Subject: What size diodes?

From: [email-masked] (Chris )
Date: Fri, 06 Dec 1996 06:11:09 GMT

Has anyone here done a ac to dc welder conversion using stud
rectifiers. I have a 200A ac welder and wonder if 4  200A 200V diodes
in a bridge configuration would handle average welding below 200A.
There are all kinds of transients and high frequency harmonics flying
around in the weld circuit that i am wondering about.

Anybody tried this?

Chris

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From: [email-masked] (Jens)
Date: Thu, 05 Dec 1996 10:34:49 -0800

>around in the weld circuit that i am wondering about.

I haven't tried it but I doubt it VERY much - try at least twice that
for capacity of the diodes and you will probably be able to keep the
smoke bottled up (it doesn't work without the smoke and once it leaks
out you can't stuff it back in)

Jens

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From: [email-masked] (Alan Muller)
Date: 7 Dec 1996 15:58:36 GMT

In article <[email-masked]>, [email-masked] (Bruce Holder) says:
>
>Chris ([email-masked]) wrote:
>: Has anyone here done a ac to dc welder conversion using stud
>: rectifiers. I have a 200A ac welder and wonder if 4  200A 200V diodes
>: in a bridge configuration would handle average welding below 200A.
>: There are all kinds of transients and high frequency harmonics flying
>: around in the weld circuit that i am wondering about.
>
As a data point, I have to replace the diodes in a Miller "Dialarc" 
250 Amp AC/DC welder.  According to Miller, these diodes are 150A 300 V.
They cost about $26 from Miller which is the same price I got from
an electronics supply house.

Hope this helps.

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From: [email-masked] (Reckless)
Date: Sat, 07 Dec 1996 11:55:38 GMT

>Chris

I'd use 450A diodes if you expect them to live, though if you just
happen to have some 200A units laying around just waiting to be used
for something, give 'em a try.

They will need to be on *BIG* heat sinks.  Average power dissipation
for diodes like that is 1.2W/amp which makes for at lot of heat.
Since each diode will only dissipate power when it is "on" which is
1/2 the time, each will dissipate 120watts at full power!

You might consider an "arc stabilizer" choke if the transients are
really ugly.  Just a thought.

Good luck,
-Reckless

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From: [email-masked]
Date: 7 Dec 1996 17:11:12 GMT

Such diodes are so slow to respond to high frequencies that nothing above
about 200Hz will even pass throungh them.  Also, diodes that are rated
200A usually mean avarage current (make sure it's not peak current), and
in a full wave bridge rectifier on a 200A welder, each diode will pass an
avarage of 100A, giving you a wide safety margin. You can even use 150A
avarage 300A peak without problems. Heat sinking and cooling is a must. I
opened up my 200A 35V mig, and it had the diodes arranged like this

a diode is represented by >-
             |>-  |    |>-  |
             |  -<|    |  -<|
            /                 \
          /                    \
         /                      \
       /                         \
      /                           \
     ------------------------------
         20" 1/2HP fan
You will have to take 4 pieces of 5x5x1/4 in aluminum or copper, mill
little 1/8x1/8 grooves in them (leaving a raised area where the diodes
sit. Put heat sink compound (that nasty white stuff) on the raised pieces
of the heat sink. Than mill pieces of suitable plastic to insulate the
pieces. Make the duct out of sheet aluminum. This is exactly how my
industial welder is made, and I think it's a bit overdone, but than you
can be prud of your work and have a welder that lasts almost forever. By
the way, read the voltage rating, and get one a bit more, just in case. My
welder, which is 35V max uses 50V diodes.

Alex.

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From: [email-masked] (Bruce Holder)
Date: 7 Dec 1996 01:45:46 GMT

I'd risk it (Fools rush in ...) unless the diodes are really expensive.
Most devices with current limitations fail by overheating, but each
diode in a bridge spends half the time cooling off. Heat sinking helps.
My guess is that the diodes you're talking about are the kind where one
terminal is a threaded stud, designed to be screwed into a heat sink.
Again because of the accumulative nature of the heat loading, I wouldn't
worry at all about transients, and I have a hard time imagining that
diodes that size would even see high frequencies.

  Still, this is all speculation on my part. Your mileage may vary.

Bruce

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From: [email-masked] (Alan Muller)
Date: 10 Dec 1996 05:06:10 GMT

In article <[email-masked]>, [email-masked] (Fitch R. Williams) says:
>
>[email-masked] (Alan Muller) wrote:
>
>
>Do you know if they are used in a bridge?
>
>The voltage rating is instructive.  The line inductance in
>high current DC circuits can make some nasty voltage spikes.
>
>Fitch
>
Yes, they are in a bridge.  I think there are some disk capacitors 
(or possibly MOVs, but they are blue) across the diodes, presumably
for shunting transients.

am

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From: [email-masked]
Date: Tue, 10 Dec 1996 18:11:11 -0600

In article <[email-masked]>,
>  [email-masked] (Fitch R. Williams) wrote:
> The current rating should be fine.  Be sure to heat sink
> them well.  The voltage rating could be low.  I noticed a
> follow up post about Miller using 300V diodes.

