# Questions about RD snubber circuits

I am not too familiar with designing snubber circuits.

• Can somebody sanity check the following sizing calculation for a snubber circuit?
• Also: Is the undampened rise rime of the voltage due to the missing capacitor an issue if the absolute value stays below the critical values for the switch?

I want to switch an inductor L1 with ESR R1 with a BJT Q1. Given that I want to limit the voltage across Q1 to a safe value $$V_{Q1} = x\times V1,$$ I am looking to size R2 appropriately.

Assuming D1 has a negligible forward voltage drop, the current in the inductor should drop as: $$I_{L1} = \frac{V1}{R1}\exp(-\frac{(R1+R2)t}{L1})$$

This means the maximum voltage across R2 is $$V_{R2} = R2 \times \frac{V1}{R1}$$

The voltage from the voltage source and across R2 add to give the voltage across Q1: $$V_{Q1} = V1+V_{R2} = V1+\frac{R2}{R1}\times V1=x\times V1$$

It follows that to limit $V_{Q1}$ to $x \times V1$, R2 is to be sized as: $$R2 = (x-1)\times R1$$

Note: please ignore the model numbers for D1 and Q1. I could not find a way to remove them.

simulate this circuit – Schematic created using CircuitLab

• Thank you Andy for your explanations. Though I was hoping somebody could check that my calculations are correct. That being said, can you may be enlighten me what the consequences of not limiting the rise time of $V_{Q1}$ with a capacitor are? Are BJTs sensitive to rapid rise times even if the absolute rise is below the max rating? – ARF Apr 29 '13 at 17:46