PicKit2 shorting with external power supply

I'm trying to program a PIC18F2550 with a PicKit2 programmer. I haven't had any trouble programming the device when using power from the USB port, but I cannot get it to work when using an external power supply. I have set the PicKit to be powered from target and there is no voltage across my rails when my external power supply is not connected, however as soon as I connect the negative supply rail (which is connected to earth) my power supply shorts and turns off (before I have connected the positive rail). How do i fix this?

Thanks

If I power the PIC from the USB port, there is a potential difference of about 11 volts between the negative USB line and the earth. Obviously this explains the short circuits, but how do I get around this? I've seen it written everywhere that it is possible to power the PicKit from an external power supply...

You need to post a schematic. What you describe cannot happen in a normal situation.

Mike.

It is happening without any input / output pins connected. I am using the setup described in Tutorial 1 – 18F4550 Development Hardware setup PIC18F.COM – Tutorials and Sample Code. The external power supply is an ATX computer power supply with an LM317 voltage regulator used to supply 5V.

If i use my laptop with the charger plugged in I can still measure a voltage between Vss (on the pickit) and earth, but when i run the laptop off battery there is no voltage measured (obviously this is expected but perhaps someone has heard of someone else having a similar problem?). Unfortunately I cant test it with Pickit2 because my laptop runs on linux.

chris414 said:

If I power the PIC from the USB port, there is a potential difference of about 11 volts between the negative USB line and the earth. Obviously this explains the short circuits, but how do I get around this? I've seen it written everywhere that it is possible to power the PicKit from an external power supply...

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Either your computer or your external power supply is driving the negative line away from ground. You should find out which and deal with it.

Normally the negative line of the USB is connected to ground. A power supply would usually be floating or have the negative grounded.