Been looking into doing what Chris suggests, and thus looking 
into diode availability.  At 100A+, 300V seems to be the _lowest_ PIV 
rating produced by several manufacturers, (Not counting the 50V Schottkeys)so it may not
be especially significant that miller uses them. I do agree however, that diodes are more likely 
blown by over-voltage, than over-current abuse. "Normal" diode applications (with
filter caps) actually pull much higher spikes than welding, as the welder 
transformer is designed especially to limit the output current.

If Chris has a source for cheap diodes ($30 each is typ. retail) then it may 
well be worthwhile to add some spike protection in the form of RC snubbers
or gas-discharge arrestors.

News server is well and truely broken, posting via deja news, and having trouble
with line lengths.  Appologies if this is badly formatted.

ciao, Kevin Ferguson
-------------------==== Posted via Deja News ====-----------------------
      http://www.dejanews.com/     Search, Read, Post to Usenet

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From: [email-masked] (Fitch R. Williams)
Date: Mon, 09 Dec 1996 14:17:08 GMT

The current rating should be fine.  Be sure to heat sink
them well.  The voltage rating could be low.  I noticed a
follow up post about Miller using 300V diodes.  If they are
using them in a bridge, which is what I would think they are
doing, there may be a message in the voltage rating.  They
are probably designing for 300V to 450V PIV spikes with some
margin.

The effect of the inductance of 40' of welding cable is non
trivial at 150 amps.  When the arc quits by instantaneoulsy
going to an open circuit, especially after a "stuck
electrode" incident, the line inductance can generate
voltage spikes that are surprisingly large and energetic as
it tries to find an outlet for its stored energy.  Usually
the arc absorbs a sizable percentage of this energy, but it
can happen in such a way that it doesn't.  

Fitch

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From: [email-masked] (Fitch R. Williams)
Date: Mon, 09 Dec 1996 14:17:06 GMT

Fitch

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From: [email-masked] (Fitch R. Williams)
Date: Wed, 11 Dec 1996 06:08:25 GMT

>am

Thanks for the data.  Based on that I think the 200V diodes
could be under rated voltage wise - I've not had a scope on
a welding system.  Have lots of data on high current DC
power systems with long cables though.  The diodes mentioned
might be OK if they had some sort of peak voltage
protection.  

The peak voltage transients that can occurr after cleanly
interrupting the current flow in a high current DC system
have to be seen to be believed.   Miller might be using
MOVs, though I'm not sure how long they would last.  MOVs
also tend to have a relatively steeply increasing voltage
with increasing current - they could be asked to dump 300+
amps for a few (maybe 40) microseconds which would make them
have relatively high voltage drop - maybe as much as 200V
depending on what ones were selected - but much less than
would otherwise be seen by the diodes.  

It also depends a little bit on the open circuit voltage of
the welder itself.  The Lincoln SA200 I used summer before
last was 75 volts open circuit, but it was a purely DC
generator, brushes and all.  My Ranger 8, with solid state
controls, AC generator, no brushes, and diodes to convert to
DC  is less, maybe half of that, but I don't remember the
number with confidence at the moment.  Its raining and cold
so I don't think I will go and dig out the schematic on it
at the moment.

The higher open circuit voltage makes the situation worse.

It wouldn't surprise me to find 20 to 25 mH in a typical set
of welding cables.  An inductor or reactor to stabilize the
arc would significantly increase that in the hot lead.  

The 200V parts might last quite a while with a lot of luck,
or get smoked early on if Murphy is on the job and paying
attention.  The diodes would do better with MOVs across them
that could take 300 or 400 amps at an environmental
temperature of say 150F for a few microseconds and not
exceed 100 volts or so.  

Fitch
Diodes have a voltage sensative smoke bushing ...

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From: [email-masked] (Chris )
Date: Thu, 12 Dec 1996 07:20:48 GMT

>or gas-discharge arrestors.
I have seen 300A 300V diodes for a cheap as $9 surplus in the past.
What could you use for a suppressor? A capacitor across each diode?

Chris

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From: [email-masked]
Date: Mon, 16 Dec 1996 17:21:11 -0600

Sorry for slow reply. News server is STILL very broken,
still reading/posting via Deja News...yech! 

> What could you use for a suppressor? A capacitor across each diode?

Yes or MOVs from dead PC power supplys or surge arrestor strips etc. Gas disccharge 
arrestors & MOVs are $3-5 new and will soak up a lot of power if repitition rate is low 
(breaking off a stuck electrode fer instance) 

You could also place an arrestor across just the bridge input, and output...

Been looking for spec on Westinghouse diode Chris e-mailed me...No luck on 
anything Westinghouse. 

Worse yet, I found some diodes at local surplus place, 150A, 600V, $4/each.

Also found some coveted anode-to-stude diodes in similar package, and 
wouldnchaknowit they are Westinghouse R5030610FJZU which I can find NO info on.
Posted quiry to alt.engineering.electrical, but nothing so far.

HiPot tester says they have ~1200V Vrb, and they drop .92V @ 25A, @ ~30C...
Lets see, how'd that go....I= Io e**(VjkT/q) ??....Maybe Fitch can lend a 
hand if his voyage into physics of air compressors left any cells unscathed for
sensible EE type stuff. (adiabatic, adiabatic, adiabatic...I'll stop if you help!)

-Kevin in Albuquerque