Ammeter Shunt Resistor

Calculator and formulas for calculating the shunt resistor for measuring range extension

Shunt Resistor Calculator

Measuring Range Extension

Calculation of the shunt resistor to extend the measuring range of an ammeter. Choose between meter resistance or total voltage as input parameter.

Choose Input Parameter
Results
Shunt resistor Rs:
Power Ps:

Ammeter Shunt Resistor

Ammeter shunt resistor circuit

Circuit diagram: Ammeter with shunt resistor

Purpose of Shunt Resistor
  • Measuring range extension for ammeters
  • Current division to protect the meter movement
  • Enables measurement of larger currents
  • Prevents overload of the measuring device
Input Modes
Meter resistance: Direct resistance value of the meter movement
Total voltage: Voltage drop across the entire measuring circuit
Important Note
An ideal ammeter should have a very low internal resistance to influence the measured circuit as little as possible.

Formulas for Shunt Resistor Calculation

Main Formula via Meter Resistance

With the ratio n = Itotal / Im:

\[R_s = \frac{R_m}{n-1}\]
\[n = \frac{I_{total}}{I_m}\]

Current ratio determines shunt resistor

Total Resistance

Parallel connection of meter and shunt:

\[\frac{1}{R_{total}} = \frac{1}{R_m} + \frac{1}{R_s}\]

Conductances are added

Power Calculation

The power dissipation in the shunt resistor must be considered in the design:

\[P_s = I_s^2 \times R_s\]
\[I_s = I_{total} - I_m\]

Practical Calculation Example

Example: Measuring range extension from 2A to 20A

Given: Meter movement with Rm = 10Ω, measuring range 2A to be extended to 20A

Step 1: Calculate current ratio
\[n = \frac{I_{total}}{I_m} = \frac{20A}{2A} = 10\]
Step 2: Calculate shunt resistor
\[R_s = \frac{R_m}{n-1} = \frac{10Ω}{10-1} = \frac{10Ω}{9} = 1.111Ω\]
Step 3: Verify current distribution
Through meter movement: Im = 2A
Through shunt: Is = 20A - 2A = 18A
Step 4: Shunt power
\[P_s = I_s^2 \times R_s = (18A)^2 \times 1.111Ω = 360W\]
Result: Shunt resistor Rs = 1.111Ω with a power dissipation of 360W

Theory and Practical Applications

Operating Principle

In an ammeter, shunt resistors are used to extend the measuring range. The shunt resistor is connected in parallel with the measuring device to divide the current. This way, only a small portion of the total current flows through the sensitive meter movement.

Important Properties
  • Current division: Current divides inversely proportional to the resistances
  • Same voltage: Meter movement and shunt have the same voltage
  • Precision: Shunt resistors must be very accurate
  • Stability: Use temperature-stable materials
Practical Applications
  • Digital multimeters: Current measurement in different ranges
  • Industrial measurement technology: High current monitoring
  • Energy measurement: Current transformers in energy meters
  • Battery management: Charging current monitoring
Symbol Directory
UtotalTotal voltage = meter movement voltage
ItotalTotal current
ImCurrent in meter movement
RmResistance of meter movement
RsValue of shunt resistor
PsPower/load capacity of shunt resistor
Important Notes
  • Shunt resistor must be precisely dimensioned
  • Consider power handling of shunt
  • Account for temperature coefficient
  • Minimize contact resistances
  • Calibration after installation
Practical Tips
  • Manganin or Constantan as shunt material
  • Kelvin connection for precise measurement
  • Heat dissipation for high currents
  • Protection from mechanical stress
  • Regular calibration

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