Precision ammeter shunt from scratch

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Precision ammeter shunt from scratch

Postby 20to20 » Sun Sep 02, 2012 2:28 pm

Today I 'bout chucked a good 500ma panel meter in the trash because I thought it was a junker when I tried to push 120ma. through it and the needle flew to the peg.

It was a $4 hamfest find with 3 others that gave me good readings when tested. I went through dissection to look for clues and came to a thought that this meter may have been part of an instrument that had an external shunt/divider circuit and that I'd never get it working right. That to use it the way I planned would take added circuitry I didn't want to build. I started playing with external resistors and high value resistors just for kicks to see if it would work at all and of course when I used a resistor about 100 x higher than would create the 120ma., the darned thing started reading within the meter range. So I messed around and found a ratio that that seemed to be a nice engineering number, started to dig around on the net for info and found posts about the precision shunt resistors built into most meters. OH! I only needed to find a .05 ohm resistor! No Sweat! I just farted 3 of those yesterday!

Long story short, I started playing with straight wire and clip lead jumpers and ultimately boiled it all down to a 6" piece of stranded #22 jumper wire across the terminals was all it took to "calibrate" this little gem of a meter to match the readings I got from the other 3 meters!

WOOOOHOOOO!!
Last edited by 20to20 on Sun Sep 02, 2012 4:53 pm, edited 1 time in total.
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Postby Geek » Sun Sep 02, 2012 2:59 pm

Such a deal! :))
-= Gregg =-
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Postby 20to20 » Sun Sep 02, 2012 4:37 pm

Geek wrote:Such a deal! :))




Yup, I can live with a small loop of wire coiled loosley behind it.

And that makes me seem to recall seeing something similar elsewhere and I not thinking anything about it at the time. Like meters with short coils hanging off the back.

I found a .05 ohm Simpson meter ammeter shunt online shaped from straight wire like some bicycle handlebars. That's when I threw the clip leads on and it came close to being right.
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Postby 20to20 » Tue Sep 04, 2012 10:45 am

Update:

After further experimentation it appears that my "finger tight" connections at the meter terminals was insufficient for a good "0" ohm bond and so the length of shunt wire grew to about 2 feet when solidly tightened. So I've gone to some solid 4 conductor telephone wire which may be #30 and doing a kind of "switchback" connection on those by tying the ends together blk/red - red/grn - grn/y to affectively make the entire length longer with a short piece of wire. I'm back down to about 9" of that. Still trimming...

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