Current Probe
It is the measurement accessory that lets you see the waveform of current over time with an oscilloscope. It measures by clamping around the cable without opening the circuit. To be able to see currents containing a DC component, a Hall-effect (AC/DC) type is needed; AC probes containing only a current transformer cannot show the DC level of PWM.
A multimeter gives you an average number; a current probe shows you how the current rises and falls within the PWM period, how high it peaks at start-up and what happens on a change of direction. A significant portion of DC driver faults can only be understood by looking at this waveform.
- Starting surge: only here do you see the real amplitude of the peak current, which rises to 5–8 times nominal, and how long it lasts (typically 0.5–3 seconds).
- Ripple: the amplitude of the current ripple within the PWM tells you whether the switching frequency suits the motor inductance.
- Bandwidth: the probe's bandwidth must be at least ten times the switching frequency; a slow probe rounds sharp edges and shows the peak value as smaller than it is.
During measurement, place the probe around a single conductor only. If you clamp the outgoing and returning conductors together, the currents cancel each other and you get a reading close to zero.
Context where this term is used: DC Motor Driver Selection Guide