This circuit is potentially dangerous! If your electricity comes with a "live" wire and a "neutral", the neutral is usually connected to the earth. Sometimes there is no neutral at all and you have 3 phase wires, with 115V to 240V between two phases. If you  form a conductive path to the earth and you touch one of those phase ("live") wires,  this can be lethal!In the first circuit, the NTCs are not insulated from the mains which means the whole circuit  is live. they have to be mounted out of reach! Only work  on a project like this if you know what you are doing, this is nothing for beginners!Even if you are experienced, you should always use a 30mA drip-switch when working on any project.

 

This is a universal thermostat to regulate 220V systems. Around 1980, I first designed it to regulate the temperature of incubators but with minor adjustments, it can also be used to regulate room temperature.The circuit is based on a 741 opamp which compares the tension of potential divider R1/R2/R3 with the reference tension of potential divider P1/R4/R5. When the temperature rises, the resistance of the NTCs drops, together with the tension at the + entrance of the opamp. At a certain moment, the opamp will flip and T1 will stop conducting, causing the heating to cease.The desired temperature at which this should happen, can be adjusted with P1, the 10K potentiometer.In the above circuit, the thermostat's optimal working point is 38°C (101°F). For room temperature, some experimenting with the resistances P1/R4/R5 needs to be done. Try to keep the sum of their values around 27KΩ.The first circuit  activates a  triac and can deliver more power than the second circuit. The left side of this one is exactly the same but insulated from the mains by an optocoupler (see parts list). The power is defined by the thyristor and the diodes D5-D8 (1N4007 = 1 Amp). For even more power whilst insulated, see Universal 115V - 240V Thermostat 2.Example of a simple print with layout.

Parts List:

Unless otherwise stated, resistor wattage = 0.25W.

FIRST CIRCUIT : R1, R2 = NTC 1KΩ R3 = 6.8 KΩ R4 = 12 KΩ R5 = 3.9 KΩ R6 = 27 KΩ R7 = 820 Ω D1, D2, D3, D4 = 1N4148

D5 = LED C1 = 470µF/25V C2 = 100nF/500V T1 = BC337, 2N1613 Tri1 = triac TIC206M, TIC216M IC1 = 741 opamp Transformer 12V sec. 100mA

SECOND CIRCUIT : R1, R2 = NTC 1KΩ R3 = 6.8 KΩ R4 = 12 KΩ R5 = 3.9 KΩ R6 = 27 KΩ R7 = 820 Ω R8 = 22 KΩ R9 = 1.5 KΩ R10 = 270 Ω D1, D2, D3, D4 = 1N4148

D5 = LED C1 = 470µF/25V D6, D7, D8, D9 = 1N4007 T1 = BC337, 2N1613 Th1 = thyristor TIC106 IC1 = 741 opamp IC2 = opto coupler CNY47, CNY57, CNY17*

(* I never tried the CNY17 but it seems to be easier to get and according to its specifications,it should function here.)

Transformer 12V sec. 100mA

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This page was last updated on 2013-09-22 at 08:40