How to see welder output on oscilliscope - Page 1
Forum section: electronics / testgear

author: jerry11
author_rank: Newbie
timestamp: September 28, 2015, 07:46:59 pm
content: Maybe I need a whack on the head for even thinking/asking about this, but I am curious and want to see if it is possible to scope the output of my TIG welder. The purpose of this is to visualize the different wave forms, frequencies, pulses, offsets... that are available with the inverter type welder. I tried this by setting up the welder with a stationary gap of about 1/16" from a heavy base metal to the tungsten. I set the amperage to 5 amps. At this amperage the base metal will dissipate the heat before melting. I connected a light alligator clip from the tungsten electrode to the probe and grounded the probe to the base metal.When I fired up an arc, the connection between the very light (maybe 28 guage?) aligator clip and probe started to smoke. I shut it within a second. I am guessing that the current went through the probe as the path of least resistance instead of starting an arc. Thankfully, the probe and the scope are fine. The welder specs show the voltage at about 25v, and with the amperage set down to 5, I thought this would be safe.Has anyone done this before, or heard of it being done? Is there a safe way to do it?ThanksJerry

author: _Wim_
author_rank: Super Contributor
timestamp: September 28, 2015, 07:54:02 pm
content: Since there are large currents involved, would it not be possible just to wrap a wire around the weldcable, and use it as a current pick-up coil?

author: tautech
author_rank: Super Contributor
timestamp: September 28, 2015, 08:14:46 pm
content: You first need to understand what you are dealing with.Watch Dave's How not to blow up you scope vid.Secondly TIG's often have HV start, and while it is low energy, LV it certainly is not and with it enabled I'd not put a scope anywhere near it without at least 500:1 probes.

author: jerry11
author_rank: Newbie
timestamp: September 28, 2015, 08:25:41 pm
content: I does have a high frequency start. I don't know about high voltage. Specs show something like 25v maximum voltage. As far as Dave's How not to blow up your scope video, it would be interesting to check voltage between the welders welding ground and mains ground.Jerry

author: PaulAm
author_rank: Frequent Contributor
timestamp: September 28, 2015, 11:05:24 pm
content: It is high voltage, probably a few thousand volts. It's the pilot arc that allows the arc to strike without touching the base plate. Air breakdown is roughly 3 KV/mm, so you can figure out the minimum it needs to be.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 29, 2015, 05:11:18 am
content: Quote from: PaulAm on September 28, 2015, 11:05:24 pmFor a uniform or soft gradient electric field.In the case of a "needle point" like a MIG wire or TIG tip, it goes down to about 700V + 1kV/mm at 1atm.From what I read about TIG welding, most popular TIG electrodes are doped with alpha emitters like thorium or lanthanum to further reduce the strike voltage and help stabilize the arc - make it easier to start and keep going.At any rate, that would still make the start voltage a few kV. And then there is the Ldi/dt when the arc breaks.

author: SteveH
author_rank: Contributor
timestamp: September 29, 2015, 05:13:21 am
content: If you want to scope the output safely you MUST disable the HF. You also need some form of dummy load. This can be as simple as a 300- 600mm length of steel strap.

author: Berni
author_rank: Super Contributor
timestamp: September 29, 2015, 07:00:39 am
content: You are lucky it only smoked your probe. The high voltage could have killed your scope too.As it was said by others you need to give it a big resistive dummy load and connect the welder to it before turning it on. That way there will never be a large voltage present.Another possibility is to look at the current using a current transformer. Usually this is done by feeding the wire with the current of interest trough a toroidal core with another coil with many turns of wire on it. If you have 1000 turns on the toroid then you will get 1/1000th the current in that coil. But be careful with current transformers since they need a low value resistor on that secondary to give that current a easy path to go and also turn the current signal in to a voltage that can be seen on a scope (Having no load can produce very high voltages that can damage the transformer or your instrument).

author: MattFL
author_rank: Contributor
timestamp: September 29, 2015, 07:38:13 pm
content: It was the ground clip that smoked right? I'm thinking the path to ground through your scope was lower impedance than the path to ground through your welder cable. So assuming your scope does not have isolated inputs, you were probably getting a large current flow from the base metal through your scope lead to ground, and the clip was the weakest part.My thoughts are; try putting a few wraps of wire around your welding cable, then with the ground clip attached to earth ground, attach your scope probe to the wire wrap and see if it will give you a good-enough view of the output wave form (for AC). Or move your probe ground clip away from the base metal, and ground it to regular earth ground. Just be sure you don't adjust the welder output to be higher voltage, relative ground, than your scope is rated for.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 29, 2015, 10:32:31 pm
content: Quote from: Berni on September 29, 2015, 07:00:39 amIf the goal is to see what happens to voltage and current during a welding cycle from arc make to arc break, putting a dummy load is not an option since it will make it difficult if not impossible to strike the arc and the make/break characteristics may not be representative of what happens under normal circumstances.Since OP connected the probe directly across the arc, he was most likely interested in measuring voltage and I'm guessing he got his DC welder's polarity backwards, shorting the output to ground through his probe.

author: bitslice
author_rank: Frequent Contributor
timestamp: September 30, 2015, 01:27:54 am
content: I got given something like this project on my placement,I didn't really have a clue how to go about it, and after a bit of fiddling with some signal isolation I gave up because everything was being swamped with noise.

author: BradC
author_rank: Super Contributor
timestamp: September 30, 2015, 01:37:32 am
content: Quote from: DanielS on September 29, 2015, 10:32:31 pmMost welders are not ground referenced.In general the correct polarity for DC TIG is Electrode Negative (EN) anyway, although welding small stuff (like razor blades) you can get better control using EP, it's just very easy to overheat the electrode.

author: verbatone
author_rank: Contributor
timestamp: September 30, 2015, 01:52:05 am
content: My dad has an old school generator welder (maybe 400V) that generated enough EMI that it would just about scramble the OTA TV signal of my Saturday morning cartoons 50 ft from the house antenna. If a TIG welder will generate anything close to this amount of EMI, i would expect a floating scope probe in the same room to pick up quite a bit of EMI. This is just a guess, but i might work. If it's saturating the probe, you may have to add a resistive load (100k?) between probe tip and ground to attenuate the pickup.

author: jerry11
author_rank: Newbie
timestamp: September 30, 2015, 03:12:03 pm
content: Quote from: DanielS on September 29, 2015, 10:32:31 pmI bet you are correct. I normally weld with the electrode negative.I am going to keep working at this. The idea of creating a field by wrapping wire around the lead and measuring that is very interesting.Thanks all for the ideas and help.Jerry