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Lessons Learned - High Side Optocouplers

Optocouplers can be used on the high side without issue.

Optocouplers let us isolate two separate power systems.  In the case of my pinball project, I am using a computer PSU to power the logic, lights, and lower voltage items like motors etc; and I am using a somewhat suspect 48V power supply for the solenoids.  To keep these systems separated, I used optocouplers on my mosfet driver board.

When designing the board, I was concerned that my transistor like optocouplers would not perform on the high side of the mosfet gate.  You cannot easily use a NPN transitor on the high side (Emitter Load).  You would normally use a PNP transistor.  The test circuit in the data sheet also showed the optocoupler in a low-side (Collector Load) configuration. 

Why did I not use a high-side configuration?  Well, say if something went wrong with my optocoupler, I want my mosfets to default to the off position, ie. ground the gate.  With a high-side configuration they would default on due to the pull up resistor.

As it turns out, unlike a normal transistor, the base current is independent and where the optocoupler is placed in the circuit has no effect on its operation. 

Optocoupler Switch Configurations via Renesas.com

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