Scope trace assistance

[Lee697]

Hi all

I set up a current shunt in series with a fuse - the 3A fuse was a sacrificial one, testing in place of a rocket igniter (much cheaper!).
The shunt is 2A/50mV, and the switching is accomplished by a BUZ71 MOSFET.

The trace seems to show the fuse allowing about 14A to pass before blowing after 40ms. Actual igniters draw 7-11A, depending on the resistance of cabling hooked to them.

I did this a number of times with other fuses (1A though) - this was the only time the large spikes appeared at the ends of the trace like this - does anyone know what these are from?

This was hooked up to an Array 14.5A PSU - I wondered if it had something to do with current limiting by the PSU itself?

Thanks for any advice...

Lee

[T3sl4co1l]

Most likely wiring inductance generating a spike when the circuit opens.  If you have a few more fuses to burn, you might set trigger level below 0V, trig mode NORM or SINGLE, and time/div around a few microseconds.  It's probably even taller than you can see here, it just didn't happen to sample the very peak.  The spike will be more exaggerated in the voltage across the fuse or ignitor, whichever one causes the circuit to open.

Tim

[Lee697]

Thanks - come to think of inductance - the shunt is coil like - I suppose that would contribute?