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# Optocouplers in series

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#### fernandomierhicks

##### Newbie level 4
Hi:

I am working on a high voltage application where current optocouplers do not meet the desired isolation voltage.

If I use a pair of optocouplers, the output of one connected to the input of the other (taking into account, minimum current requirements of input stage, and using a floating supply source for the different circuits at different voltages), would this effectively double the isolation voltage?

Thanks

Fernando

If we examine DC case only high voltage divides 50&50
but with higher frequencies and voltage spikes with high dV/dt
there is 3 meaningful capacitances. ( Inductive parts too, not drawn )
Consisted of wire to wire and wire to groud effects.
If values C1 and C2 are not equal , voltages over isolators are not equal too.
then there is supply to ground capacitance C3.
I drew only capacitances , but inductive effect between wires and parts
make same thing to happen.

View attachment DoubleOptoCoupling.pdf

I suggest to use some HV-isolator type
or optocable between devices

I remember that cheap 4N25 has 7.5kV versions.

and then
for ex. OPI1268S
• 20KV Isolation
• 30 KV/uS dv/dt immunity
• 2 Mbit/s transfer rate

KAK

Sorry, I think I used the term optocoupler wrong, what I want to do is measure small currents at high voltages. So I am using a isolation amplifier, that works on the same principle as the optocoupler.

If thereare no high voltage spikes, and it will be constant DC, would there be any problem using the optocouplers in series?

Need some info for system, for clarifying it for myself.

What is the highest voltage in measured circuit. (DC or AC frequency = ??? )
What is the max. / min. measured current value.
What is the needed accuracy.

I presume measuring principle is shunt resistor.

Regards KAK

its 2500 Volts Dc currents in the micro and nano amperes range, accuracy, as best as possible, possibly in the microamperes scale. I found a isolation amplifier capable of isolating 2500V (AD210BN) I will give it a try and post the results...

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