I need to switch a load, controlled by a microcontroller. (Actually, bypassing a load with a short-circuit.) My question is how to implement the switch.
The circuit will have available to it the following rails, in addition to the 2 V-50 V supply: +5 V, +12 V, -12 V.
I am struggling to simplify my design of the switch. The problem is interfacing the controller with the "switch".
The following is one idea I've had:
However it strikes me that three transistors for this one "switch" could be simplified.
I'm using a MOSFET so the on-resistance would be low, but a BJT would be OK. But either way, I can't see a simpler circuit that would allow the switch to be controlled by a microprocessor.
For example, to turn the P-channel MOSFET on, when the 2 V-50 V rail is at 2 V, the gate would have to be pulled down to around -2 V or lower. And the simplest way of interfacing that with a microprocessor would be to have one BJT pulling Vgate to -12, and another BJT sourcing current into the last one's base from +5 V, which could then be controlled by the microprocessor.
Of course, if the constant current sink is programmed to sink a small current, and the 25-50 V rail is at 50 V, the MOSFET's Vgate would have to be taken to about +50 V to turn it off. My circuit would do that by turning off the BJTs.
Is there a simpler way that I've missed?