An unusual Oscilloscope phenomenon

I wish... I'm working on interfacing a current sensing coil from a welding transformer. During welding there is a couple of hundred volts everywhere. This project starts to give me a headache :-)

Try differential transfer, shielded twisted pair or Twinax soldered to the burden resistor, from there to your measurement box, place a 2nd signal transformer there. Make sure the twisted pair is source- and/or end-terminated properly, usually 100-120ohms. And never, never use this connection as the burden resistor even if there is a fire extinguisher at hand ...

I already went for a differential input. Solves most of the problems, but not all but I'm pretty sure the inputs where overdriven as well. I fixed that, but didn't field-test it. A 2nd signal transformer won't fly because the pulses may be quite long (up to 100ms).

The problem is that nobody seems to know what the pulses look like exactly. The transformer is pulsed by a 1000Hz PWM signal. This makes me assume the current sensor sees pulses as well. But the shape probably depends on the induction in the welding circuit while carrying many kA...

That might be worth something to explore. The burden resistor (1k is specified) is now on the measurement board itself. I'd like to keep it there though to keep the installation procedure easy. The wiring in the test setup is pretty short (2 meters) but bad (not twisted, unshielded). I'll order some shielded twisted pair tomorow. I already figured this must be part of the problem.

Recipe against headache in cases like this: Proper matching of literally everything, shielding, a glass or two of Beerenburger.

Unless the input diff-amp has a huge common mode range even for fast stuff such as spikes I am not sure you can do it without another transformer at the receiving end. Or rather, I've seen cases where it didn't work.

If it's fed 1kHz then 1kHz should come out.

Not good. Increases noise, big time, because the cable is mismatched because even regular cord won't have a Z of 1000ohms. And if the cable ever comes off ... tungggg ... phssssss ... *PHUT*

Ook niet zo goed. In an environment like this you can't work without shielded twisted pair, definitely not across two meters. That's like trying to listen to a Mozart concerto on a Harley-Davidson.

They probably use it in the security CCTV system that they need to ensure that nobody steals their $10k toilet seats.

Can you get decent yakisoba in SFO? I also like plain ol' ordinary fried rice, but I can make that at home; I wish I had the recipe that the Java Curry Shop in Shinjuku used - best curry I've ever had! :-)

Yes, Japanese curry. :-)

Thanks! Rich

At least toilet seats have a purpose. More like port holes on a ship that will never go into service ($2B+ DDG, please!).

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.