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Looking for something to control temperature accurately and easily? This switching temperature controller can either heat or cool and can hold a temperature constant. Best of all, it’s cheap and easy to build. There are many processes which require temperatures to be closely maintained, from film and photo developing through to home brewing and even egg incubation. When temperatures change, things go haywire: colours change, the brew goes off, chooks cook . . . It’s fairly easy to control temperature when things get too cold – simply heat them up until they get to the right temperature and turn off the heater. It’s not quite so easy if things get too hot and require cooling. But this controller can do either – heat or cool – depending on your application. SWITCHING TEMPERATURE CONTROLLER Heats OR Cools Design by Branco Justic* Article by Ross Tester 54  Silicon Chip T he heater can be any standard resistive heating device such as a jug or electric heater element, an incandescent globe or even a resistor. The specifications depend on the application: more on this anon. Cooling, on the other hand, is done via a Peltier-effect device. For those who haven’t come across these before, see the separate panel for an explanation on what they are and what they do. Suffice to say at this stage they are a solid-state device which absorbs or gives off heat when a current passes through their junction. Normally, designers of semiconductors go to great lengths to minimise the effect. However, in a Peltier-effect device the action is exploited. The heat can only be absorbed from, or released to, the area surrounding the device. So the device either cools, or heats, the surrounding area. Provided certain precautions are taken, they can be quite effective coolers or heaters. They do require significant current (several amps) but typically can raise or lower the temperature by 50°C or more. There are three Peltier-effect devices specified for this project; you choose which one you want. They are rated at 42, 60 and 75W and draw 4A, 6A and 8A respectively for a ∆T(or difference in temperature between the two sides of the device) of 65°C. Unlike most temperature controllers which simply switch a heater on or off to maintain temperature, this controller switches the heater or cooler by varying the duty cycle. This form of control is not only very accurate; in this application it’s also a requirement of the Peltier device which must be switched on and off at a minimum of 2kHz. Repeatedly switching on and off DC would result in mechanical stress to the device and its possible damage or destruction. That’s not to say you cannot use a Peltier device on DC. If the device is turned on and left on for relatively long periods, DC is fine. It’s only when used in a temperature control application where the device is switched on and off many times over relatively short periods to maintain a constant temperature that mechanical stress really becomes a problem. And while on the subject of Peltier devices, there is nothing to stop you using one as a heater, if you wish. But A typical Peltier-effect device. Actual size is 40mm square and about 4mm thick. When connected to power, one side of the device becomes about 65°C warmer than the other. This can be used for heating or cooling. given their cost and the low cost of a resistive element, we know which we’d prefer! The circuit Operation is most easily understood if you break the circuit down to its basic functions. Fig. 1 shows the circuit diagram for the controller. Transistors Q1, Q2 and Q3, along with ZD1 and associated components, form a series voltage regulator supplying a reference voltage of about 7.5V to op amp IC1. Q1 and Q2 are in parallel and are both driven by Q3, effectively forming a Darlington transistor. The stable voltage at the emitters of Q1 and Q2 is important because it gives the controller its accuracy. Temperature setting and sensing is performed by a preset pot and thermistor connected to the input of IC1a, one of the four op amps in an LM324 quad package. The voltage at the inverting input (pin 13) is set by the preset pot and held stable by the 10µF capacitor. The voltage at the non-inverting input, though, varies with temperature due to TH1, a negative-temperature-coefficient (NTC) thermistor. While nominally 68Ω in resistance, the NTC thermistor decreases its resistance with an increase in temperature (and conversely, of course, increases its resistance with a