Electronic – Snubber Capacitor Voltage Rating in the Real World


What are voltage ratings for the capacitors inside the common off the shelf ac snubber networks, such as 0.1uF + 100 Ohm ?

It would seem that larger inductive loads can easily drive the capacitor voltage into the kilovolts. Is it ok because it only lasts a short time?

I am more interested in the real world practices than in theory. I don't know how many people in the field actually try to find or measure the load inductance and current to calculate the capacitor voltage before grabbing a snubber and wiring it in. Or is it common practice?

It is not obvious from datasheets what actual voltage ratings are on the capacitors inside those snubber networks. It is only stated for the networks themselves. And I have not taken one apart.

This applies as well to the numerous snubber examples on the web. Almost none mention capacitor voltage, only capacitance.

Best Answer

You are forgetting the capacitor is, well, a capacitor. In order to get it up to failure voltage you need to charge it up that high.

When the switch opens the voltage across the capacitor will not change instantaneously like the coil voltage does. Instead, the current in the coil passes trough the resistor to charge the capacitor. The voltage across the coil is therefor limited. The energy from the coil is then dissipated through the resistor and into the capacitor.

The capacitance then needs to be large enough to absorb the energy in the coil without exceeding it's rated voltage.