# shunt with a capital SSSHHHHHH!



## Roy Von Rogers (Mar 21, 2009)

gsmith191145 said:


> I am having problems with my ammeter not reading. Before I replace the meter I want to make sure the shunt is not bad. The meter and the shunt is 30 years old, just fyi
> 
> Can a shunt go bad?
> 
> How can a bad shunt be diagnosed?


 
Put an ohmeter on it to make sure it isnt open, depending on the value, you may not have a meter accurate enough to read it. Do you know what value its suppose to be ??

Is it analog or digital ?

Roy


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## JRoque (Mar 9, 2010)

Hi. Not sure if you really meant "ammeter" but you're not measuring "amps", you're measuring the voltage drop across the shunt so set the meter to (milli?)volts. Amps = Volt / ohms. To use the ammeter it would need to take the place of the shunt but the current flow will likely exceed it's rating.

JR


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## major (Apr 4, 2008)

gsmith191145 said:


> I am having problems with my ammeter not reading. Before I replace the meter I want to make sure the shunt is not bad. The meter and the shunt is 30 years old, just fyi
> 
> Can a shunt go bad?
> 
> How can a bad shunt be diagnosed?


Hi g,

Current viewing shunts are pretty robust. It is unlikely they will fail. But I have seen some el-cheapos. The 2 failure modes which come to mind are mechanical damage and corrosion.

The current shunt is just a calibrated low resistance resistor made of materiel which does not change resistivity with temperature. Typically, the shunt will give a 50 millivolt (mV) drop at rated current (like 500 amps) and the mV drop will be proportional for all current values below that rated current. Then at 200 A the voltage drop across the shunt will be 20 mV.

Then to display the current, the meter is actually a voltmeter, or millivolt meter with a 50 mV "full scale" often times labeled on the meter as FS=50mV. And the scale (numbers and line markers) on the meter is calibrated in amperes. So when you look at the meter, it reads 200 amperes, but really it is seeing 20 millivolts on the terminals on the back of the meter.

It is much more likely that the meter is faulty than the current shunt. Meters tend to be fragile. Also there can be a problem with the sense wires, the leads coming from the shunt to the meter. Signals of the magnitude of less than 50 mV may also be subject to electrical noise.

You can test a current shunt by passing a known current through it and measuring the voltage drop across it with a good quality voltmeter made to read in the millivolt range.

Regards,

major


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