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30 Amp Panel Mount Meter, 75mv movement?

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gary350

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Meter movement is 0 to 30. Fine print on the right side says, 75mv movement.

.075 / 30 = .0025 ohms

If I am correct a .0025 ohm resistor across the meter will make it read 30 amps. Right?

Where do I get a .0025 ohm resistor.

 

crutschow

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ronsimpson

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There is the real question:
Is this a 30A meter. (which has 75mV drop at full scale)
OR
Is this a 75mV meter. and has 30A markings.
 

gary350

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Meter says 75mv movement manufactures always put that on the meter. 0 to 30 is the scale. 30 amps on this meter would probably smoke it. I see some meters have shunts? I know what shunts are in transformers but I dont know about meters? It is advertised as a 30 amp meter but it probably wont do 30 amps with 75 mv movement.
 
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Reloadron

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You buy a 75mV 30A shunt resistor.
They are specifically designed for that purpose.
I agree with Carl.

If you have a 30A meter, it already has it built in.
Not necessarily, why would you say that? If I have a 400 Amp panel or switchboard meter shall I assume it passes 400 Amps? Looking at the posted meter do you figure the rear terminals are rated for 30 Amps? Looking up that line of meter design points to meter movements that read 20 Amp and below having an internal shunt and those greater than 20 Amp requiring an external shunt.

The meter face also shows what looks to be a small shunt pictured with 75 mV drawn off of it. I see the meter movement as a .075 Volt Full Scale meter movement with a 0 to 30 Amp scale. Less any external shunt I also see the meter movement as a 2.5 mA FS meter movement. I also see the meter movement as having a DC resistance of 30 Ohms. :)

For the curious here is a data sheet for Simpson basic panel meters including current panel meters. Scroll down the data sheet to the current meters.

† DC current meters are self-contained for ranges up to and including 50 amperes.
Higher range DC current meters (50 mV) listed above are calibrated for 5’ leads and
require external shunts.
All depends on the meters but when we start getting into the higher current meters, in this case exceeding 50 Amps, they require and external shunt.

Finally for the truly curious at heart this is a very good read and explanation of the subject.

Ron
 
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MikeMl

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If you have a 30A meter, it already has it built in.
Not necessarily. Lots of meters utilize remote shunts. That way you mount the meter on the dashboard, and the shunt in the engine room. The sense wires that drive the meter can be #22awg instead of #6, and quite long...
 

KeepItSimpleStupid

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One of the questions was: What's a shunthttp://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0CCsQFjAC&url=http%3A%2F%2Fwww.crompton-instruments.com%2Fdownloads%2F2013%2FEPP-2092-03-13_SHUNTS.pdf&ei=jbLZVMa8JMOuggSe1YPYBA&usg=AFQjCNEWkuCNba3QcEh5giz9Fcyc02kXdw&sig2=lTsZCA1Jrnhc9iIaNcOZdQ&bvm=bv.85464276,d.eXY? Here's some: http://www.google.com/url?sa=t&rct=...=lTsZCA1Jrnhc9iIaNcOZdQ&bvm=bv.85464276,d.eXY

Note the big and little or outer and inner screws respectively. Between the inner screws the resistance is known.

If the needle moves appreciably when you shake the meter, it's likely 75 mV voltmeter. A low ohm shunt will act as a keeper.
 

crutschow

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You can test the meter by applying a test voltage through a resistor. A volt or so through a 10k resistor will tell you whether the meter has a built-in shunt or not. If it has a shunt there will be no noticeable meter deflection.
 

MrAl

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Hi,

I would do a quick test too. For some meters you can use an ohm meter to test, but this one might pin and be damaged by most ohm meters, so as Carl suggests use a voltage divider. Make the output voltage around 50mv and connect the meter. If the meter deflects to near the "20" marking, then it's a voltage meter not a current meter, so you need to use an external shunt. If the meter barely moves or not at all, then it has the internal shunt and so you need to keep an eye on the temperature of the meter itself if you use it at currents of say 5 amps or greater.

If it does need a shunt, consider yourself lucky because the ones with internal shunt often burn up if the shunt is not connected properly. 30 amps is a lot of current that you dont play around with because even small resistances will develop a lot of heat. Consider a 0.010 ohm shunt at 30 amps develops 9 watts of heating which is a lot in a small space, so bad connections to the shunt heat up FAST and melt the plastic. Even the right resistor or shunt will develop 2.25 watts at full current, which either has to be external or internal, and a confined 2.25 watts is just as bad as a non confined 10 watts roughly.

This meter does look like it requires an external shunt, but the test will prove or disprove this.
If it turns out that it does need it, buy an actual shunt not a resistor. Shunts are sold as shunts, and state the type of signal such as "50 amps, 75mv" or "50 amps, 50mv", etc. You (may) need a 30 amp 75mv shunt but you dont have to limit yourself to that value either if you really need a different top end on the range.
If you need 60 amps for example, you can get a 60 amp 75mv shunt, which means all the readings will be double what they are marked, so if it reads 30 amps with a measurement then it's really 60 amps. If you use a 15 amp 75mv shunt then when it reads 30 amps it is really 15 amps.
I used a Lafayette 100ma meter for reading 0 to 20 volts for years. Took out the internal 'shunt' resistance (a small coil of wire) and added an external series resistor to calibrate it for 20v full scale. When it read 60ma for example that would mean 12 volts.

If your meter has the internal 'shunt' then the only way you can go is 'up'. You can use another external shunt to increase the current full scale, but you wont be able to go down on the scale (say to 15 amps full scale).

Let us know how you make out with it, and good luck.
 

Tony Stewart

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Not necessarily. Lots of meters utilize remote shunts. That way you mount the meter on the dashboard, and the shunt in the engine room. The sense wires that drive the meter can be #22awg instead of #6, and quite long...
Good Point.

To test if the shunt is internal or not without electrical test is simply a gentle shake.
Since coil EMF is shunted, it's motion is braked and will not move or move very slowly as if dampened by a large shock absorber.

A coil with large series resistance will move easily from imbalance and low EMF current, but a coil shunted with near zero ESR will be well dampened and not move with vibration.
 
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