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# [SOLVED]Designing circuit for providing isolation

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#### ans94

##### Newbie level 5
I have to measure voltage across capacitor which acts as a dc source in a cascaded h-bridge inverter and input it to the dsp (digital signal processor). I have designed a voltage measurement circuit which senses the pulsating dc voltage across the capacitor and steps it down to 3.3V (since the max voltage input to the dsp is 3.3V ) using a voltage divider circuit and a differential opamp. Now before giving the 3.3V output to the dsp, I need to isolate it. What is the possible method to do so? Can i use an optocoupler? I have gone through few optocouplers but am not sure which one to use and how to use it.

Can you supply a circuit, its not all exactly clear....

This is the circuit.

You don't need an opto isolator, the differential amplifier will give you complete isolation.

Just the standard four resistor diffential amplifier circuit:

Make R1 and R2 both 200K
Rg and Rf should be 2K4

That will give you 3.0 volts out for 250 volts in, allowing for a bit of measurement headroom if the 250 volts goes a bit high.

ans94

### ans94

Points: 2
A differential amplifier with symmetrical voltage divider would be also my standard solution. It should be mentioned that it involves a leakage current according to the common voltage between high voltage and measurement circuit.

The circuit must be designed to tolerate the leakage current, e.g. with respective grounding, the amplifier should also provide a margin for additional common mode voltage and suppress high frequency common mode interfernces.

Assuming the HV is the rectified mains and the uP is on an isolated o/p, then you do need to isolate the signal, you can use a freq to voltage converter and put the freq across a small Tx or opto, with a freq to voltage converter on the uP side...

I assumed he was rectifying the square wave output from an inverter.
Its the only way you can get 250v dc from 250v ac.

But it could (?) be mains,and that changes quite a few things if it is.

ans94

Points: 2