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I have a formula here for finding the suitable value of shunt capacitance to be put on an SCR snubber.

\$C=\frac{1}{2L}(\frac{0.546V}{dv/dt})^2\$

where L is the snubber inductor, V is the DC supply voltage and \$dv/dt\$ the maximum allowable surge voltage. My question is about the derivation of the said equation\$-\$if it is a correct one. The formula looks bizarre to me when considering the equation for getting the right inductor, \$V = L\frac{di}{dt}\$, which is more straightforward. Moreover, I'm puzzled why choosing the capacitor value would depend on the series snubber inductor.

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  • \$\begingroup\$ L is the load inductance \$\endgroup\$ – Marko Buršič Aug 24 '17 at 9:01
  • \$\begingroup\$ Based on my experience I think it's the snubber inductor. \$\endgroup\$ – DorkOrc Aug 24 '17 at 9:07
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    \$\begingroup\$ There is no snubber inductor, just RC snubber. The snubber circuit is connected in parallel with SCR. \$\endgroup\$ – Marko Buršič Aug 24 '17 at 10:28
  • \$\begingroup\$ Yes, and an inductor would be connected in series to protect the SCR from di/dt. So essentially it is also in series with and becomes part of the load, as you said it is load inductance. \$\endgroup\$ – DorkOrc Aug 24 '17 at 12:39
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Interesting question.

The inductor stores energy in the current: 0.5 * L * I^2

The snubber capacitor needs to store that same amount of energy, as voltage across the capacitor: 0.5 * C * V^2

Equate the two energy equations, and have fun.

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