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Registered Member #509
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If your voltage varies and you need a wide input range, you can also use a depletion mode mosfet current source with something like a IXTP08N100D2 mosfet. The SOA looks like its good for 30mA drain current all the way upto 1kV.
The drain connects to your +500v, you put a variable resistor in series with the source, and connect the gate to the other end of the variable resistor. The opto isolator is fed from the gate/resistor junction also.
As your voltage rises, current flows though the fet/resistor/diode, and voltage is dropped across the resistor, biasing the gate below the source, partially turning off the mosfet, biasing it into a relatively constant-current reigion of operation. Use figure 4 in the datasheet ( ) figure out what current range your opto needs, pick pot/variable resistor to give you that voltage drop at that current. I.e. for 20mA max, a -3.0vGS would be needed, so ~150 ohms would be around what you want. get a 300-500 ohm pot and twiddle as needed.
I used this technique to make an isolated digital input for a DAQ system, (added a series diode to protect against reverse polarity too) and I got it turning on at 2.2v and it ran fine upto 300V, the optocoupler current varied from ~2mA to 6mA iirc.
But a low turn on threshold may not be desireable, so if a resistor works fine, simplicity rules.
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how to detect 500vdc using opto coupler
