All the information you need to build a simple Precision Temperature Controller - using a few cheap off-the-shelf components.
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Temperature Controller - Test Procedure

Free Circuit



Once you're satisfied that your layout is correct - and you have made a careful and thorough check of the underside of the board - it's time to power-up the circuit and test its operation. Note that the low voltage section of the controller requires 12v AC. If you use a DC Supply - the circuit will not work.

Below - R12, R13, MOC 3021, TIC 206D & Fuse - are all mounted inside the output module. And the heat source - here a 40-Watt tungsten bulb - plugs into the socket. Test the output module - before you add the control board. When you connect a small 9v battery to the red and black wires - the bulb should light.

Fit a 3k8 resistor in place of the thermistor. And connect the output module to the control board. The resistor simulates about 30°C (86°F). Rotating R11 clockwise should turn the light on. And rotating it anticlockwise should turn the light off. With a single-turn pot - control is fairly coarse. So your aim is simply to confirm that - at some point in its rotation - R11 actually turns the bulb off.

Remove the 3k8 resistor - and replace it with the thermistor. Turn R11 to the left - until the bulb begins to dim. If you've turned R11 all the way to the left - and the bulb still hasn't dimmed - don't worry. It just means that your ambient temperature is below about 20°C (68°F). Place the thermistor about one-centimetre (½ inch) from the bulb. As it warms the thermistor - the bulb should begin to dim - and then probably turn off.


Hang the bulb inside a well insulated box. Then fill a 500ml (1 pt) plastic drinks bottle with water. And use a largish lump of BluTack - to hold the thermistor in close contact with the bottle's surface. The idea is to insulate the thermistor from the surrounding air - while allowing it to respond to changes in the temperature of the water. Placing the sensor behind the bottle - helps shield it from the bulb's radiated heat.

Turn R11 slowly to the right - until the bulb just about reaches full brightness. Then close the box - and wait. It will take a while for the temperature to stabilize. But as it rises - the bulb will slowly dim. If you can't actually see the bulb - plug a second 40-Watt tungsten lamp into the output module - and watch it dim instead.

Be patient. Warm water rises. So to begin with - the heat energy may not be distributed evenly throughout the liquid. Note also that - as the preset temperature approaches - the heater's output is at its lowest. So it will take relatively longer - to add the final degree or so.

When the temperature has been stable for several hours - it's time to try a different setting. To raise the temperature - turn R11 to the right (bulb gets brighter). And to lower the temperature - turn R11 to the left (bulb turns off). Note that the ambient temperature - is the lowest temperature you can achieve.

If You Find a Problem

If the controller is not working correctly - inspect the board carefully. Where you've cut the board to size - look for small loose strands of copper left behind by the saw. And check for short-circuits caused by component leads touching each other. It can also happen that the stripboard itself is faulty. I have seen cases where the copper tracks have not been completely severed from one another during manufacture.

If you've built your circuit using the specified components - and you've followed the step-by-step construction guide - then the bug is probably caused by something minor - a component connected the wrong way round - a missing or unwanted solder link - an incomplete cut in the track etc.

If you can't see anything obvious then adopt a systematic approach to faultfinding. Begin by double-checking that all of the cuts in the tracks have been made, that they are all - In The Right Place - and that they sever the track completely. Use a magnifying glass - and backlight the board. It only takes the smallest strand of copper to cause a problem.

When you're satisfied that the tracks have been severed in all the right places, check that you have made - and correctly placed - all five solder links. Mark each one with a felt-tip pen - or something similar - so that it can be easily identified later.

Next, carefully examine the full length of each track. If you backlight the board during the examination - it makes potential problem areas easier to spot. Look for unwanted solder links. Your felt-tip markings will tell you which ones should be there - and help you to identify any that shouldn't be there.

If all else fails and you still haven't found the cause of the problem - work your way through the assembly instructions on the Support Page. Check each individual component and link - to make sure that it's present and correctly positioned.

Print out the drawings and mark off the components as you go. Examine each individual component carefully. Take your time. If you do it right - you'll only have to do it once. Pay particular attention to the orientation of the diodes, the transistor and the electrolytic capacitors.

Voltage Readings

Here are a few voltage readings that may help with faultfinding. All are approximate. The red figures are fixed voltages. The green figures (at pin 6) should vary with both the temperature - and the setting of R11. The blue figures are readings I took with my meter on the 20v DC scale. They are inaccurate indications - of voltages that change one-hundred (120) times a second. Don't expect to get exactly the same readings. But your meter should at least tell you wheather the voltages are present - or not.

Temperature Controller - Circuit Simulation