High voltage capacitor question

Hey All,
I have an old Miller Gold Star welder that may have failing high
voltage caps in the high frequency circuit. I think I can still buy
these caps from Miller but they are expensive and if the caps aren't
the problem then I am wasting a good chunk of change. However, I can
get for 6 bucks each similar caps. So I would like to know if these
caps would be appropriate replacements. The original caps are MICA and
are rated at .002 uf @ 6000 volts. The new, cheaper caps are .001 uf @
30,000 volts. I know that I can parallel the caps to get the .002 uf
to match the originals, and the voltage is no problem seeing as how
the voltage rating of the new caps is 5 times the what the old caps
are rated at. But the originals are mica and the new ones are film
caps. Could this be a problem? When welding aluminum the caps are used
constantly because the high frequency is always on. So the caps can
see a lot of on time when I'm welding aluminum.
Thanks,
Eric

 

[Owen Cook]

On Monday, February 23, 2015 at 11:10:32 AM UTC+11, et...@whidbey.com wrote:

Hey All,
I have an old Miller Gold Star welder that may have failing high
voltage caps in the high frequency circuit. I think I can still buy
these caps from Miller but they are expensive and if the caps aren't
the problem then I am wasting a good chunk of change. However, I can
get for 6 bucks each similar caps. So I would like to know if these
caps would be appropriate replacements. The original caps are MICA and
are rated at .002 uf @ 6000 volts. The new, cheaper caps are .001 uf @
30,000 volts. I know that I can parallel the caps to get the .002 uf
to match the originals, and the voltage is no problem seeing as how
the voltage rating of the new caps is 5 times the what the old caps
are rated at. But the originals are mica and the new ones are film
caps. Could this be a problem? When welding aluminum the caps are used
constantly because the high frequency is always on. So the caps can
see a lot of on time when I'm welding aluminum.
Thanks,
Eric

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

 

[Michael Black]

On Sun, 22 Feb 2015, Owen Cook wrote:

On Monday, February 23, 2015 at 11:10:32 AM UTC+11, et...@whidbey.com wrote:
Hey All,
I have an old Miller Gold Star welder that may have failing high
voltage caps in the high frequency circuit. I think I can still buy
these caps from Miller but they are expensive and if the caps aren't
the problem then I am wasting a good chunk of change. However, I can
get for 6 bucks each similar caps. So I would like to know if these
caps would be appropriate replacements. The original caps are MICA and
are rated at .002 uf @ 6000 volts. The new, cheaper caps are .001 uf @
30,000 volts. I know that I can parallel the caps to get the .002 uf
to match the originals, and the voltage is no problem seeing as how
the voltage rating of the new caps is 5 times the what the old caps
are rated at. But the originals are mica and the new ones are film
caps. Could this be a problem? When welding aluminum the caps are used
constantly because the high frequency is always on. So the caps can
see a lot of on time when I'm welding aluminum.
Thanks,
Eric

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

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

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


Owen

 

[George Herold]

On Sunday, February 22, 2015 at 7:10:32 PM UTC-5, et...@whidbey.com wrote:

Hey All,
I have an old Miller Gold Star welder that may have failing high
voltage caps in the high frequency circuit. I think I can still buy
these caps from Miller but they are expensive and if the caps aren't
the problem then I am wasting a good chunk of change. However, I can
get for 6 bucks each similar caps. So I would like to know if these
caps would be appropriate replacements. The original caps are MICA and
are rated at .002 uf @ 6000 volts. The new, cheaper caps are .001 uf @
30,000 volts. I know that I can parallel the caps to get the .002 uf
to match the originals, and the voltage is no problem seeing as how
the voltage rating of the new caps is 5 times the what the old caps
are rated at. But the originals are mica and the new ones are film
caps. Could this be a problem? When welding aluminum the caps are used
constantly because the high frequency is always on. So the caps can
see a lot of on time when I'm welding aluminum.
Thanks,
Eric

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.

