How to use a Polyswitch Self Resetting Fuse and light up led when fuse has been disconnected

Question

I'm designing a circuit that will be connected to usb of a pc. How can I isolate the supply when a short has been detected on any part of the circuit. An led should light up indicating there's a short somewhere in the circuit.

Answer 1

I would consider putting a PNP transistor accross the polyfuse such that current flows thru the base when there is more than one junction drop accross the fuse. In normal operation the drop accross the polyfuse shouldn't be that high. You can put a large enough resistor in series with the base such that the leakage thru the device is small enough not to matter. After all, a polyfuse will have considerable leakage on its own, so leakage really isn't the issue.

Connect the emitter of the PNP to the power side of the polyfuse. The base goes to the device side with 10K Ohms in series. The collector will now source some current when the fuse is open and a device is trying to draw current. You could drive a LED and resistor directly from the collector if you only want a few mA. You could also use a additional NPN transistor to amplify the current and drive a LED with a higher current.

Answer 2

I think you could do this with an LED across the poly switch in series with a 1k or so resistor. If the output is shorted, the LED conducts and lights up dimly. If the polyfuse is conducting normally, then no light is emitted as the voltage drop will be very small. Make the LED+resistor combo draw no more than a few mA short circuit; that should be fine for a simple indicator. This only works with short circuits though.

Answer 3

You could put an LED and resistor in parallel with the polyswitch, but that isn't really doing much to isolate your load if the fuse opens (as current will flow through the LED and resistor).

What will you use to power the LED? If you can get the datasheet for the USB hub IC you may find it already has overcurrent outputs that you can make use of.