High voltage capacitor question

If they are silver mica they will fail eventually due to migration of the silver.

What else is in the circuitry? I would just buy 2 of the cheaper caps and try them.

Owen

I thought that was specific to a certain type or brand, and a specific period in time.

Indeed, the most common time I've seen commentary about "migration" is for capacitors inside IF transformers, which makes their replacement a whole lot more troublesome. I gather they didnt' bother packaging those capacitors, because they are in the IF transformers, but having to replace them means taking out the transformers, and that's a lot of work.

Michael

Hi Eric, First I know nothing of Welders and such. What's the frequency? One thing I've heard about silver mica is that they have a smaller dissipation factor at high frequency. This may no longer be true. (The dissipation factor is kinda a measure of the capacitors resistance... energy loss.)

Do you have dissipation numbers for both caps?

George H.

Greetings Owen, There is not much in the circuitry, just the caps, some resistors, an inductor or two, and the spark gaps. The welder is probably 40 years old. Thanbks, Eric

Greetings George, The manual doesn't have the dissipation numbers, nor does it say what frequency the thing operates at. I would love to increase the voltage of the high frequency, or maybe just find a way to couple the high frequency to the welding cable more efficiently. When learning to TIG weld back in the 80s I used both Lincoln and Miller machines. The machines were rated the same as far as what could be welded with them but the Lincoln machines had much better high frequency output. When using the Lincoln machines I would step on the pedal while holding the TIG torch well away from any grounds I could see purple arcs being emitted from the tip of the tungsten electrode. On the other hand when trying the same trick with the Miller machines no arcing was visible. When actually welding the high frequency is used to establish the welding arc. When welding with DC the high frequency turns off once the welding arc is established. When welding with AC the high frequency stays on all the time. The Lincoln machines were great to use because the tip of the tungsten electrode could be over an inch away from the work and it woul still establish an arc. The Miller Gold Star machines wouldn't do that. Often the electrode tip needs to be as close as 1mm to establish an arc. Sometimes even that wasn't close enough and the electrode would need to be dragged gently on the work and then lifted away. Once away from the work trying to establish an arc usually works. But sometimes even that won't work and power needs to be applied while scratching the electrode across the work. Touching the electrode to the work dirties the tungsten electrode, is not good weld practice, and is only done when necessary. So if I could get higher voltage at the electrode I would be very happy. I have thought about replacing the old Miller but it is a special model because it has a current range from 1 to 300 amps and I weld enough at about 5 amps on some really thin stuff that I keep the machine. Cheers, Eric

Hi Eric,

Woosh... That's the sound of most of your response going over my head. (Grin)

Do you have part numbers or links to the caps?

I found this on digikey,

There is nice spec sheet if you click on the link. As long as your replacement caps are good for 1/2 the current of the ones you are replacing....

George H.

If you have an IR thermometer "gun", try running the welder in ever-longer bursts and measuring the temperature of the new caps. If they stay cool, you're probably, well, cool.

Miller probably recommends just one brand/model of cap because that's what the circuit is tested with, and they can't honestly guarantee a cap that's selected at random without testing.

Thanks Tim, I do have an IR thrermometer. So I'll see if the things get warm. Eric

Greetings George, There are no part numbers on the caps themselves other than the Miller P/N. I am going to try to get at the wires that power the xmfr for the high frequency. If I can do that easily and if the caps I want to buy have a current rating that equals or exceeds the measured current I will be buy them. I won't be buying those caps from Digikey though. Over 300 frog pelts each! I spoke with a Miller repair guy yesterday and he said the most likely problem is the high frequency/high voltage is partially leaking to ground somewhere before it gets to the electrode. This is why it is weak but there. Now I checked the path of the high voltage as well as I could looking for places where it could be jumping to ground before I thought about replacing anything. What I didn't think to do, and probably should have, is to remove the covers on both sides of the welder, turn off the lights, and start the high frequency. That way I will be able to see any leaks to ground. A great idea and I will be trying it this weekend. Thanks, Eric