °C. The thermistor is housed in a plastic tube which is sealed to prevent corrosion of the leads. R12 100<:l + v - - + - - + - - - - + - - - - - - - + - - -+--"Ni~-+-- - - - - - - . - - - - - - + - - - , C7 10 + A 16VWJ C9 100 D3 1N4148 + 16VWJ: R16 56k R7 10M 02 2N2907A C C10 .01!. D4 1N4148 C5 1D .,. ':' + 16VWJ 03 ':' 2N2907A C C11 01:t T B EOC VIEWED FROM BELOW ~ EC B ,; A2 ':' D1 1N4004 .~. +v 32VAC + 31~~11~~~1 m CJ TEMPERATURE CONTROLLER + C4 + 400mW -:- J '":" Fig.1: op amp IC1 is wired as a high gain amplifier but really functions as a comparator. When the thermistor (RTtttl cools down, IC1 's pin 6 output switches low and enables oscillator IC2b via inverter IC2a. The output pulses from IC2b then trigger Triac Q4 via IC2c/Q2 and IC2d/Q3 to turn on the heater. D1, Qt and their associated components provide a regulated 13.6V supply. JUNE 1989 61 Fig.2: here's how to install the parts on the PCB and wire up the external hardware. Note the use of shielded cable for the connection to the 6.5mm jack socket. There is no need to isolate the metal tabs of Ql and Q4 from their respective heatsinks - just make sure that the heatsinks don't touch since they sit at different voltages. CAPACITORS D D D D No. 1 3 2 1 Value 0 .1µF .01 µF .001 µF 680pF IEC 1 OOn 1On 1n 680p EIA 104K 103K 102K 681K RESISTORS D D □ □ D D □ D D No. 1 1 1 2 4 2 5 3 1 Value 10MO 1MO 100k0 56k0 10k0 4 .7k0 1 kO 1000 4.70 the NTC thermistor, RTHl, with the reference voltage set by the temperature controller, VR1. When the thermistor cools down, the voltage across it increases. This causes the voltage at pin 2 to rise above that at pin 3. When this happens, IC1 's output switches low. The output line from IC1 is filtered with R8 and C6 and then fed to Schmitt trigger stage IC2a. This inverts the output from IC1 and enables oscillator stage IC2b. 62 SILICON CHIP 4-Band Code brown black blue gold brown black green gold brown black yellow gold green blue orange gold brown black orange gold yellow violet red gold brown black red gold brown black brown gold yellow violet gold gold 5-band code brown black black green brown brown black black yellow brown brown black black orange brown green blue black red brown brown black black red brown yellow violet black brown brown brown black black brown brown brown black black black brown yellow violet black silver brown IC2b is a Schmitt trigger oscillator with the frequency set by Rl 1 and C8. When its pin 2 is high (corresponding to a low output from IC1), the oscillator runs at about 500Hz. IC2a also drives the two LEDs via lkO resistors. When IC2a's output is high, the " Heater On" LED is alight and the "Temp Correct" LED is out. When IC2a's output is low, the "Temp Correct" LED lights and the "Heater On" LED goes out. The rest of the circuit is used to trigger the Triac Q4. Transistors Q2 and Q3 drive the gate of the Triac via C14, a O. lµF capacitor. Q2 "sources" current into the gate via C14 and charges it in one direction and then Q3 charges C14 in the other direction. IC2c and IC2d, together with diodes D3 and D4 and associated components, form a complementary drive circuit for Q2 and Q3 which ensures that there is a "dead time" between one The PC board is secured inside the case using machine screws and nuts. Smear the mating surfaces of Ql, Q4 and their heatsinks with heatsink compound before bolting the assemblies together and take care with component orientation. transistor turning off and the other turning on. This "dead time" is good insurance because if the two transistors happen to turn on together they will provide a direct conduction path between the positive and negative supply rails and thereby destroy themselves. Construction As mentioned previously, the Temperature Controller is housed in a dip-together plastic case. Inside is a printed circuit board which accommodates all the circuitry. After checking the board for any track defects such as shorts or open circuits, you can start the board assembly by installing the resistors, diodes and small capacitors. Make sure that the polarity of the electrolytics and diodes is correct. Now solder in QZ and Q3 and the two IC sockets. Note that the IC sockets have a slight "nick" at one end which matches the moulding of most ICs, so as to indicate correct orientation. You can now solder in Darlington transistor Ql and the Triac (Q4). Note that both these units are laid flat and mounted on U-shaped heatsinks which are each secured to the board by one screw and nut. No mica insulation is needed between the