# Analog amp gauge and stunt



## Sunking (Aug 10, 2009)

Not really. You need a 500 amp 75 mv shunt

A shunt is just a calibrated Power Resistor and your meter simply reads the voltage developed across the resistor when the current flows through it. Your shunt produces .075 volts full scale with 50 amps. 

So you have a 50 amp 75 mv which means it is .075 volts / 50 amps = .0015 Ohms and can handle 50 x .075 volts = *3.75* watts maximum heat.

A 500 amp 50 mv shunt is .0001 Ohms @ 25 watt resistor. That shunt produces .050 volts full scale with 500 amps. 

Simple 5th grade math and Ohm's Law










You have a 75 mv volt meter gauge and need to use a 75 mv shunt. You might be able to use the gauge (volt meter), but would need to change the shunt to 500 amp 75 mv. One issue with that.

1. Your scale is off by a factor of 10 where full scale 50 amps = 500 amps. No big deal or show stopper.


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## dcb (Dec 5, 2009)

jonaplin said:


> I have a 50amp 75mv analog gauge. Can I use it with a 500amp 50mv shunt?
> I intend to use it to monitor a 12v battery system in a travel trailer.
> Please let me know.
> Jon
> [email protected]


Umm, actually if you change out the shunt like you say, then yah, but the reading will be off, but it is a reading still.

so that at 500 amps, it will make 50mv, which if the meter was calibrated for the 50a/75mv full scale deflection, should read 2/3 of 50a, or 33.333 amps. You can always sharpie some new index marks on it too, it doesn't sound like you are looking for super accurate resolution or anything.

fyi the shunt resistances should be 
.0015 (.075v/50a) .0001 (.050v/500a)

but you should only have sensing wires from the meter and gut the old .0015 ohm shunt, or the resolution and load distribution just gets worse.


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## jonaplin (Jun 15, 2016)

Just to be clear.
My battery system is a pair 12v marine deep cycle. I need to know how many amps the trailer uses when off the grid.
Gauge is 50amp 75mV
Shunt is 500amp 50mV

So if I understand the correctly a shunt the is => 50amp at 75mV should work. Yes this is external shunt.

Please correct me if I have it wrong.
Thank you for your time.
Jon


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## Sunking (Aug 10, 2009)

jonaplin said:


> Just to be clear.
> My battery system is a pair 12v marine deep cycle.


Means nothing. It could be 120 volts, makes no difference what voltage you are running. You want to know current, not voltage.



jonaplin said:


> I need to know how many amps the trailer uses when off the grid.
> Gauge is 50amp 75mV
> Shunt is 500amp 50mV


OK so use what you need full scale. With a 12 volt battery system you are likely talking fairly low current of less than 50 amps. So if you use a 500 amp shunt, and only use 10 amps is going to barely deflect the meter and the reading will be pretty much meaningless and very inaccurate. 10 amps would barely register on a 500 amp scale. You want your amp meter scale to match the system scale. So at maximum level is 500 amps, you use a 500 amp scale.

On another note, if you are using 50 amps on a 12 volt off-grid system, you are using the wrong voltage. You should be using 24 or 48 volt. You got yourself stuck inside a 12 volt toy box.


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## miscrms (Sep 25, 2013)

The gauge is just a volt meter, set up to read 50A when you put 75mV across it. It will linearly read 50/75 =~ 0.666A for each mV between 0 and 75.

The shunt is just a low value precision resistor that converters Amps to Volts by the formula V=IR. You know your shunt is sized for a 50mV voltage drop at 500A. So R = V/I = 0.05/500 = 0.0001 Ohms, or 0.1 milliOhm. It will put out a Voltage linearly related to current based on this ratio of 0.1mV per Amp up to 500A / 50mV.

So yes, you can use this shunt, but the scale will be wrong. If you are drawing 50A, the shunt will put out 5mV (50*0.1). When that 5mV is put across the gauge it will read ~3.33A (5*0.666). Or, said another way, to get the corrected current draw, you would multiply whatever the gauge says by ~15 (50/3.33 or 1/(0.1*0.666) ). That might be fine if you are trying to read 100s of Amps, but will make it difficult to read 10s of Amps with much accuracy as everything from 0 to 100A will be squeezed into the 0 to 6.66A part of the gauge.


