🚀 go-pugleaf

Nico Coesel wrote:
> 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.
> 

Then there's cost reduction potential :-)

No way to clamp anything so you don't need all these precision parts?


>>>> 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.
> 

Until the spot contact lets go at a not so good moment, or one of the 
panels to be welded had a bad spot there, or a cable breaks, and so on. 
Never assume things to remain as perfect as they are on paper or in the 
engineering lab.


>>>>> 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.
> 

Good! I'd test for faulty conditions as well, like the ones above.

-- 
Regards, Joerg

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