About 20 years ago I used to have an electronic repair business. One day a
guy brought in this diode assembly he said was from a welder. It consisted
of a couple of metal plates with about 100 or more plastic diodes like is
used in home electonics mounted between them. When I told him it would take
a couple of days of solid work to disaasemble and replace all the diodes hes
was no longer interested as he could buy a new assembly cheaper. What I
would kike to know is who would use a device like this in their welders and
does it work OK.
I don't know who would have used it. For it to work reliably, the diodes
would have to be fairly closely matched so one doesn't take too much
current, fail, then another does the same thing until they are all
Nowdays, welders typically use 4 large diodes in a full-wave bridge. People
have built their own converters doing this, or getting surplus car
alternator diodes cheap and paralleling about 3 or 4 sets with load
balancing resistors. If you want to take a chance you could probably get by
with one set since stick welders are current limited and if you run at less
than 100A they would probably be OK considering the low duty cycle most
hobbyist welders have. Sticking the rod down for an extended period of time
could shorten the diode life.
I personally made a converter box using 12 35A, 600V plastic bridge
rectifiers. I use equal lengths of #18 wire on all arms of each bridge which
gave about 0.04 ohm total balancing resistance. My welder at a 225A max
setting will put out about 300A shorted (rod stuck). Another thing that is
required is a choke (reactor or inductor) to keep the current flowing when
the rectified voltage is at 0. A few turns of light welding cable around the
core from a microwave oven transformer coild will do for this but an air gap
has to be left in the E and I pieces of the core or it will saturate. A MOV
or equivalent transient surpressor on the output is good for reducing
transients generated when the arc is interrupted. Also, 0.01uf capacitors in
parallel with each diode of the bridge is good practice as well. With a 600V
PRV in my case it is probably less important.
This is probably a whole lot more than you really wanted to know.
I searched the Groups with Google and found a bit of info on making a choke
for this application but none of it was very scientific. One person said a
few turns of the welding cable "around a bolt" would likely be enough. Also,
there is inductance in long welding leads as it is.
What I did was get a couple of discarded microwave ovens from the local dump
and remove the transformers. Fortunately, microwave oven transformers (MOT)
are not real high-class transformers. They (thank goodness) don't alternate
the E and I pieces. The E pieces are stacked up, the coils fitted over them,
and the I pieces are welded onto the open end of the Es. They are easily
disassembled by hacksawing the welds.
I removed the coils and wrapped about 5turns of #2 welding cable around 1
core and about 4 turns around a second core, that is, they are in series. If
I were to do it again I would use #4 cable to get more turns and it is
easier to handle.
You cannot put the I pieces back onto the E pieces without an air gap since
the high current will cause saturation of the core. I left about a 1/8" gap
(put a thin piece of plywood between the pieces).
It seems to work fine but I never did any scientific test on it. One could
use a scope to determine that a phase shift is present and the voltage and
current don't go to zero at the same time. I just noticed that I had a good
stable arc for my hobby welding which is typically from 80 to about 130A. I
also bought some DC only rod and found it worked well too. I thought that
was a good test since it doesn't have the material in the flux that keeps
the arc going when the AC goes through zero volts.
I think in some of the more expensive welders they vary the choke's
inductance with the current setting.
I got a copy of the article that I had mentioned in an earlier post,
and was disappointed that the author did not use a choke at all --
just rectifiers. He claimed that the resulting waveform gave most of
the benefits of DC and allowed the use of DC rods. I think when I get
around to trying this that I will go the MOT route. Billh, where did
you wrap the wire -- around the central "leg" of the E piece, or
around one or both of the end pieces?
I wound it around the center leg of the E. Never even thought of wrapping it
around the outer legs; I guess I don't think out of the box! I also wrapped
a few layers of electrical tape around it first to reduce the sharpness of
the edge - probably not required but I felt better.
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