🚀 go-pugleaf

Joerg <invalid@invalid.invalid> wrote:

>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.

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

>>> 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 

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.

>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.

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

>> 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 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 :-)

-- 
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
                     "If it doesn't fit, use a bigger hammer!"
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