🚀 go-pugleaf

Nico Coesel wrote:
> 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 :-)
> 

I liked this stuff ;-)


>>>> 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.
> 

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


>> 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.
> 

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.


>>> 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 :-)
> 

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

-- 
Regards, Joerg

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