Understanding Current Probe vs Differential Probe (Across Shunt) in Oscilloscope Power Measurements
Scenario
You are measuring current in a circuit using an oscilloscope.
You have two options:
- Current Probe (e.g., clamp, Hall effect, Rogowski)
- Differential Probe across a shunt resistor
Configuring your oscilloscope or analysis software as if your diff probe was a “current probe” (unit: AMP) gave incorrect results. Why?
Key Differences
| Probe Type | Senses | Output Voltage Means | Software Conversion Needed? | Example Scaling |
|---|---|---|---|---|
| Current Probe | Magnetic field | Proportional to current | No (if scaling correct) | 1V/A, 10mV/A, etc |
| Diff Probe+Shunt | Voltage difference | Voltage across shunt | Yes, V/Ω | — |
How They Work
1. Current Probe (Clamp, Hall, Rogowski)
- Senses: Magnetic field around the conductor (no circuit interruption)
- Outputs: Voltage directly proportional to the measured current
- e.g., 1V out = 1A flowing (if probe designed for this scaling)
- Oscilloscope Setup: Set channel unit to AMP, enter correct probe scaling.
- Software: No conversion needed if probe scaling is set—voltage is already "current".
2. Differential Probe (Across Shunt Resistor)
- Senses: Voltage drop across the shunt resistor.
- Outputs: Voltage proportional to current, but must use Ohm's law:
I = V / R_shunt- e.g., 10mV across 0.01Ω shunt = 1A
- Oscilloscope Setup: Channel unit should be VOLT (not AMP).
- Software: Enter shunt resistance (Ω) so software converts voltage to current.
Common Pitfall
Setting your oscilloscope channel to “AMP” when using a diff probe + shunt does NOT make the measurement correct.
- The scope only changes the displayed unit, not the actual conversion.
- Unless the scope’s probe settings also know your shunt value, results will be incorrect.
Correct Setup for Differential Probe + Shunt
- Scope channel unit: VOLT
- In analysis software:
- Select Probe Type: Shunt
- Enter shunt value in Ω
- The software will compute current as
I = V / R_shunt
Summary Table
| Setup | Scope Unit | Probe Type | Value | Software Conversion | Correct? |
|---|---|---|---|---|---|
| Clamp Current Probe | AMP | Clamp | (V/A) | None (if scaling set) | Yes |
| Diff Probe Across Shunt | VOLT | Shunt | (Ω) | I = V / R_shunt |
Yes |
| Diff Probe + Shunt, unit: AMP | AMP | Clamp | 1.0 | None | No |
Practical Advice
- Use current probe + “AMP” only if you have a real current probe.
- For shunt+diff probe, always use VOLT and set the correct shunt value in your analysis software.
- Double-check your software scaling and units!
If you see weird current readings or power calculations, check your probe type, scaling, and software settings.