Hey Guys and gals,

I am trying to measure the voltage across this inductor (0-40VDC) and scale to 0-5VDC for a micro controller input.


For this application i would typically use a voltage divider but in my case I cannot ground the "negative" side of the inductor so I am at a loss.

Any suggestions?

Thanks!

 

You take a differential measurement requiring two probes (two voltage measurements).
V1 with respect to GND
V2 with respect to GND
Difference V1 - V2 or V2 - V1.

 

so two different dividers? because that seems pretty logical.

 

You can use an op amp in a differential circuit such as this:

The gain is R2/R1, so for an attenuation you would make R2 smaller than R1.
Thus to reduce 40V to 5V you could make R1=40k and R2=5k, for example.

 

Hello,

@crutschow , he can't use the circuit as he is using an isolation opamp.
He must make two equal voltage dividers and put them on the + and - input of the opamp.

Bertus

 

he can't use the circuit as he is using an isolation opamp.
He must make two equal voltage dividers and put them on the + and - input of the opamp.

He did show an isolation amp but he made no mention of that in his post so I assumed it was just an artifact of the schematic.
However if he does need isolation then he should do as you say.

 

sorry I dont know what you mean by isolation amp

 

sorry I dont know what you mean by isolation amp

An isolation amp has galvanic isolation between the input and output (no ohmic connection between the two).
This allows the input and output to have separate grounds that can be at different voltage levels (often up to a thousand volts or more difference). It used when you have a sensitive circuits that you want to completely isolate from noisy or high voltage circuits.
I assume you don't need that here.(?)

 

that's correct I don't need it here, although it does seem like it would be nice

 

that's correct I don't need it here, although it does seem like it would be nice

Isolation tends to add complexity and cost so "nice" is usually not a sufficient engineering criteria to include it..

 

hahaha agreed. So i'll give your differential opamp a go and see how it works. thanks for your help