What is a snubber circuit, and when is it needed?
A snubber is a small resistor-capacitor network used to suppress the voltage spikes and high-frequency ringing that appear at the moment of switching. Typical RC values are chosen in the 5–20 ohm and 1–22 nF range according to the motor inductance. Its purpose is not to destroy the energy but to slow the rate of voltage change and thereby lower the peak value.
The leakage inductance and the capacitance values in the circuit together produce an oscillation at the moment of switching; that is the ringing you see on top of the voltage edge on an oscilloscope. This oscillation causes two problems: stressing the voltage limit of the semiconductor, and broadband EMI emission.
When to think about a snubber:
- A long motor cable; the cable inductance amplifies the ringing.
- An RF receiver or sensitive analogue measurement nearby.
- Noticeable brush arcing on a brushed motor and the noise that comes with it.
- A relay or contactor used at the driver output.
Application notes:
- The RC snubber is placed at the motor terminals, with as short a connection as possible.
- The resistor turns the spike energy into heat, so its power rating must not be neglected.
- The capacitor must be of a type suited to the switching voltage and the ripple current; types made for this job are preferred over general-purpose ceramics.
- Adding a snubber alone does not fix a wiring mistake; first apply the rules of twisting, shielding and 50 mm separation.
Because snubber values depend on the real leakage inductance of the circuit, the final choice must be based on measurement; the ranges given are a starting point.
The full topic: DC Motor Driver Selection Guide