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Electrically isolating my programmer

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Speakerguy

Active Member
I have a meLabs U2 USB programmer. It apparently is not isolated, as when I disconnected my power supply from my project board by removing the ground lead it kept working. Apparently electric current was plenty happy flowing through my programming cable, PIC programmer, USB bus, computer PSU, and back to neutral at the wall receptacle. Now yes I should have yanked the + power lead, but anyway I would like to make a simple battery powered isolator board to put between the PIC programmer and my target board.

So first question, which if any of the ICSP signals are bidirectional?
 

Mr RB

Well-Known Member
What's the problem? The programmer was powering your target board?

That's the method I prefer, so the target PCB can be programmed with one connector to the programmer.

Or are you saying there was a different issue?
 

dknguyen

Well-Known Member
Most Helpful Member
Similar thing happened to me with the ICD2. YOu couldn't have the RS-232 port from the PC plugged into the project and the ICD2 plugged in at the same time or else it would read as a short-circuit.

But why are you even having this problem in the first place? Aren't you wall-powering your project from a wall-wart or some kind of transformer for isolation? Actually...isn't the USB port also isolated from the mains through the PC's PSU transformer as well?

Both PGD and PGC are bidirectional if the diagram from the ICD User's Guide is accurate (it implies that each line has both a transmitter and a driver).
 
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dougy83

Well-Known Member
Actually...isn't the USB port also isolated from the mains through the PC's PSU transformer as well?
I would expect that the neutral and active mains lines would be isolated. I'm pretty sure most desktop computers have all connector shields connected to the computer case, and the case is connected to the ground pins of the motherboard. Obviously the case is connected to the earth return pin of the mains connector.
 

Sceadwian

Banned
You could modify a PC power supply to be totally floating but when connected normally they share the households ground connection.
 

Hero999

Banned
I don't know if that's a good idea. The insulation between the primary and secondary side of the PC power supply might not be good enough to be safe. Often the output is earth-bonded in case there's a short from the primary side to secondary.
 

Hero999

Banned
Most modern laptop power supplies I've seen are isolated anyway.
 
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