Need help measuring welder current with DMM

Discussion in 'General Electronics Chat' started by Mike33, Mar 16, 2012.

1. Mike33 Thread Starter AAC Fanatic!

Feb 4, 2005
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25
Hi!
Ok, another obscure sort of subject here for the extra-knowledgeable!!

I'd like to calibrate my arc welder to some degree...it can put out anywhere from 50 to 105A DC, and the scale it has on it is just not accurate. Of course, my DMM will only handle 10A max, and I don't have the means of obtaining a clamp-on meter or anything.

I've thought up a few schemes, from measuring the VOLTAGE at the plates being welded and working back, using the resistance (out, because this is probably a constant-current device, I'm thinking) to placing something like copper pipe in series with one of the electrodes and setting up a shunt system with the DMM (risky if you don't get it just right!). I do have an old D'Arsonval meter movement that I might be better off trying this with....I can easily measure its internal resistance, etc.

Any thoughts on how to do this, in a way that is fairly safe and may give somewhat reliable results? I'm leaning toward the shunt idea....a piece of copper pipe would give the low resistance needed, and can be cut to adjust its resistance....or maybe that's just a "pipe dream"?
I just need to mark 3 or 4 locations of known amperage to refer to

Thanks!

2. #12 Expert

Nov 30, 2010
18,076
9,684
The copper shunt idea seems valid. A long time ago I used 30 feet of some forgotten gauge of copper wire as a 30 amp current shunt. At least AWG wire can be expected to have a certain ohms per foot.

Here's another idea. Measure the primary current with an amp-clamp. Problem: That isn't measuring the delivered current. Well, the current at the primary should be proportional. Perhaps a combination of the 2 methods can get you the range of values you need to measure.

3. shortbus AAC Fanatic!

Sep 30, 2009
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Pretty sure that you'll find that your current is continuously varying as you weld. Even though it is "set" on a number on the machine, as the 'arc length' varies so does the current drawn from the transformer.

4. KMoffett AAC Fanatic!

Dec 19, 2007
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285
I agree with shortbus. So mush of welding is finding out what works by trial and error, then using that to repeat "that" operation, as opposed to "X amps for Y thickness and Z rod/wire" will always work. That's just a starting place.

Ken

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5. Mike33 Thread Starter AAC Fanatic!

Feb 4, 2005
349
25
Really good answers, guys - thanks! I suppose the 'changing current with work' concept is very valid. It may not even be worthwhile to get these readings.
I don't have an O-scope (well, a small cheap one I got for \$50), or clamp ammeter, so it would be a bunch of guess work to rig up a meter movement.

Maybe the vague current indication scale is just that - giving you a vague starting point, since the current will be varying as you weld!

I was more curious if it can be done by shunt - it probably can, but may not be worth the effort!

6. #12 Expert

Nov 30, 2010
18,076
9,684
I bought my amp-clamp for \$35. Same model I bought 30 years ago and wore out! In my job an amp-clamp is essential, and I think \$35 is a bargain.

In addition, if your welding machine delivers pulsed DC straight off the rectifiers, an amp-clamp will measure the output current! I figured this out from making a jump starter for cars. The readings might not be deadly accurate, but like I said, it's proportional, and an amp-clamp rated at 300 amps is standard for the \$35 model.

7. Rbeckett Member

Sep 3, 2010
205
32
What you want to do is really pointless. Arc voltage varies with stick out length. Longer stick out is higher volts and vice a versa. If your fairly close when you start then watch your arc length and you will be better off. I have 7 or 8 welders and I have not attempted to calibrate any one of them. I get the scale close for a starting point and fine tune by eye and ear from there. Save the money and buy some more rods. Just my .02 but I have probably burned 10 tons of rod and wire in 35 years. Hope this helps. If you need further explanation contact me by PM and I will help you figure it out.
Bob

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8. chuppandi New Member

Mar 17, 2008
1
0
Hi can you pl. let me know what you finally managed.

My problem is like this.

I get some porblem with some components during welding even when done by some quality welders. They claim its only due to the current fluctuations.

I would like to measure the current live during welding to be fairly sure that it does not falls below say 100A.

Is there any method available. Like Shunt resistor. I guess i can even afford a standard one if avl. in the market.

Or if using Standard Clamp-on meters can i get those values out to monitor or process further??

I would like to make an alarm mechanism by comparing with Arduino so that whenever the current drops i can get an indication.

Plzz help...

9. mcgyvr AAC Fanatic!

Oct 15, 2009
5,394
1,194
Rbeckett said it best.. Its pointless you measure/indicate the current level. Proper welding "skill" is whats required here.