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Low Voltage Detector

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HI,
I need help in an Op-Amp Based Low Voltage detector Circuit i want to know how this is working and instead of using two Op-Amps cant we detect low voltage by using one just having a simple comparator circuit kindly help me understand this circuit i found it inside an inverter using lm339 and transformer is giving 9Volts after rectification i am attaching a schematic which i drew on paper.
Thanks In Advance.
DSC00096.gifDSC00096.gif
 

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hi 7,
Its possible to build a low voltage mains detector using a single LM339 or LM339 comparator.

Do you already have any components , mains step down transformer etc,??
 
hi,
yes i have all the components but i want to know why two comparators are used in above circuit and how.
if someone can explain the circuit it would be a new thing to learn..
thanks.
 
A low-voltage detector needs a voltage reference which is missing from the posted circuit.
 
hi Mike,
this is the complete schematic i found i was just missing a resistor which i have now edited rest is the same.
any idea whats happening in this circuit.?
 
An LM339 has four comparators in a 14 pins case.
An LM393 has exactly the same comparators but only two of them and is in a smaller 8 pins case.
 
Let me understand this? You want the PIC to be able to detect that the input AC voltage is less than X? What is X?

Does the PIC have a comparitor? Does the PIC have an A/D?
 
no the PIC used in Inverter have no A/D and comparator module so its just a "1" or "0" logic...
and yes i want PIC to detect when AC fails or when mains Voltage are lower well i dont know exact value of "X" but at normal potentiometer gives 4.6 volts to comparator and its is compared against 2.27 Volts which is fixed by 18k and 15k resistor network...
so i guess that when ac voltage goes lower to may be 160 or 150V from 220V then it gives 1(high O/P)....
 
Do you have an inverter and you want it to feed something when the grid power fails?
Do you have a relay and relay driver for the "something"?
 
yes i have an inverter which is perfectly working and this is the circuit which is used in that inverter to detect low ac voltage when it happens pic starts inverter process and relay triggers untill mains restore...
i want to examine this circuit and how its working..
thanks
 
Here is one. Plenty of hysteresis. Adjustable over a wide range. Can use either a rail-to-rail low-voltage opamp or a comparator.
 

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here's one I made years ago for a UPS and also for a 12 volt battery to start a generator, I used it to hold a relay contact open and drop out when the voltage got low, the contact would close and send a signal to head quarters to let them know that things needed serviceUntitled.png
 
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