Low-high voltage protection circuit with time delay.

Submitted by alaaj99 on Tue, 04/18/2017 - 19:46

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


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.


Hi alaa99,

Before I start answering the questions I have one doubt, you have said "he/she used only three comparators out of four available to reduce the cost of it (witch is nice)."

But, LM324 is a quad op-amp package weather you use the 4 op-Amps or not you have to pay for it, so where is your point of saving money? Or is there any other circuit that would use this additional op-amp..

Anyway now lets see how I can answer your questions!!

1. The designer has used a 0.68uF 400V capacitor to avoid ripples, you cannot reduce the voltage of the capacitor from 400V. Because the ripple voltage may go as high as 400V. Reducing the Cv will lead to explosion of the capacitor. So, DO NOT DO IT!!


2. The Zener Diode Z-D1 is used for protection purpose, the circuit should work even without the zener. If you cannot find a 1W zener use two 0.5W in parallel. But, it has few small risks the breakdown voltage of both the zener might not be exactly identical, so one might get activated before the other and over current might kill it. But, YOU CAN TRY it by using two 0.5W zener in parallel and test the circuit. 


3.You can control the cut off voltage of the circuit be changing the reference voltage of the op-amps. This reference voltage in your circuit is set using a potential divider. You can change the values by putting a potentiometer in the place of R7 and R14 (calculate the required pot value for your circuit)


4.Sorry I am not sure why D8 and D10 are used. I cant find the purpose of them there. But they must be for some protection purpose only. Use them as given in the circuit. Do not replace them with LED's the voltage drop across them will be different than that of the diode.


5.R7, R9,R14,R15 and R10 are the resistors which are used set the reference voltages for the op-Amp. 


6.LED should have a resistor only to limit current through it. In our case the LED is being sourced by an Op-Amp. The maximum current given by the op-amp (Source current) is not high enough to harm the LED. Hence resistor not needed. 


7. Yes you cannot use the same circuit for 12V relay without major modifications, You have to add a circuit after the 14th pin of the op-amp to convert the output voltage to 12V and then feed it to the Relay coil.


Hope this helps, LEt me know how your project turns out....



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Tuesday at 12:29 AM