 

[George Herold]

On Monday, February 23, 2015 at 12:30:22 PM UTC-5, et...@whidbey.com wrote:

On Mon, 23 Feb 2015 06:36:10 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

On Sunday, February 22, 2015 at 7:10:32 PM UTC-5, et...@whidbey.com wrote:
Hey All,
I have an old Miller Gold Star welder that may have failing high
voltage caps in the high frequency circuit. I think I can still buy
these caps from Miller but they are expensive and if the caps aren't
the problem then I am wasting a good chunk of change. However, I can
get for 6 bucks each similar caps. So I would like to know if these
caps would be appropriate replacements. The original caps are MICA and
are rated at .002 uf @ 6000 volts. The new, cheaper caps are .001 uf @
30,000 volts. I know that I can parallel the caps to get the .002 uf
to match the originals, and the voltage is no problem seeing as how
the voltage rating of the new caps is 5 times the what the old caps
are rated at. But the originals are mica and the new ones are film
caps. Could this be a problem? When welding aluminum the caps are used
constantly because the high frequency is always on. So the caps can
see a lot of on time when I'm welding aluminum.
Thanks,
Eric

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 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,
http://www.digikey.com/product-search/en?pv13=125&FV=fff40002%2Cfff800ed%2C380132%2C38054c%2Ce34001c&k=capacitor+mica&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

http://tinyurl.com/pdvb75q

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.

 

On Sun, 22 Feb 2015 20:06:10 -0800 (PST), Owen Cook <xemoth@gmail.com>
wrote:

On Monday, February 23, 2015 at 11:10:32 AM UTC+11, et...@whidbey.com wrote:
Hey All,
I have an old Miller Gold Star welder that may have failing high
voltage caps in the high frequency circuit. I think I can still buy
these caps from Miller but they are expensive and if the caps aren't
the problem then I am wasting a good chunk of change. However, I can
get for 6 bucks each similar caps. So I would like to know if these
caps would be appropriate replacements. The original caps are MICA and
are rated at .002 uf @ 6000 volts. The new, cheaper caps are .001 uf @
30,000 volts. I know that I can parallel the caps to get the .002 uf
to match the originals, and the voltage is no problem seeing as how
the voltage rating of the new caps is 5 times the what the old caps
are rated at. But the originals are mica and the new ones are film
caps. Could this be a problem? When welding aluminum the caps are used
constantly because the high frequency is always on. So the caps can
see a lot of on time when I'm welding aluminum.
Thanks,
Eric

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

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

 

On Mon, 23 Feb 2015 06:36:10 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

On Sunday, February 22, 2015 at 7:10:32 PM UTC-5, et...@whidbey.com wrote:
Hey All,
I have an old Miller Gold Star welder that may have failing high
voltage caps in the high frequency circuit. I think I can still buy
these caps from Miller but they are expensive and if the caps aren't
the problem then I am wasting a good chunk of change. However, I can
get for 6 bucks each similar caps. So I would like to know if these
caps would be appropriate replacements. The original caps are MICA and
are rated at .002 uf @ 6000 volts. The new, cheaper caps are .001 uf @
30,000 volts. I know that I can parallel the caps to get the .002 uf
to match the originals, and the voltage is no problem seeing as how
the voltage rating of the new caps is 5 times the what the old caps
are rated at. But the originals are mica and the new ones are film
caps. Could this be a problem? When welding aluminum the caps are used
constantly because the high frequency is always on. So the caps can
see a lot of on time when I'm welding aluminum.
Thanks,
Eric

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 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

 

[Tim Wescott]

On Sun, 22 Feb 2015 20:06:10 -0800, Owen Cook wrote:

On Monday, February 23, 2015 at 11:10:32 AM UTC+11, et...@whidbey.com
wrote:
Hey All,
I have an old Miller Gold Star welder that may have failing high
voltage caps in the high frequency circuit. I think I can still buy

< snip >

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

+1. The biggest issue I can think of is that the new caps will see more
current than they can stand, and will overheat. Knowing what current the
caps have to carry would be exceedingly helpful.

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

On Mon, 23 Feb 2015 13:41:48 -0600, Tim Wescott
<seemywebsite@myfooter.really> wrote:

On Sun, 22 Feb 2015 20:06:10 -0800, Owen Cook wrote:

On Monday, February 23, 2015 at 11:10:32 AM UTC+11, et...@whidbey.com
wrote:
Hey All,
I have an old Miller Gold Star welder that may have failing high
voltage caps in the high frequency circuit. I think I can still buy

snip

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

+1. The biggest issue I can think of is that the new caps will see more
current than they can stand, and will overheat. Knowing what current the
caps have to carry would be exceedingly helpful.

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

 

<snip>

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,
http://www.digikey.com/product-search/en?pv13=125&FV=fff40002%2Cfff800ed%2C380132%2C38054c%2Ce34001c&k=capacitor+mica&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

http://tinyurl.com/pdvb75q

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.

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