🚀 go-pugleaf

Nico Coesel wrote:
> Joerg <invalid@invalid.invalid> wrote:
> 
>> Nico Coesel wrote:

[...]

>>> 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 :-)
>>>

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


>> 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 
> 
> 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). 
> 

100msec? Is it a DC welder? Anyhow, 100msec is feasible if you use a 
small doorbell transformer, the kind with two compartments where you can 
scrap one of them out with wire cutters and without having to 
disassemble the core. Calculate it so it's well under the saturation 
limits at 10Hz. Commercial current transformers can also go that low if 
you use one that's totally oversized for your current.

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.


> 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...
> 

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


>> 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 ...
> 
> 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. ...


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*


>                                               ... 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.
> 

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.

-- 
Regards, Joerg

http://www.analogconsultants.com/

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