A couple of days ago I was making some changes to a circuit. This included updating the firmware of a PIC chip using my PicKit3. But somehow during this process I managed to kill my programmer; the V+ and GND pins that should be connected to the target circuit are now causing a direct short (around 8.3Ohm resistance).
(Un)fortunately my PSU clamped the current to 0.3A so nothing popped and I am having problems figuring out the cause (and how to prevent this in the future).
Sofar I can think of 2 possible causes:
1- Circuit was changed
I changed the circuit and replaced 2 switches with a single pot. Attached are diagrams showing the connection to the PIC. The old situation using the switches worked as a charm but the pot allows a direct connection to GND to happen. This pin was shared with the ICSP_CLCK pin of the programmer.
I checked the PTC (marked PTC3 on the PicKit3 schematic) for this line and it is showing a resistance around 11Ohm, which I guess this is about normal for a 50mA fuse. The Zener (marked TR2) also seems to work when running a diode-tester, although supply-voltage never exceeded 5V so I don't think these conducted much anyway when the issue occured.
Is it possible to fry the PicKit3 by just connecting ICSP_CLK to GND like this? I would guess the PTC catches the current before it gets too high but that did not happen at 0.3A it seems; perhaps it did not get hot quickly enough.
2- Connecting the PicKit3 to a live circuit
Initially the PicKit3 was disconnected while I wanted to flash the new firmware I created. I then connected the PicKit3 but forgot to switch of the circuit first. The programming itself worked fine but after that, when I stopped and restarted the PSU, the PicKit3 would short the circuit.
As programming itself worked after connecting the pins, I am hoping this was not what caused the issue (but my gut fears this is the probable cause).
I've been probing around the PCB but sofar I had no luck in finding what exactly happened. The weird part is also that everything else seems to work fine; when I connect the PicKit3 to USB the LEDS light up the same as before (if I remember correctly) and MPLAB seems to communicate with it normally too. Currently I am tempted to connect it to the PSU again and let it rip with 0.5A or something to see which component gets hot. But I am hoping someone has a better idea that might perhaps even allow me to repair the kit.