# Help me wire in an ammeter with a shunt



## scotty (Oct 12, 2012)

I purchased an un shunted 30 amp ammeter so i could learn how rto wire it in with a shunt
I WILL ENCLOSE A PHOTO BELOW.

The shunt is rated at 80 amps and has a 75mv something. I see the meter has a 75 mv movement.

The shunt has 2 terminals at each end. One large and one small.
Each set of terminals on either end of the shunt is on a metal block and electrically shorted to one another.

I realize that the shunt must be in series with one leg of the supply voltage (( In this case 220 volts.)) The thing that has me confused is the 2 diferent size terminals.
Normally i would just say that the larger terminals are for the hot wire and the smaller terminals for the meter. BUT BUT Resistances in series tells me that the meter will be the path of least resistance and blow out when current flows.

HELP PLEASE!!!!!!!!!


----------



## scotty (Oct 12, 2012)

Ok research and help from someone has got me on track.

Turns out that the shunt must match the meter. I just ordered a 30 amp 75mv shunt to match my 30 amp 75mv meter.


----------



## scotty (Oct 13, 2012)

I had a great revelation this morning when checking out my ebay. The shunt that i puraches is really 30 amps not 75 amps as i thought. I used the magnifying glass to re check. I guess my eyes are really bad. HMMM


----------



## JohnT (Oct 19, 2012)

For those of us that still remember having a tv and a phone that actually had a dial... What is a shunt?


----------



## scotty (Oct 19, 2012)

JohnT said:


> For those of us that still remember having a tv and a phone that actually had a dial... What is a shunt?


 

Well an ammeter unlike a volt meter is hooked in series with a wire that hasthe current flowing through it.

Meter movements are very sensative and react to microvolts.

the shunt is placed in the line that you want to install the meter. The meter is connected to the shunt. The heavy current passes through the sunt and a small part is diverted to the meter. If the shunt is designed for the same curren readin as the meter has in full scale, you will get an accurate current reading without blowin out the meter movement.

Thats the best i can do.


----------



## JDC (Oct 19, 2012)

The Shunt is a precise resistor of a low ohmic value that has a known voltage drop at the rated current - in the case above 75 Millivolts (not micro) at a current of 30 amps. The meter movement is actually done by the current flow throught the meter (that is micro amps) - the 75 mV would represent the amount of voltage required to push X micro amps. The larger current you are measuring (up to 30 in this case) is "shunted" through the lower ohmic valued shunt resistor and the resultant voltage drop is measured by the meter that has an intrinically much greater ohmic value - as the two are in parallel (meter and shunt) there is a small amount of current that passes through the meter - but is the meter current is too small to impact the circuit. The larger terminals are for the current to be measured and the smaller for the meter - the ohmic value of the shunt is also very small so that is does not impede the circuit to be measured - in this case .0025 ohms (R=V/I)


----------



## scotty (Oct 19, 2012)

boy did i need tha info--now i willread and re read it till it sinks in. all day today i have been trying to imagine hoe that circuit worked so as not to destroy the meter. Thanks JDC


----------



## JDC (Oct 22, 2012)

The shunt is very low ohm, the meter has many turns and has high impedance - most meters are 100 microamps for full deflection (full scale) in your case that takes 75 milivolts to produce that


----------



## scotty (Oct 23, 2012)

Thanks again. The thing that really disturbs me is that i cant visualize whats happening-I can see other electronic components but i guess i'm just getting old.

At any rate i feel secure in the way it is to be wired. Thanks again


----------

