Automatic dummy load circuit

[Fish-in-a-barrel]

I'm betting this is a solved problem and I just don't know the right words to get google to spit up a solution.

I want to switch on a fixed dummy load if the actual load in my project board dies. This is to prevent the boost converter from melting if it has no load. I feel like there should be a 1 transistor solution to this, but I can't find a configuration that works. This is what I've come up with instead:

I'm using a 2.2k resistor after the load to give me enough voltage to drive an NPN BJT that I'm using to essentially invert the signal to another NPN that switches on the load. LEDs are just for style.

This works on the bread board, but I feel like it's more complicated than it needs to be. Any recommendations?

 

[Tony Stewart]

Confirm OVP voltage and minimum current. ?180V/75k

I think you got the best solution if it acts as an uncontrolled current source. But with feedback or TVS clamp you may also secure it.

 

[Roff]

I get it now. The switch is emulating the failure.

 

[Fish-in-a-barrel]

I get it now. The switch is emulating the failure.

Yeah, I could have clarified that.

The design voltage and current is around 180V and 25mA. A TVS seems like something to consider.

 

[rjenkinsgb]

What about a simple shunt regulator, that conducts if the voltage gets slightly higher than normal?

NPN transistor, emitter to 0V, a resistor between collector and V+ to dump excess power eg. your "safety load" in your schematic.

Add a resistor base to emitter (1k?) then a 180V zener (or chain) - or TVS - with another 1K in series, between V+ and base.

If the voltage start to creep up for any reason, the transistor will start to conduct and dump the excess power between itself and the load resistor.