Egg Incubator Thermostat

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jaro1

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Hello, could you please advise me, I would need to remove this fan from this thermostat, which is functional. The old fan has reached its end of life, and a 12v PC fan will come in its place. But the old fan was connected in series with zener 6 8V, which served as a resistor, had a resistance of 68R, 50W, and it was powered at the same time. After disconnecting the fan, the thermostat does not work, it is not powered. So it would have to be connected somehow so that the zener can survive, or adjust the power supply, I just don't know how. Thank you all
 

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Solution
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Hi jaro1,
indeed, in your circuit the fan was used as kind of pre-resistor. So w/o fan, there is no dc/ac current path to "N" (green line in attachment) for supply of the op-amp circuit. Brief calculation shows you need ~3mA current flow.
You could use a MKT Cap (similar to yellow one) with 47nF/275V, or a resistor of 80k/1W, replacing the fan. My recommendation is the Cap solution.
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Hi jaro1,
indeed, in your circuit the fan was used as kind of pre-resistor. So w/o fan, there is no dc/ac current path to "N" (green line in attachment) for supply of the op-amp circuit. Brief calculation shows you need ~3mA current flow.
You could use a MKT Cap (similar to yellow one) with 47nF/275V, or a resistor of 80k/1W, replacing the fan. My recommendation is the Cap solution.
 

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Solution
Thanks for the feedback & that it's working for you.
My I ask what was the reason to add two resistors?
As you are using quite high cap value of 220n, maybe current is too high for 6V8 Zener, therefore you used 220R?
 

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