High voltage differential amplifier 2.0

Thread Starter


Joined Mar 18, 2022
as a continuation to the 2 previous threads :

I would like to gather all the necessary information , requirements and questions:
We have a battery and an engine.
This component needs to connect the two parts in the high voltage system during a pre-charge procedure, by closing 2 relays (K1,K2) after the charged part gets to 95% of the 450V battery.(only the relays are presented here)
The relays are open until 95% of 450V and will stay close until a discharge process which isn't shown here.
We are solving this by comparing 2 differential op-amp with small voltages compared to the 450V.(left side of the circuit).
The differential op-amps are supplied with 12V from a different source that should supply the op-amps before the high voltage from the battery gets to them, in cases of malfunction it may not be supplied (question ahead).
InkedInkedScreenshot from 2022-03-19 19-32-19_LI.jpg
First question:
1)We are searching for a proper op-amp to be used as a differential op-amp (gets a supply of 12V from another source), gets inputs of 0-5V. reasonably priced ,for its function it doesn't have to be super precis. (we got dizzy from the varity).
What Op-amp would you use? and would it be good for this configuration?
2) If the 12V supply to the op-amp will fail, what effect will it have on the circuit? what will be the inputs of the op-amp in this situation?
If it is a problem- how would you handle it in the simplest way?

3)What will happen if the 450V battery will disconnect for some reason from the op-amps (purple arrow wire disconnect) and the motor (green arrow) still have high voltage (this is after/near the closing of the relays)?
Will the op-amp be ruined?
Perhaps using diodes will solve this problem and if yes at what configuration?

I probably missed some info and your question will help me to complete the full picture.
Thanks a lot guys! you are all really helping us.
best regards, Hadar.



Joined Jul 11, 2016
The back-emf spike from the K1 goes through the Miller to the input of the U1 . . . with 20pF it won't be much . . . still a bit careless as along with forward recovery it might pose a threat.

? Are the cathodes of your capacitor and battery connected (by a low impedance path <<< 10MΩ . . . as it didn't show out previously) ?


Joined Jul 11, 2016
It remains fuzzy how your extension-module ( dynamically ) , gets connected to / interacts with the rest of the circuits.
( including ) How it's power is tied to /!\ (a voluntary interpretation of) "dependent/relevant" circuits (e.g. battery , motor)

it's about the basics . . . there might be 100-s of volts in between each of these circuits