Re: Convert an AC to a DC Welder

In article <fnsiav$5fg$2@xxxxxxxxxxxxxxxxxxxxxxxxx>, zekfrivo@xxxxxxxxxxxxxxxx (GregS) wrote:
In article <fnrpah$ab5$1@xxxxxxxx>, someguy@xxxxxxxxxxx wrote:

On Wed, 30 Jan 2008 09:34:24 GMT, "Arfa Daily"
<arfa.daily@xxxxxxxxxxxx> wrote:


<someguy@xxxxxxxxxxx> wrote in message news:fnpbr9$319$1@xxxxxxxxxxx

I recently read that an AC welder can be converted to a DC welder, and
all thats needed are either two or four very high amp diodes. The
welder puts out up to 240amps. I never measured the voltage, but I
think it's around 25 to 30. (I'll have to check). Two diodes will
make a half wave rectifier, but if my memory is right, I'll only get
half the voltage, whereas using 4 diodes as a bridge rect. I will get
the full voltage output.

Now, my question is this. How do I figure the amperage of the diodes
that I would need (for both methods), so I have at least 250amp
capacity. I'd guess 300 would be better to cope with heating. Of all
the years I puttered with electronics, I was never good at math. My
guess is that each diode should be 150A to achieve 300A. But I might
be wrong.
Can someone please help.

Also, where could I get something like this?

Thanks


I made a converter using a bridge made up of diodes of 1/2 the required
amperage,
and a series coil made out of a 2KW variac core with a cut going all the way
through the torroid. I used parallel winding for the output through the
indictor.

greg

Before I did the mod, I just asked around like you did. The slotted inductor makes
the current more constant. I don't imagine it would work well without it.
It seems like you have the same basic figuring as I in making a bridge.
If a bridge is outputting 150 amps, then each diode average current is 75 amps.

greg


One diode will create a half wave rectifier. Two diodes will create a full
wave rectifier, if the winding that you're feeding them from, has a centre
tap, as the one in a welder almost certainly won't have. With a non-tapped
winding, four diodes will be needed, connected into a bridge. You're kind of
right in that two out of the four diodes will be conducting during any given
half cycle, but the two diodes are effectively in series with either side of
the load, and hence each other, so all four diodes have to be ratd for the
full current that you wish to be able to draw.

A half wave circuit does not give you half the voltage. Rather, it gives you
half of the total energy that would otherwise be available in the whole
waveform, by chopping off one half.

As far as where to get such diodes, I'm sure that there will be others on
here that know a lot more than I do about the specifics of electric welders,
who will be able to better help you on that part of your question. FWIW, I
had a feeling that long ago, the rectification and control elements of these
units, changed to SCRs (thyristors) as they are readily available in sizes
that can control electric trains (!) but I could be way off-beam there.

Arfa



Now it's all coming back. It's been years since I did much with
electronics. My eyes are not good enough to work on circuit boards.
Most of the stuff I did work on was the old tube stuff with large
parts and terminal strips. That stuff was easy to see and really
built to last. Then I got into the old transistor stuff which was
still ok to work on but harder to trace. When IC's came on the
market, I pretty much quit working on the stuff. Just far too hard to
test the stuff.

Anyhow, as soon as you said this, I recalled the old power
transformers with their center taps. I can almost remember the whole
power supply for the old tube guitar amps, which was one of my
favorite things to play with. So, yes, without the center tap, I
would need a bridge rect. Looking at the link to the part and price,
I had no idea they would cost $60 each. At $240 I may as well buy a
new welder with AC and DC. Oh well, it was a good idea while I
thought about it.

By the way, I know that a welder is low voltage but high current. I
wanted to measure the voltage with a standard digital volt meter.
Then I pictured the whole meter getting it's circuits welded into a
blob of junk. Is it safe to apply the leads of a common voltmeter to
a welder? Logic tells me that it should be, but I am still not
comfortable with it. For example, I have always been told to
disconnect the NEG battery cable from the newer cars when welding
anything on the car, or the car computer can burn up, as well as the
alternator diodes. Knowing that, makes me question using a test
meter.

By the way, what are SNUBBERS ?

Thanks
.

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