High end P type switch.

Ian Rogers said:

One of my issues, I will now check, is upping the Zener resistor will screw up my RDon

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All explained now

Anyhow, the zener shouldn't mess anything up, it's simply to prevent too high voltage between source and gate. If you're using a zener, then there's no need to worry about calculating the respective values of the two resistors - and all you need for the top one is a high value resistor to bleed any charge away. So you can ignore the top resistor, and simply calculate the bottom resistor based on the current you want through the zener.

ronsimpson said:

The key is the emitter resistor to keep the NPN from turning on full.

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Going back to Ron's version -

A reasonably stable base voltage plus an emitter resistor = constant current!

Choose the gate-source resistor to give eg. 10V at that current and you should not need the zener, the gate voltage should be the same for any (adequate) supply voltage.

Add a small cap across the gate resistor if you are concerned about transients.

rjenkinsgb said:

Going back to Ron's version -

A reasonably stable base voltage plus an emitter resistor = constant current!

Choose the gate-source resistor to give eg. 10V at that current and you should not need the zener, the gate voltage should be the same for any (adequate) supply voltage.

Click to expand...

Interesting variation on the theme, nice one

I drew this up while waiting to hear that the Mosfet was driving a very small load and I didn't think it was worth the complexity but, depending on the opto, here is a universal solution.
It keeps 5mA across the zener and 10mA to the opto - regardless of the input voltage from 12VDC to 48v (or more).
The "load" (opto) can be any forward voltage (must be at least 2v below supply voltage). Also, your R(on) of the Mosfet can't cause the Mosfet to be the current limiting element in the path of the opto.

Here at 48v supply





Here at 12vdc supply...

Is that what we need to build? 10mA for a opto isolator. Supply 12 to 48V.
Here is 10mA with a supply of 6V through 100V.

I think Ian Rogers asked for a high-side opto switch on the opto but I have no idea why. The other side of the opto (phototransistor) doesn't care if the source light is grounded or not.

gophert said:

I think Ian Rogers asked for a high-side opto switch on the opto but I have no idea why. The other side of the opto (phototransistor) doesn't care if the source light is grounded or not.

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From what I can gather the opto is in existing equipment, and is already grounded.

Your solution in post #24 is impressive, but really needs to be inside an IC

Nigel Goodwin said:

From what I can gather the opto is in existing equipment, and is already grounded.

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Could get the 10mA from the 5V supply that the switch is connected to. Very simple circuit.