Precision ammeter shunt from scratch
Posted: Sun Sep 02, 2012 2:28 pmToday 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!!
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!!