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## jonaplin (Jun 15, 2016)

Thank you for your reply. I am sad to say I am more confused now then when I started.
I thought a gauge that reads 50amp max attached to a system that is less then 50amp would work. I also thought a shunt that could handle more then 50amps would work. I did find out the mV are important so I was thinking that gauge that would read 50amp max at 75mV would work with a stunt 500amp at 75 mV. even though am only putting less then 50amp across the stunt.
The shunt that came with the gauge it's terminals are to small for connecting a battery cable. 
However, it looks like I am wrong on all accounts. Now that I spent money on this stuff I'm not sure what will work. 
Thank you again for the help.
Jon {aka Lost and confused}


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## Sunking (Aug 10, 2009)

Jon you can run 50 amps through a 500 amp shunt. There are no safety issues doing that. As I said a Shunt is just a Power Resistor of a fixed Ohm value. They come in three voltage values of 20, 50, and 75 millivolts. 

They are passive devices and convert current to a voltage. In Ohm's Law Voltage = Current x Resistance. A shunt is a Resistor of a fixed Resistance. So by using a Shunt we can determine the Current by reading the voltage developed with the current flowing through a known value. 

So if you have a 500 Amp Shunt @ 75 mv means that shunt develops 75 mv with 500 amps flowing through it. If you run 50 amps through it the shunt will read 50 amps x ..00015 Ohms = .0075 volts. On a 75 mv scale = 50 amps.

You have one major incompatibility. You have a meter scaled for 50 mv shunt, not a 75 mv shunt. So if you run 50 amps on a 500 Am 75 mv shunt you are going to read 33 amps rather than 50. 

Short story here if you have a 50 mv meter, you need to use a 50 mv shunt. Further you want the shunt sized in maximum system amps. So if your max expected load current is 50 amps, you use a 50 to 75 amp shunt. You could use 500 amps, but you loose accuracy and resolution.

Edit Note.

So why not use your 50 amp 75 mv meter you have? On a 12 volt system is good up to a 500 watt Inverter. FWIW you should not be using anything larger than 500 watt Inverter on a 12 volt battery


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## jonaplin (Jun 15, 2016)

the gauge face plant says 1 - 50 scale and just below the "A" for type of meter it states 75mv.
it came with a stunt stating on it 50A 75mV. 

to me these look paired correctly. the only trouble is the connection to it is to small for battery cables.

that said, it sounds like I need to find a shunt the 50A 75mV that has a connection the works for the batteries.

is this correct?


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## Sunking (Aug 10, 2009)

jonaplin said:


> the gauge face plant says 1 - 50 scale and just below the "A" for type of meter it states 75mv.
> it came with a stunt stating on it 50A 75mV.
> 
> to me these look paired correctly. the only trouble is the connection to it is to small for battery cables.
> ...


What is preventing you from connecting the shunt to the battery?

If your battery and cables are too small tells me you have bigger problems.


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## jonaplin (Jun 15, 2016)

the battery cables are standard battery cables. no problem there.
the problem is the shunt that came with the gauge has the small screw connection. so to connect the stunt to the battery cables i would have to step down the cable to the shunt size. im not sure how the would effect the trailer. since the battery cable size is something like 4/0 and the shunt would take something no bigger then about 14 gauge. that to me seems to be to big a difference. i'm not able to find a 50A 75mV shunt that has large cable lugs.


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## major (Apr 4, 2008)

Hi jona,

Give this place a call. They have helped me. Good quality at reasonable price. Ram Meter. http://www.rammeter.com/current-shunts 

major


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## miscrms (Sep 25, 2013)

That's pretty big wire for the amount of current you are talking about. Have you traced it through to see if it stays at that size all the way up to your fuses/circuit breakers and distribution panel? If it necks down before it hits the main breaker, or between the main breaker and the distribution point, that might be a more reasonable place to put your shunt.

Rob


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