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Transistor to switch on and off op amp?

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lee321987

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Hello.
I've built a constant current source based on an LM324 op amp.
(schematic attached)
I'm trying to turn on and off the LM324 with a transistor (T1).
What I expect is when i cut off power to the base of T1, the output of the LM324 (pin 1) becomes 0V, but the voltage actually goes up.

Please tell me what i'm doing wrong.

 
Last edited:

Hello.
I've built a constant current source based on an LM324 op amp.
(schematic attached)
I'm trying to turn on and off the LM324 with a transistor (T1).
What I expect is when i cut off power to the base of T1, the output of the LM324 (pin 1) becomes 0V, but the voltage actually goes up.

Please tell me what i'm doing wrong.


You’ve observed a very normal behaviour of a device that stays connected to +supply and the –supply is missing, or is dis-connected, AND all other stuff is referenced to –supply.
Try to make the opposite: use PNP transistor and remove the +supply from the opamp ..

:wink:
IanP
 
Last edited:
You’ve observed a very normal behaviour of a device that stays connected to +supply and the –supply is missing, or is dis-connected, AND all other stuff is referenced to –supply.
Try to make the opposite: use PNP transistor and remove the +supply from the opamp ..

:wink:
IanP


Thank you.
But the base will be controlled by a PIC microcontroller (5V max input).
Since the supply voltage to the LM324 is 12V, does that mean the only way to do this is with two transistors?
Yep -- did some googling, and I guess i have to use an NPN on the base of the PNP.

Thanks a lot for the help!
 

You could use a NPN (as common emitter) to drive the PNP (or a relay) IanP said.
 
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