# High voltage differential amplifier

#### MoranZ

Joined Mar 16, 2022
8
Hi everyone
Im designing a PCB for a pre-charge circuit of an electric motor with a 500V battery and I need to measure the voltage on main contactor 1:

I was thinking of using a simple op-amp as a differential amp circuit (i couldnt find a differential amp for this voltage) like this one:

it should get me a Vout = Vin/100.
my question is- do you think about any problems that can come up with this method for such high voltages?
and if a simple op amp like LM399 or LM741 can fit?
(datasheets:
thanks!

#### ericgibbs

Joined Jan 29, 2010
18,668
Hi Moran,
Welcome to AAC.
Where are 'A' and 'B' points connected to the switches.?
E

#### ci139

Joined Jul 11, 2016
1,898
500V is not so high voltage as over the 2kV ones would be.
The 20MΩ would set the bias current through the.. (speculative) apx. 2MΩ (. . . a self-check forced) ..differential input impedance to about the 23µA.
The Iio Iib are 20(300) 80(800) nA(max.) . . . a zero drift options might be considered . . . and/or the bias compensation schemes for the bipolar op amps

The charging speed/rate of the capacitor may set the requirements to your specific metering design . . . about the ?minor? delays from cap. terminals to op amp output . . . to the charger disengage chain

PS! -- At the powered on metering circuit with your proposed biasing scheme the 23µA never flows through the input terminals -- it's just for a fuzzy margins check
+ The possible bias current compensation is relevant when an INA setup is selected

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#### MoranZ

Joined Mar 16, 2022
8
Hi Moran,
Welcome to AAC.
Where are 'A' and 'B' points connected to the switches.?
E
hi Eric
the A point is connected to the + side of main contactor 1 and the B point to the - side

#### Ian0

Joined Aug 7, 2020
9,531
Where does the supply for the op-amp come from? It will work as you have drawn it provided that the common-mode voltage at the op-amp inputs is within its power supply voltage

#### DickCappels

Joined Aug 21, 2008
10,127
If your digital voltmeter is rated to safely handle 500 volts (safety concern only) then feel free to float your meter. Just be careful about touching the circuit. I have floated a DVM at 17kV without incident. At 500 volts corona should not affect your readings.

#### MoranZ

Joined Mar 16, 2022
8
Where does the supply for the op-amp come from? It will work as you have drawn it provided that the common-mode voltage at the op-amp inputs is within its power supply voltage
do you think that a situation when there is no supply voltage to the op amp can damage it?

#### Ian0

Joined Aug 7, 2020
9,531
do you think that a situation when there is no supply voltage to the op amp can damage it?
Unlikely, but a few diodes between the inputs and the supply won’t go amiss. It depends mostly on exactly which op-amp you chooses.

#### drjohsmith

Joined Dec 13, 2021
850
could a thermister replace the pre charge relay ?

#### ApacheKid

Joined Jan 12, 2015
1,471
do you think that a situation when there is no supply voltage to the op amp can damage it?
I read recently that applying input voltages when there's no power voltage, can damage (some?) op amps.

#### MoranZ

Joined Mar 16, 2022
8
Unlikely, but a few diodes between the inputs and the supply won’t go amiss. It depends mostly on exactly which op-amp you chooses.
which parameter of the op amp is important for this case?

#### drjohsmith

Joined Dec 13, 2021
850
I read recently that applying input voltages when there's no power voltage, can damage (some?) op amps.
The max / min voltage spec of the chip as you say ,
is normally specified with respect to the power supplies,
QED, if zero PSU, and voltage is applied to the inputs, your out of spec for the chip

@MoranZ
what is the complete circuit for this op amp ?
where is it powered from ?
where is it connected in the circuit ?

#### ci139

Joined Jul 11, 2016
1,898
which parameter of the op amp is important for this case?
↑ Dont do this mistake ↑ . . . In the electrical circuit everything is a function of everything -- you can suit (better or less better) any op amp to your need --OR-- fix your circuit and see which op amp survives it. . . . ? ask a more non-generalized question

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#### MoranZ

Joined Mar 16, 2022
8
The max / min voltage spec of the chip as you say ,
is normally specified with respect to the power supplies,
QED, if zero PSU, and voltage is applied to the inputs, your out of spec for the chip

@MoranZ
what is the complete circuit for this op amp ?
where is it powered from ?
where is it connected in the circuit ?
hi
here is a more detailed circuit of the op amp:

the supply for the amp is 12V.
thanks

#### MoranZ

Joined Mar 16, 2022
8
↑ Dont do this mistake ↑
In the electrical circuit everything is a function of everything -- you can suit (better or less better) any op amp to your need --OR-- fix your circuit and see which op amp survives it.
? ask a more non-generalized question
yeah you are right, perhaphs i didnt explain myself so good..
i need to measure the voltage over the "main contacor 1" and close it when the voltage is less than 50V
my question is- is it a good idea to use and op amp as a differential amp like i drawed it?
are there any problems that can occure in the moment of closing the pre-charge switch or if somehow the op amp doesnt get power supply?

thanks

#### drjohsmith

Joined Dec 13, 2021
850
hi
here is a more detailed circuit of the op amp:
View attachment 263195
the supply for the amp is 12V.
thanks
Is the amplifier always powered before the power on the left is applied ?

#### MoranZ

Joined Mar 16, 2022
8
Is the amplifier always powered before the power on the left is applied ?
not always, is that a problem?

#### drjohsmith

Joined Dec 13, 2021
850
not always, is that a problem?
As above,
silicon circuits have a max / min input voltage limit
this is specified in most chips with reference to the chips power supply
so if the power supply is zero
any voltage on the inputs is probable outside chip specification
The current into the inputs in this situation can be sufficient to pop the chip.

#### ericgibbs

Joined Jan 29, 2010
18,668
Hi MZ,
I see you want to switch on the contactor when the Vdiff = <50 Volts, when do you want to switch Off.?
E