decrease in temperature). Therefore, if the temperature rises, the thermistor’s resistance falls and the voltage at pin 12 of the op amp will drop slightly. If the temperature falls, the voltage will rise. The op amp has a gain of roughly 221, set (mostly) by the 220kΩ and 1kΩ resistors and the slight change in voltage at the input results in a much larger change in voltage at the output. For example, if the voltage rises just 10mV at the input, the output voltage will rise by more than 2V. This voltage is applied to the pin 9 inverting input of IC1c and to the non-inverting input of IC1d, pin 5. You will note, though, that there is another op amp in the circuit, IC1b. It is connected as a sawtooth oscillator, with an output voltage varying between about 1/3 and 2/3 of the rail voltage at about 2.2kHz. When power is applied, the + input of IC1b is held at nearly 4V via the voltage divider (R8 & R9) across the regulated supply. C6 is discharged but immediately starts to charge via R11. When C6 reaches the input threshold voltage of the of IC1b, it discharges via R11 and the whole process begins again. This sawtooth waveform is applied to IC1c and IC1d. IC1c and IC1d are comparators – that is, they compare the voltage between their + and - inputs and turn their high or low accordingly. If the voltage is higher on the + input than the - input, the output goes low. If it is higher on the - input than the + input, the output goes high. Perhaps this is most easily explained by referring to Fig. 2. The output of each comparator then is a pulse waveform at 2.2kHz with a duty cycle (or on to off ratio) which is in inverse proportion to the output voltage of IC1a. Depending on whether heating or cooling is required, this waveform is used to switch Mosfet Q5 or Q6. A short duty cycle means very little power is applied to the Mosfet gate while a long duty cycle means it is being powered most of the time. Hence it cools (or heats) for most of the time. In the cooling circuit (IC1c), a green LED (LED2) connected to ground gives a visual indication of the degree of cooling. Even though the circuit is beAUGUST 1999  55 NOTE: INSTALL EITHER HEATER OR COOLER BUT NOT BOTH. ing driven at 2.2kHz (essential for the Peltier-effect device) you cannot see the LED turning on and off this quickly. The heating circuit is slightly more complex, due in part to ensuring that the gate of the P-channel Mosfet (Q5) is not over-driven; however, it operates in much the same way – the main difference being it is opposite in effect. The 9.1V zener and series diode ensure that the gate cannot be taken more than about 10V below the source, when transistor Q4 is turned on. A red LED (LED3) in series with the base of Q4 shows the degree of heating. One area not yet mentioned is the power supply. This depends to a large extent on the amount of heating or cooling required – naturally, this is limited when you use a 12V supply. PULSE-WIDTH MODULATION EXPLAINED Op amp 1 is connected as an oscillator, producing a sawtooth waveform across the capacitor. This is connected to one of the inputs of op amp 2. The other input has its voltage fixed at a certain level by the voltage divider across the supply. As the sawtooth waveform voltage rises, it reaches this threshold voltage and the op amp output goes high until the sawtooth waveform voltage again falls below the threshold. If the threshold voltage is high, op amp 2’s output is high for a very short period compared to its low-time each cycle. If the threshold voltage is low, the op amp output is high for a significantly greater length of the cycle. The difference between high and low time is called the “duty cycle”. OP AMP 1 OP AMP 2 Fig. 1: the temperature controller is capable of either heating or cooling, depending on which device is installed. Fig. 2 (right): how pulse-width modulation works. At top is a simplified circuit which you can see corresponds to IC1b and IC1d in the circuit above. Below are the waveforms showing the inputs and the output for a high voltage and a low voltage. The duty cycle, or on time to off time, is in inverse proportion to the input. 