Re: Repair of a 7S14

Ed Breya

Yes, the PV MOSFET drivers should work great as current sources, with plenty of voltage - you would just have to shunt regulate them with LEDs. Only a single PV would be needed for each channel. Don't bother with regular transistor optocouplers. You'd need about five per channel to get enough total voltage, making the total parasitic capacitance too large - I've tried it. The transistor optocouplers do however, have significant output current in the tens of uA, but only one Si junction voltage, so you need a bunch to make decent voltage. You take the output from the B-C junction, and run the LED as high as possible. This means you have to use the big six pin type to access the base. The four pin ones only connect the C and E. These generators can be quite useful in certain situations.

My most recent use of this kind of deal was in the Keithley 417 electrometer upgrade project, where I made an adjustable, bipolar voltage source that floats on the output signal, in series with the very large feedback resistors, in order to null out the input bias current. The optocoupler circuit was by far the simplest method.


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