Circuit Operation help

[Mahmoud Arwani]

Hey guys!

My first post here!

Doing a project, making a PCB tester. Basically the way it works is I have a number of test points on the PCB in testing. I will be putting voltages between these points and read the voltages across power resistors via ADC. The main devices on the PCB being tested are SCRs and a diode, so it is straightforward what I'm looking for (SCR on/off etc..)

The problem.
I'm having issues making a circuit to switch from these 3 states.
- start as open circuit
- allow switching +24V through limiting power resistor
- allow switching -24V through limiting power resistor

Been recommended to use IGBTs as the -+24V may be increased in the future. Using an STM32 MCU - will need a driver circuit to run IGBTs I presume.

Any suggestion on how I can do so are very welcome.

Current problem is I need to know how the 2 attached circuits work, the 500k resistor on the end is just for simulation purposes.

Thanks in advance !

 

[duke37]

It helps if you use positive and negative rails rather than voltages and grounds floating in space.

1. If a sufficient positive voltage is put across R4, then Q1, Q2 and M1 are turned on. Q2 acts as a level shifter so that M1 can run from -24V. What is X1 50W? Why is it there?

2. This works in a similar way to 1. with opposite polarity. Q1 is the level shifter so that M1 can run from +24V The component 50W X1 should not be there. Is this a load resistor?

 

[Mahmoud Arwani]

Issue now for the N-Mos circuit is the output changes from -14.6V to -24V. How do I bias it in a way that makes it switch from 0 to -24V?