56  Silicon Chip Parts List 1 PC board, 114 x 77mm 1 plastic case#, with label, 85 x 120 x 28mm 1 14-pin IC socket 1 U-shaped heatsink, 32 x 28 x 13mm 2 3mm x 10mm screws & nuts 4 lengths figure 8 cable (see text) Virtually the same size as the finished project, this photo shows how and where all components are placed. Note the electrolytic capacitor at the bottom of the PC board – it is a PC mounting type but is mounted lying down. We have shown both Mosfets & heatsinks installed for clarity: normally there would be only one. The circuit as shown is suitable for supplies up to 50V or so with only one resistor change (R15). The voltage ratings of C3 and C6 should also reflect the higher supply voltage – they should be at least 30% and preferably about 50% higher than the supply. As with most circuits, you can use higher voltage rated capacitors if you wish but these tend to be more expensive. Regardless of the supply voltage, it needs to be fairly well filtered. Remember, too that a heating element or Peltier device will each draw significant current – quite a few amps, in fact. The switching Mosfets (Q5 & Q6) are both rated at 12A with a maximum dissipation of 88W. The heater, or P-channel Mosfet could therefore be used to control loads up to 1kW with adequate heatsinking (certainly much larger than the heatsinks specified). For even higher loads, higher rated Mosfets could be used or even paralleled. A heating element of about 4Ω and a 12V supply would be acceptable with the heatsinks supplied. Larger heatsinks would allow a 2Ω element (72W). With a higher supply voltage, much higher load powers can be produced while maintaining the same dissipation in the Mosfet. A 50V supply and a 16.6Ω heater element would be about 150W; an 8.8Ω load would be about 300W. In practice, a 36W heating load (12V<at>3A) would produce acceptable heat dissipation from Q5, mounted on the PC board and using the heatsinks specified. What type of heating element? That’s up to you: series or parallel combinations of low voltage light globes are one idea. Or perhaps you could use an electric jug element stretched out to full length and cut to a suitable length. A 1kW jug element is about 60Ω in water – cut in half (30Ω each) and twisted Resistor Colour Codes         Value 220kΩ 100kΩ 47kΩ 10kΩ 2.2kΩ 1kΩ 680Ω 10Ω 4-Band Code (1%) red red yellow brown brown black yellow brown yellow violet orange brown brown black orange brown red red red brown brown black red brown blue grey brown brown brown black black brown 5-Band Code (1%) red red black orange brown brown black black orange brown yellow violet black red brown brown black black red brown red red black red brown brown black black brown brown blue grey black black brown brown black black gold brown Semiconductors 1 LM324 quad op amp (IC1) 4 2N5551 NPN transistors (Q1, Q2, Q3, Q4) 1 Power Mosfet – either IRF9530 P-channel (Q5) or BUK453 N-channel (Q6) 1 GIG power diode (D1) 1 1N4148 signal diode (D2) 2 9.1V zener diodes (ZD1, ZD2) 1 4mm yellow LED (LED1) 1 4mm green LED (LED2) 1 4mm red LED (LED3) 1 Peltier-effect device (see panel) Capacitors 1 1000µF 25VW# electrolytic (C3) 1 220µF 16VW electrolytic (C4) 1 100µF 25VW# electrolytic (C6) 2 10µF 16VW electrolytic (C1,C2) 1 0.1µF 16VW ceramic or polyester (C5) 1 .0022µF 16VW ceramic or polyester (C6) Resistors (0.25W, 1%) 2 10Ω (R2, R3) 2 680Ω (R1, R16) 1 1kΩ (R6) 6 2.2kΩ (R4, R5, R12, R13, R15, R17) 1 10kΩ (R10) 3 47kΩ (R8, R9, R11) 1 100kΩ (R14) 1 220kΩ (R7) 1 100Ω horizontal trimpot (VR1) 1 68Ω NTC thermistor (TH1) A kit, not including Peltier device, is available from Oatley Electronics for $15 plus p&p. #Some components in the Oatley kit may be recycled from existing equipment.­ Capacitor Codes   Value IEC Code EIA Code 0.1µF 100n 104 .0022µF   2n2 222 AUGUST 1999  57 together would give 15Ω; cut in quarters (15Ω) and all twisted together would give about 4Ω, and so on. The cooler, or N-channel, Mosfet should be more than adequate to handle any of the specified Peltier devices. If you want to use more Peltier devices (in parallel) you will probably need better heatsink-ing and perhaps a higher rated Mosfet as well. Fig. 3: the PC board component overlay. Compare this to the photograph when assembling the board and you shouldn’t have any problems. Again, both Mosfets are shown installed – you choose the one you want for heating or cooling. Construction All components with the exception of the thermistor (TH1) are mounted on a PC board measuring 114 x 77mm. This is designed to fit into a small plastic case measuring 120 x 85 x 32mm. The cases supplied in the kit are recovered from surplus stock so are not new but still perfect for the job. A label fixes to the front of the case with the power, cool and heat LEDs showing through. This label is printed on paper and will need some covering to protect it. (We use adhesive plastic). Begin construction by checking the PC board pattern