Pickit2 Gotcha!

[Mosaic]

This probably applies to many µC programmers (ICSP).
1) The Vss is grounded to the PC chassis via the USB. I suppose for EMI reasons.
https://electrosome.com/pickit2/

I happened to be testing an isolated circuit with a scope...once, after reprogramming the µC I forgot to remove the ICSP plug which is ok most of the time EXCEPT when there is another ground making a loop. That happened to be my scope probe ground clip. As I powered on, I got a powered ground loop from the probe clip thru the scope into the UPS, back into the PC, thru the USB, thru the pickit2 ..into the ICSP header on the PCB and into the circuit ground...burning out 3 IC's and several traces , costing me a day of repairs!

I am wondering if I can remove that chassis ground from the Pickit2 to prevent such accidents.

 

[Pommie]

Wouldn't it be better to prevent the ground from not being a ground? As it stands at the moment, any grounded item will cause a problem. Something like a soldering iron or even a finger will cause problems.

Mike.

 

[Mosaic]

Well soldering on a powered board isn't something I'd normally do at all.
I don't see how a finger would cause a problem?

 

[be80be]

Your finger can ground t0o don't think it would pop a trace. But I would run my scope and computer from the same outlet then all grounds will be the same.
Back to the finger I've seen 1 inch sparks just opening a door here lately oh and it shocked the you no what out of me.

 

[Mosaic]

The prob was because the scope ground clip was attached to a 36V circuit supply (isolated). The PICKIT2 ground was attached via ICSP to the same circuit. That caused a massive short thru the ground in the circuit burning assorted parts.
I am wondering if by removing the PK2 voltage ground to chassis ground such mistakes can be avoided.

 

[Pommie]

The PK2 needs the ground in order to communicate. The solution is, if you are going to make your scope ground 36V, isolate your scope.

Note, to isolate the PK2 would require isolating your computer, monitor, router, printer etc.

Mike.

 

[Ian Rogers]

Dave has done an EEV blog on exactly that same problem..

 

[Diver300]

A USB isolator would work. It is a lot cheaper than a scope isolator.

I've blown a couple of PICkit2 programmers when the earth contacts on the wall socket didn't make connection, and some of my test setup was in one socket, some in the other. The earth leakage current from whatever was plugged in, along with the capacitance of everything meant that when I plugged in the PICkit2, the Vpp pin took a big hit and a transistor blew.

I manage to fix the programmers.

 

[MaxHeadRoom78]

Your PC, P.S. is connected to earth ground in the Mobo and generally in the P.S. itself.
This is where the USB power is derived from.
Max.

 

[killivolt]

Your PC, P.S. is connected to earth ground in the Mobo and generally in the P.S. itself.
This is where the USB power is derived from.
Max.

Hi, Max.

A USB isolator would work. It is a lot cheaper than a scope isolator.

I've blown a couple of PICkit2 programmers when the earth contacts on the wall socket didn't make connection, and some of my test setup was in one socket, some in the other. The earth leakage current from whatever was plugged in, along with the capacitance of everything meant that when I plugged in the PICkit2, the Vpp pin took a big hit and a transistor blew.

I manage to fix the programmers.

I looked at a few USB isolators and all I could see were DC to DC converters. I'm not sure if these will share a ground or not?

Do you know of any that won't and maybe post a URL.

kv

 

[MaxHeadRoom78]

No personal experience with the converters, I would assume they are isolated?
Max

 

[killivolt]

Maybe someone else will weigh in on it; I plan on doing some experiments with my scope and this is one of my biggest fears.

kv

 

[killivolt]

https://www.amazon.com/Olimex-USB-ISO-USB-isolator/dp/B007B2EWBI

I found this one; pretty cheap most of them I found were around $100us

Let me know if this is not what I need?

Say's it fixed a ground loop. I would think that if the grounds were all tied together as most USB hubs are; you wouldn't be able to stop the hum created by Powered Speakers or other.

There was only one review and it was positive under that condition; the person who purchased it said it stopped the hum.


kv

 

[be80be]

You can remove the power wires from a USB cord get a cheap USB hub and power that with a 5 volt cellphone charger

 

[killivolt]

You can remove the power wires from a USB cord get a cheap USB hub and power that with a 5 volt cellphone charger

Nice to know but; I thought the Data was referencing the same ground? If not then it's of no issue.



kv

 

[be80be]

This is better https://www.rs-online.com/designspark/electronics/blog/you-can-build-your-own-isolated-usb-cable

 

[NorthGuy]

D+/D- line is relatively hard to isolate. You need an isolating programmer. Solves all the problem.

 

[dr pepper]

There are a couple of supploers on ebay that sell usb isolators, you'll need a power supply too, thats also isolated.
Also handy for pc usb 'scopes.