Thermostat for room heater

Electric heating is a very nice thing. Not only is it clean and rather safe, it also is easily controllable. But many room heaters have no thermostats, or just some simple mechanical bi-metal type of kludge, mounted right in the heater, that maybe manages to keep the average temperature of the heater relatively constant, but never that of the room!!!

Here is a circuit that implements phase-control of heaters, adjusting their power smoothly from zero to around 95% of the rated power, keeping the room temperature to within 1 or 2 degrees, while using few components.

Here is the full resolution version of this schematic. How do you like it? I created this design just yesterday (2000-6-10, southern winter!), so I'm still mighty proud of it, while enjoying the regulated temperature in my room at this very moment!
Notice that this is entirely an AC design. The TRIAC, DIAC, capacitor, resistor, potentiometer and thermistor are all AC devices. I did not have a suitable MOV at hand, so I used two Zener diodes in back-to-back fashion. The poorest aspect is that I don't know of the existence of any AC transistors, so I had to pair two complementary ones, joining the collectors through diodes, thus creating my AC transistor from 4 parts! It works well, but if the transistors are not well matched, this will result in the TRIAC firing earlier in one half cycle than in the other, superposing a DC component on the AC line, which in certain (few) cases can lead to trouble. Try to select nicely matched complimentary transistors!
Let's suppose that a positive half-cycle starts. As the voltage rises, the 22k resistor will feed the circuit. The upper Zener diode will clamp the voltage at 42V, with the lower one just conducting. The 100k pot is adjusted in such a way that the base-emitter voltage is just enough to make the PNP transistor conduct. The capacitor will slowly charge up, reaching 32V sometime within the half cycle. The DIAC will then fire, dumping the capacitor's charge into the TRIAC, which will also fire, and conduct for the rest of the half cycle, keeping the control circuit powered down.
For the negative half cycle, everything will be the same, except that the Zeners reverse roles, and the NPN transistor conducts. The unused transistor in any case stays off, with 0.6V inverse voltage across the base-emitter junction, and its collector insulated by the diode.
If the room temperature rises just a little, the resistance of the NTC will drop a little bit. The small reduction in base-emitter voltage will make the transistors conduct dramatically less current, so that the TRIAC will fire much later in the cycle, strongly reducing the heating power. If the temperature rises enough, for example by sun shining into the window, the capacitor will never charge to 32 V, the TRIAC will never fire, and the heater will stay completely off. On subsequent cycles the capacitor will charge positively and negatively, never accumulating enough voltage to fire the DIAC, eliminating odd-cycle firing, which is a problem in some other designs.
At lower temperatures, the transistors will saturate, firing the TRIAC very soon after the waveform reaches the 42 V level. This affords around 95% of heater power.

Practical construction and use

The 22k resistor dissipates more heat as the heater power is reduced, reaching a maximum of about 2W when the heater is fully off. The TRIAC causes about 1.5V drop, so with a 2kW heater it can dissipate almost 15W ! It needs a heat sink. You can bolt it to the metal case of the thermostat, but in this case be sure to follow safety rules. I suggest using an internally insulated TRIAC. They are easily available.
Both the TRIAC and the resistor, and to a much lesser degree the other parts, generate heat that will act upon the thermistor, if you don't take care in placing it properly. While you could install the thermistor remotely, and mount all other parts inside the heater (at a low, cool place!), the long wires between the thermistor and the transistors would lead to RF pickup, and the heater would power up and down erratically in response to radio transmitters, cell phones, etc! Better keep the thermistor and the rest close together, and mount them in such a way the there can be easy vertical air flow through the case, and that the thermistor is at the lowest spot in the case, with the heat-producing components at the top. You may even include some small aluminum shield between the thermistor and the hot areas! The idea is that the thermistor should feel the room temperature, and nothing else.

This printed circuit board, when installed in the proper position, works quite well. Of course, you also can install the thermistor away from the board.

This circuit could also be used to control the temperature of tropical aquariums, chemical processes, etc, as long as their requirements can be satisfied by a 2 degree tolerance. For color photographic work, specially slide developing, this is NOT good enough. You would need real PI or PID control for such a task. This requires a few more components!

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