Low-high voltage protection circuit with time delay.

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Thread Starter


Joined Apr 7, 2016
Please help me understand this circuit. It's a circuit designed especially for protecting the fridge, AC units, etc.

Moderator's Note : The circuit was deleted.

The designer of this circuit (not me) used LM324, and he/she used only three comparators out of four available to reduce the cost of it (witch is nice).
Inside red rectangle there is the power supply for this circuit, inside the blue square there is the Time Delay for the circuit (3-5 min).
My questions :
1. C1 is a 680nf filming capacitor for voltage drop & current limit. Should the designer used a higher value for a 24v relay circuit?
(220v - 30v) \ 47ma = 4043 ohm ??!!
2. Z-DI is a 27v1W zener. I can't find a 1w or higher in my local market (and can't order online), what can we do? Using a resistor ? Using LM7824 ?
3. If I want to control the high & low voltage values of this circuit using a 2 potentiometers, where should I put them?
4. What's the purpose of D8 & D10? and can't we replace them with LEDs to indicate Low voltage & High Voltage?
5. What's the purpose of R7, R9, R10?
6. Why LED1 green has no resistor?
7. If I want to run this circuit on 12v relay, I must change the whole circuit?
I apologize for bad English and the many questions, you're the only community I can get help from.


Joined Aug 23, 2012
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