How to stop arcing between components on high voltage PCB?

[userx2]

Hello,

I am repairing a high voltage arc starting circuit on a PCB for a TIG welding machine.

It uses a HF oscillator with step up transformer and several voltage doubler stages made from diodes and capacitors.
All of it is thru hole stuff.

The problem is that the voltage multiplier circuit is fried because there was arcing from the final output resistor back to a capacitor in the multiplier chain.

This is because the PCB design is not good and the gaps between the physical components are inadequate. That resistor being very close to those other parts.
It arced straight through the resistor body insulation, through the air and into the blue disk capacitor next to it, also through its insulation.

I can't change that design but I am hoping to prevent this problem from re-occurring by means of some suitable barrier between these parts.

Without spending any more $ what could I use for this purpose?

BTW, 10x 1nF Y1 capacitors, 4 resistors and 8x 4kV diodes cost me US$54 at Dig Key. I cannot believe how expensive components have recently become!!

Regards
X

 

[barry]

since you can’t change the gap, the only other alternative is to increase the resistance between points. there are materials (conformal coating, e.g.) that you can use to coat exposed metal.

just do a search for high voltage coating

 

[userx2]

I was more thinking of something physical. Paper won't work, I am not sure about heatshrink. Those high heat insulation sleeves used around wires seem porous.

Ohh, maybe a piece of FR4 PCB between the components. That would increase the insulation over open air right?

Regards
X

 

[KlausST]

Hi,

I aslo reommend a potting compound.

But if there was arcing before, then you have to carefully clean everything (but the new components), because vapuorized metal may generated an invisible thin but conductive coating over everything nearby (including PCB, insulators, ...). This may be the cause for arcs, even if components are replaced. Simply cleaning with water, or alcohol may not be sufficient.

Klaus

 

[Easy peasy]

grind away all the carbon impregnated pcb before applying insulation compound - even plain clear nail varnish works surprisingly well ...

 

[userx2]

There is heaps of black damage and the PCB is burned through in one place.
But I think that came after the arcing.
I will fix and work around that. Yes, I have been also been using nail varnish for 40 years on PCBs. I am not sure about its capabilities for HV though.

Regards
X

 

[wwfeldman]

if the arc was across the surface of the PWB, you can cut a piece of the PWB so the arc has to go through air instead of along the surface

whatever you use as a potting material or a conformal coating material has to be put onto a clean surface and it has to "wet" the surface well.

a high voltage supply i worked on would arc between along the interface between the PWB and the insulating transformer oil the board sat in. since the oil did not "wet" the PWB material well, the only thing that saved the board and project was to cut out a piece of the PWB between the two points that arced