for any obvious defects. If so, either correct or replace the board. There are six holes on the PC board which may need to be enlarged – the four mounting holes (in the corners) all need to be drilled out to 5.5mm (7/32in) while the two holes for the Mosfets (in the middle of the large copper areas) should be 3mm (1/8in). Start by inserting all resistors in their appropriate positions, soldering as you go. The three links on the board can be made from cut-off resistor pigtails. There are seven capacitors to be inserted, of which all but two are polarised electrolytics. One of these, the 100µF electrolytic (C6), is a PC board type (ie, both leads emerge from the same end) but is actually mounted lying down on the board. A dab of super glue or silicone sealant underneath it would help keep it in place. If you need to fit a higher voltage rated capacitor here (which will normally be larger), there is plenty of room to do so. Next mount all the small semicon58  Silicon Chip ductors, taking special care with the diodes ZD1, ZD2 and D2. Sometimes they look almost identical to the naked eye – you may need a magnifying glass to properly identify them. Fortunately the power diode, D1, normally looks quite different! Solder in the pot (VR1) and the IC socket but don’t insert the IC just yet. Then solder in the three LEDs so that their tops are 25mm above the surface of the PC board. The yellow LED is LED1 (power), the green LED2 (cool) and the red LED3 (heat). The last component to mount is the appropriate Mosfet, Q5 or Q6. Again, these look virtually identical so be careful. It mounts flat onto its heatsinks with the legs bent down. Before mounting, hold its three legs with a pair of needle-nose pliers and bend the ends of the legs down 90°, 5mm away from the Mosfet body. Check a second time that you have the right Mosfet in the right spot: the BUK453 is for cooling, the IRF9530 is for heating. Before soldering, slip the heatsink underneath and secure both the heatsink and Mosfet with 3mm screws and nuts. No insulation is necessary between the Mosfet and heatsink but a small dob of heat transfer compound wouldn’t go astray. You could, of course, install both Mosfets and install either the heater or cooler (but not both). Conversely, if you will only ever require cooling (or heating), all components after IC1d (or IC1c) could be left out. Solder in a suitable length of figure-8 cable (or two individual wires) for the thermistor, the heating element and the Peltier cooler, along with suitable red and black wires for power connection. Ensure that the cables have a high enough current rating to cope with the current drawn. To complete the PC board, insert the LM324 IC into its socket, making sure it is the right way around. Put the project aside for a while. Enjoy a cup of coffee before you check over all your component placement and soldering. Checking it out Don’t connect your Peltier cooler or the heating element just yet. However, you will need to connect the thermistor to its leads. Apply power and confirm that the yellow LED comes on. Measure the voltage across C4 – it should be around 8V – and if you have either an oscilloscope or frequency meter, check that there is a 2.2kHz output from pin 1 of IC1b. You can also check that the heating and cooling LEDs come on as you vary VR1 over its travel. If everything checks out OK, turn off and connect the heating element or Peltier device to their appropriate leads. Note that the heating element should not be polarised but the Peltier device is: the black lead connects to the Mosfet drain for correct use. Now you can check that the appropriate devices really do heat or cool as they should. You will probably find that it takes a lot longer for a Peltier device to cool than a heating element to heat – that’s the nature of the beast. Finishing off As mentioned before, the case supplied with the kit was intended for another device. It has a number of holes and cutouts down one side which are handy to take the external leads through. You will need to drill three 4mm holes through the lid of the case (and the label) for the three LEDs to poke through. It’s easiest to do this with the label fixed to the case – we used spray adhesive. The label itself might need some protection – we use plastic contact on our projects (see the article in the April 1999 issue). With the LEDs soldered in place as noted above, they should just poke through the holes in the front panel when the PC board is mounted in the case lid. The board sits on small rebates in the case mounting posts and does not require any further securing. Take all of the external wiring through any suitable holes in the side of the case and pop on the bottom, securing it with the four screws provided. The thermistor needs to be mounted in very close contact with the item being temperature controlled but away from the Peltier device. If it’s a liquid, ideally the thermistor needs to be actually immersed in it but this is often impractical or dangerous WHICH PELTIER DEVICE? As well as the kit of parts, Oatley Electronics currently have three Peltiereffect devices available to suit this project. All measure 40mm x 40mm and have a ∆T of 65°. 