🚀 go-pugleaf

Joerg <invalid@invalid.invalid> wrote:

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

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.

>>> 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.

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

>>>> 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.

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

-- 
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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