need help with a ltspice circuit

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keony5

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I need to create a circuit that monitors +12V . The circuit compares a +12V and a reference voltage of 5V. The circuit should then send a ground (0V) to
a 20V buzzer (I have not put it in the circuit yet) causing the buzzer to sound when the 12V is removed. Knowing this Im trying to make a comparator op amp. I need v+<v- so that I can get 0v out of my output because I need to send that to a 20v buzzer . Right now I have v+>- so the 12v which gives me -6.857 mv as my output. Could anyone please help.
 

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Welcome to ETO!
keony5 said:
causing the buzzer to sound when the 12V is removed.
Click to expand...
When the 12V is removed the comparator has no supply so can't operate. Can you power the comparator from the 20V buzzer supply?
Unless your buzzer consumes very little current you should use the comparator to drive a switching transistor, because the comparator itself can source or sink only a few mA.
 
V1 is power for U1 and buzzer and to make 5V. Most ICs like U1 can not pull up. They need a resistor from output to supply.
 
Curious. What is the meaning of MV at the end of what I believe is the output value?
 

Questions:

1. Is the op amp supply voltage source the same voltage source as the 12 volts being monitored?
Or is it a separate supply? If its a separate supply, what is the voltage level?

2. Why not use a comparator instead of an opamp?
 
atferrari said:
Curious. What is the meaning of MV at the end of what I believe is the output value?
Click to expand...
LTSpice is saying MEGA Volts... probably because the generic op amp model is being used, and with no feedback, the sky's the limit... there is a way of doing this with an LM311 that makes more sense than using an op amp (some of which have serious limitations on what the differential voltage can be at the inputs).
 
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