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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:

diagram
  1. Connect the meter in series with a DC power source (battery) and a variable resistance R1.
  2. Starting from highest resistance, vary R1 until you attain a full scale reading.
  3. Connect a second variable resistor R2 across the meter. Vary R2 untilyou attain a half scale reading.
  4. After disconnecting R2 from the circuit, measure its resistance. It will be equal to the meter resistance Rm.

Note: Insure that R1 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 R1:

formula

Current Shunt

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

diagram

R = Rm/N - 1
Where:R = shunt value in ohms
N = new full scale reading divided by original full scale reading
Rm = meter resistance in ohms

Voltage Multiplier

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

diagram

Where:R = series multiplier value in ohms
Efs = full scale reading in volts
Ifs = full scale current of meter in amps
Rm = meter resistance in ohms

Ohms per Volt

formula
Where:Ifs = full scale current in amps

Last update: 16 Aug. 2003