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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.

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  • \$\begingroup\$ Are you sure they are calling out an "ordinary" DC rated capacitor and not an X-class mains safety rated capacitor? \$\endgroup\$ – ThreePhaseEel Nov 27 '17 at 3:23
  • \$\begingroup\$ Ignoring losses in the resistors (read Trevor answer below), simply equate the C energy with the L energy: 1/2 C * V^2 = 1/2 L * I^2. But those resistor losses , during the charging time ..... are the magic \$\endgroup\$ – analogsystemsrf Nov 28 '17 at 4:26
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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.

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  • \$\begingroup\$ Right. So what are common voltage ratings being used? So many examples of snubbers on the web, and almost none bother with the voltage. I know to do this right you have to calculate it. I am asking for real world practices. \$\endgroup\$ – mcu Nov 27 '17 at 2:30
  • \$\begingroup\$ @mcu it really depends on the motor and how much inductance it has. If the cap is too small uF wise, it will charge quickly to a high voltage. I'd say twice supply voltage would be a ball park maximum you should be aiming for. BTW For a DC motor, where snubbers are more commonly used, this all happens so fast within a commutation cycle, it really does not charge up much at all. \$\endgroup\$ – Trevor_G Nov 27 '17 at 2:36
  • \$\begingroup\$ So you are saying I must know the load inductance before attempting to use a snubber, so I can calculate the capacitance needed? I just wonder how many people skip this step and use what snubber feels right. I may be all wrong on this. \$\endgroup\$ – mcu Nov 27 '17 at 2:41
  • \$\begingroup\$ @mcu, most folks probably just guess, or copy from some schematic on the web. Which, for general small motor work is probably overkill and fine. Driving a large kW motor though would require a bit more careful study. \$\endgroup\$ – Trevor_G Nov 27 '17 at 2:45

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