Polyfuses and negative voltages

[seb1982]

Hi everyone! 

I'm having a complete brain fart!  Obviously, voltage is relative - that's like lesson one!

But I'm planning to use a 1.6A polyswitch in series on a positive voltage rail to protect it.  There's also a negative voltage rail as well as a ground.  Am I right in thinking that the polyswitch will work exactly the same on the negative voltage rail as it does on the positive?

I'm sorry for such a basic question, but I've tied my brain in knots by thinking about it too much!   

[c4757p]

Yes.

[seb1982]

Clear, concise and to the point! 

Thanks. 

[c4757p]

A polyswitch is just a special thermistor, which is just a special resistor. And remember that the component doesn't care what ground is, it just cares what it sees applied to it. Applying -20, 0 across a resistor is just like applying 0, 20 across it - it just sees that one side is 20V higher than the other.

One thing to keep in mind is that you don't just need to block currents rail-ground, but also rail-rail. Many polyswitches have pretty low voltage ratings and many dual rail systems have pretty high voltages. Make sure it has a good safety margin around the total rail-rail voltage.

(Clarification: Ideally, if the fault current is rail-rail, both polyswitches will trip and share the blocking voltage. They are resistors, after all, even when tripped. I don't like to count on that, though.)