Shunt

Some info on Meters and Shunts for current measurement.

Simply put an Shunt is a conductor with a known, low resistance. Shunts are rated for the current they are expected to pass [A or amps] and the voltage they will drop at full current [mv or millivolts]. Factory made shunts will have a pair of connections for the circuit you want to measure the current in. It also has a pair of connections for the meter you will use to indicate the current draw.

For a simple, very cheap shunt a 1 foot length of #10 copper wire can be used as a 100mv shunt[.001018 ohms]. Very crude and likely not exact but a fair indicator for current draw coupled with a 100mv meter. I don't think you would want to run much more than 50 amps through a #10 wire for long as my current capacity chart tops out @ 55 amps for #10 depending on insulation material.

Of course to read the current through a shunt you need a meter. The meter needs to be matched to the shunt you will use. Shunts may come with a matching meter. The meters will have a mv [millivolts] rating 50, 100, 150 mv are common meter ratings. If you are matchng units from seperate sources you need to know the ratings of your shunt and meter. You can also read the shunt current with your regular analog or digital meter set on the proper millivolts scale for your shunt.

In a perfect world you would have a shunt with the same mv rating as the meter.

In my world I am usually brewing something up from whatever I've got in the junque pile.

Anyway, back to how these two devices work together:

Whenever current flows through a conductor there is a voltage drop that directly corresponds to the amount of current flowing in the conductor. Ohms law determines this drop. (E = I X R). Or in terms of our shunt the voltage [E in volts] dropped across the shunt equals the current [I in amps] multiplied by the resistance [r in ohms]. so a 100 milliohm shunt will read 100 millivolts drop at 100 amps. Of course other combinations are possible but the 100/100/100 is easy to see mathematically and a common combination. For the mathematically challenged "milli" means .001 so 100 millivolts is .1 volts, etceterra. 100 amps X .001 ohms = 100 millivolts [.100 volts]

The current rating of a shunt is pretty much determined by its size and generally is rated by the manufacturer. In the case of a 100 millivolt shunt matched to a 100 millivolt meter one amp of current will deflect the meter 1 millivolt. and full scale on the meter would mean 100 amps of current flow in the shunt.

Shunts are generally calibrated by the manufacturer and barring alteration or modification should not change in electrical properties even if abused.

Meters, however, have fine internal wires with moving parts and can be damaged or destroyed if improperly used or overdriven with voltage.

For use in RE systems don't be too concerned with calibration. Generally we just want a good idea of whats coming in and going out.