heatsink and the transistor (or Triac) which means the heatsinks are "live" at about + 45DC [for Ql) or 32VAC (for Q4}. For this reason, the two heatsinks must not touch. When you have completed the board, the two ICs can be inserted into their respective sockets and the board can then be set to one side. The case now needs to be drilled for the board mounting holes and the front and rear panels need to be drilled for the various pieces of hardware. You may also care to label the controls and this can be done with white Letraset. Now install the hardware items into the case, then install the PC board and complete the wiring. Check your work carefully against the wiring diagram. Sensor lead Now you will need to make up the sensor lead for the thermistor. For this you need a 6.5mm jack plug and a length of light duty twin flex or shielded cable. Connect the jack plug to one end of the cable and the NTC thermistor to the other. The thermistor can be protected from damage by fitting a length of heatshrink tubing over it. With all assembly complete, plug in the sensor cable and apply 32 volts AC to the input of the controller. Measure the DC voltage between the collector (centre pin) of JUNE 1989 63 PARTS LIST 1 plastic instrument case , 185 x 125 x 50mm 1 PC board, code OETC89, 102 x 85mm (available from Oatley Electronics) 1 1oon wirewound potentiometer (VR 1 ) 1 instrument knob to suit pot 1 6800 NTC thermistor (Rrn 1 ) 1 8-pin IC socket 1 1 4-pin IC socket 2 U-shaped transistor heatsi11 ks (included in kit) 1 6 . 5mm jack socket 1 6 .5mm jack plug (for thermistor cable) 4 banana jack sockets (2 red, 2 black) 4 banana plugs 2 5mm LED mounting bezels Semiconductors 1 SC151 D, E or BTl 39-600 15-amp Triac (Q4 ) 1 80679 NPN Darlington transistor (01) 2 2N2907 A NPN transistors (Q2 , Q3) 1 TL071 op amp (IC 1) 1 4093 quad Schmitt NANO gate (IC2) 1 1 N4004 silicon power diode (01) 3 1 N4 1 48 signal diodes (0 2 , 03 , 04) 1 15V 400mW zener diode (ZD1) 1 4 . 7V 400mW zener diode (ZD2) 1 5mm red LED 1 5mm green LED Capacitors 1 4 70µF 63VW electrolytic 1 1 001,tF 35VW electrolytic 2 1 OOµF 1 6VW electrolytic 1 0. 1µF disc ceramic 3 .0 1µF disc ceramics 2 .001 µF disc ceramics 1 680pF disc ceramics Resistors ( ¼ W, 1 10MO 1 1MO 1 1 OOkO 2 56k0 4 10k0 5 %) 2 4.7k0 5 1k0 3 100!2 1 4 . 7!2 Miscellaneous Hook-up wire , solder, screws , nuts, washers, heatsink compound. 64 SILICON CHIP You can use either shielded cable or light-duty twin flex to make up the sensor lead. One end of the cable goes to the thermistor leads while the other goes to a 6.5mm jack plug. where the fault lies. If you have no voltage from Q l it is likely that you have r ever se connected the zener diode, ZDl . Now check that you can get the red LED to light by rotating the control knob clockwise. Rotating it anticlockwise should extinguish the red LED and light the green LED. Heater elements The NTC thermistor looks like a ceramic capacitor but is labelled 68rl 10%. It's resistance varies with temperature. Ql and the Al terminal of Q4 [this is OV in the circuit). It should be about -t- 45V. Now measure the voltage at the emitter of Ql. It should be about + 13.6V. If these voltages are not correct check the circuit around Ql to see If you have to make up heating elements for temperature contr olling a liquid, the best approach is to use modified electric jug elements. These are cheap, come with preformed leads, and it is easy to take off resistance wire to obtain the exact ohms value you r equire. If the liquid to be heated is corrosive, the heating element should be sealed in a Teflon bag. We 'll have more to say on this topic in a future issue. ~ Where to buy the parts A complete kit of parts for this project is available from Oatley Electronics, 5 Lansdowne Parade, Oatley West, NSW 2223. Phone (02) 57 9 4985. Address all mail orders to Oatley Electronics, PO Box 89 , Oatley NSW 2223 . The kit can be purchased as follows: The complete kit, as per prototype but Without instrument case , is $29 .90 . The instrument case is $8 .00. A short form kit with the PC board , 80679 transistor and NTC thermistor is also available for $12 .00. Postage and pac king for any of these combinations is $ 2 .50. A suitable 32V 4 A transformer is also available for $26.90 plus freight. Limited numbers are available. If you have trouble getting your Temperature Controller to work , post it to Oatley Electronics at the above address. They will fix it for you at a minimum service charge . For more details, phone them on (02 ) 5 79 4985.