comparator schematic
Vipul,
If I may...I would like to make some observation and recommendation.
1. As I understand you would like to make a circuit that monitors your battery and you want this circuit to cut the battery off when it goes below 10V....right?
2. Your initial problem is, "your circuit is working fine without the transistor, but when transistor is connected everything got messed up".
I think Babesh and Millwood had pretty much answered this problem but I would like to elaborate on this. Referring to the latest diagram you uploaded, and assuming your battery has an initial full charge value of 12V. When the output of the comparator is HIGH (as it will be due to the voltage divider ratio of your resistors R1&R2 which will be around 5.7V@Non_inv input COMPARED to Inv input which is at your zener voltage), this will effectively turn the transistor ON with full force of so much base current.
Let's just say the Voh of the comparator is approximately equal to 12V,
then Ib = (12V - 0.7V)/470ohm
Ib = 24mA : this is far more than the saturation requirement!!!
BC547 only needs Ib=5mA so that Ic=100mA
Let's say we didn't break the transistor with this base current, the next
question will be : Will the transistor turn ON?
You never mentioned what relay you are using, this is critical since the coil
resistance will dictate the level of collector current. Assuming you are using
a 12V relay with a coil resistance of 200ohm and that somehow the
transistor is ON, you will approximately have around 60mA of collector
current!!!...You will be draining the battery and the "Domino Effect" will
happen, Vcc for LM358 goes down, zener voltage will not be maintained,
Voltage divider ratio goes down proportionally, transistor goes ON/OFF,
relay will cease working, then you shout " WHAT THE F**K !! ".
3. Obviously, there are design flaws...most of it, but the idea is there.
>>Resistor ratio is cumbersome : make R1=R2, life will be easier.
>>R3=10Kohm which limits the current going to zener may not be able to
maintain Vz. If Millwood was able to bench 1Kohm satisfactorily for
Vz=4.7V(1N750) then it's fine. But in case you have a problem with Vz,
R3 must be the culprit. It would be nice if you put a trimmer starting at 1K
going down to a few hundred ohms. If you check the datasheet for 1N750,
you will see that Izt is at 20mA.
>>LM358 as comparator is fine but I recommend a real COMPARATOR, say
LM311....and btw, you don't need a pull-up resistor for LM358 since it is not
an open collector.
>>I suggest you use a 7805 (5V regulator) to power up your comparator,
zener, and relay. This way, you will have a monitor circuit that is not point
by point reliant on the changing source which is the battery. Output of
comparator will be at 0V-5V. You will be able to control The Izmin more
accurately and you will be able to maintain the the relay coil voltage.
PLEASE SEE THE ATTACHED SCHEMATIC...
IF YOU HAVE FURTHER QUESTION ON THIS, FEEL FREE TO ASK