4 Amp – Qmax 42W $25.00 6 Amp – Qmax 60W $27.50 8 Amp – Qmax 75W $30.00 Contact Oatley Electronics on (02) 9584 3563, Fax (02) 9584 3561 or email oatley<at>world.net (or visit their website, www.oatleyelectronics.com) * Branco Justic is the Manager of Oatley Electronics. (the metal leads could contaminate or be damaged by the liquid). The thermistor could be “potted” for protection but this could inhibit its ability to detect temperature changes. This part SC is left to you! WHAT IS A PELTIER-EFFECT DEVICE? The “Peltier effect” occurs when current flows across the junction of two dissimilar metals or semiconductors. In one direction, heat is absorbed into the junction; in the other direction, heat is given off. This effect can be used to make a solid-state heater or cooler. They are usually called Peltier-effect devices or Peltier devices but you may see them referred to as thermoelectric modules. A typical Peltier device is composed a number of P-type and N-type Bismuth Telluride dice “sandwiched” between two ceramic plates. While both P-type and N-type materials are alloys of Bismuth and Tellurium, both have different free electron densities at the same temperature. P-type dice are composed of material having a deficiency of electrons while N-type has an excess of electrons. As current flows through the module it attempts to establish a new equilibrium within the materials. The current treats the P-type material as a hot junction needing to be cooled and the N-type as a cold junction needing to be heated. Since the material is actually at the same temperature, the result is that the hot side becomes hotter while the cold side becomes colder. Typical Peltier devices draw between 4A and 10A <at> 12V but there are “industrial” types drawing 100A or more. In a resistive load, the heat created is proportional to the square of the current applied (I2R). In a Peltier device, the heat created is actually proportional to the current because the flow of current is working in two directions. Therefore, the total heat ejected by the module is the sum of the current times the voltage plus the heat being pumped through the cold side. Typically, the difference between hot and cold sides can be 65°C or more. The ability to add or remove heat is mainly a function of the current-handling capability of the dice. With no moving parts, Peltier devices are rugged, reliable and quiet. They are typically 40 x 40mm square or smaller and approximately 4 mm thick. The industry standard mean time between failures is around 200,000 hours or over 20 years for modules left in the cooling mode. While not polarised in the true sense, most devices have a red and black lead attached, signifying the positive and negative connection. The convention is that with the device lying flat and the leads pointing towards you with the red on the right side, the lower plate is the “hot” side. Reversing the power connections has no effect except for swapping which of the two plates becomes the “hot” side. The Peltier device works as a heat pump. In a cooling application it takes heat from the surrounding area (or more correctly anything in intimate contact with the cold side) and passes it through to the hot side. Normally the hot side is itself thermally bonded to a heatsink, often fan-cooled, to disperse the heat into the atmosphere. Because the two ceramic plates of the device are bonded together and one side expands as it gets hot while the other contracts as it gets cold, thermal stresses occur. If cycled on and off too often, damage or failure may occur. For this reason, where Peltier devices are to be turned on and off repeatedly, they are fed with a pulse-width modulated waveform instead of DC. To finish, some trivia: heat one side of a Peltier device and you’ll generate a tiny electric current – the “Seebeck” effect. AUGUST 1999  59