# DC Meter Formulas

Meter Resistance Current Shunt Voltage Multiplier Ohms per Volt
There may be times when a defective panel meter needs repair or replacement. As original meters can be difficult to find, a replacement may have to be made from non-original stock. To help with this process, the following information is provided.

__Meter Resistance__

To determine the internal resistance of a meter, perform the following steps:

- Connect the meter in series with a DC power source (battery) and a variable resistance R
_{1}.
- Starting from highest resistance, vary R
_{1} until you attain a full scale reading.
- Connect a second variable resistor R
_{2} across the meter. Vary _{R2} untilyou attain a half scale reading.
- After disconnecting R2 from the circuit, measure its resistance. It will be equal to the meter resistance R
_{m}.

Note: Insure that R_{1} is of high enough a value to prevent allowing too much current to flow through the meter. Use the following formula to estimate a safe value for R_{1}:

__Current Shunt__

Use the following formula to calculate the shunt resistance for a current meter:

R = R_{m}/N - 1

Where: | R = shunt value in ohms |

| N = new full scale reading divided by original full scale reading |

| R_{m} = meter resistance in ohms |

__Voltage Multiplier__

Use the following formula to calculate the series resistance for a volt meter:

Where: | R = series multiplier value in ohms |

| E_{fs} = full scale reading in volts |

| I_{fs} = full scale current of meter in amps |

| R_{m} = meter resistance in ohms |

__Ohms per Volt__

Where: | I_{fs} = full scale current in amps |

*Last update: 16 Aug. 2003*