An unusual Oscilloscope phenomenon

Shielding sounds doable. The Beerenburger is out of the question though :-)

Yup. It's a DC welder. I already tried a small mains transformer but that doesn't work out very well in a test. Besides that would exceed budget and space constraints.

I think I have a pretty decent diff-amp. It can handle about 800V p-p common mode.

The cord coming off doesn't seem to be a problem. All similar systems I've seen so far have the burden resistor on the measurement board, not on the welding transformer. But I'll try it anyways. The old ways are not necessarely the best ways :-)

There is a fairly mindless thread about ground loops on news:sci.electronics.repair right now.

Every CT I've built has had the resistor as close to the transformer (i.e., on it) as possible. I've had quite awful results putting the resistor on any kind of length. Come to think of it, that may be more due to the resistor itself having length, which would be a problem. Still, better safe than sorry...

'Course, I wind my own, encapsulated in masking tape, so embedding a burden resistor is no big deal. ;o)

Tim

'Course, I wind my own, encapsulated in masking tape, so embedding a burden

I liked this stuff ;-)

A doorbell transformer blows the budget on a welder? Man, and here I thought I was uncle Scrooge.

In a welding environment 800V may not be good enough. Plus you'd have to check the CMRR for fast pulses, like when the arc starts and stops.

Often they sure aren't. A burden resistor at the other end of a cable is usually a bad idea.

Which "welding" environments did you have in mind?

The usual. >100 amps, electrodes touching, arcing, getting stuck once in a while, thick cables floating around the room, ground potential of equipment bouncing, and so on. I once told someone to get his ham radio gear out before we start welding a broken gate. "Nah, this stuff is tough" ... "Better get it outta here" ... "No, really, it's ok, just get cracking on the gate" ... "Ok then" ... bzzt, bzzt, bzzzzzzzzzzt ... beep ... *PHUT*

Umm, have you ever put a volt meter across the cables? I was under the impression that that stuff matched the telephone and was in the vicinity of 40volts.

I politely suggest that if it was around 800volts, then black and crispy would be a welder who made the wrong circuit.

Hint, it is the current that causes the field.

Its the collapsing field that generates the HV spikes that can, and does knock careless welders on their asses.

Podcast:

Do you still get the ringing if the nose of the probe is shorted to the tip? I thought it was the inductance of the ground lead which contributed mostly to the ringing.

The problem is that there is an existing board. The new board must be more or less budget neutral. I blew most of the budget on 0.1% resistors and instrumentation amplifiers.

Its for a resistive spot welder. There is no arc. Just tens of kiloamperes! An arc would be very bad for the particular application.

I hope I can get some testing on a real rig this week.

Hint: There's lots of inductances involved, wanted ones as well as undesired ones. Inductance -> current -> current suddenly stops -> inductance says "I don't think so!" -> tsssk ... *PHUT*

And mains connected power supplies. That's what croaked in the radio. The spikes on a mains circuit of sufficient length (and thus impedance) look rather terrifying.

Then there's cost reduction potential :-)

No way to clamp anything so you don't need all these precision parts?

Until the spot contact lets go at a not so good moment, or one of the panels to be welded had a bad spot there, or a cable breaks, and so on. Never assume things to remain as perfect as they are on paper or in the engineering lab.

Good! I'd test for faulty conditions as well, like the ones above.

If you live anywhere near high voltage power lines, be very afraid if they ever collapse. The induced voltages in length of cable are interesting.

I remember a report and a photo where some guy installed a cable TV hookup at a house near power lines. It cinged the whole side of the house.

Yes.

Nope. As I've said, it works without the groud lead even attched, or even on a bit of coax.

Dave.