Electronic – Do capacitors provide ESD protection even if they are already charged

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I've read about ESD protection with capacitors and it seems that if one knows its limitations it could in some cases be a cheap alternative for or addition to designated ESD protection circuits.

The basic idea is that the charge of the ESD source (i.e. Human Body Model) will be shared across the source's capacitor and the "protective capacitor". The final voltage could then be estimated using simple calculations. (enter image description here)

But what if the "protective capacitor" is already fully charged when the ESD event occurs? For example, let's say it is a charged input capacitor of a voltage regulator and a ESD event occurs at its input connector. Does it then still provide any protection?

Any clarification would be appreciated.

Best Answer

But what if the "protective capacitor" is already fully charged when the ESD event occurs?

It still provides protection; if more charge is put into the capacitor from an ESD surge then, the capacitor charges up more and the voltage increases. Of course, if the voltage rises above the maximum rating of the capacitor then it will likely damage the capacitor. And, if the voltage rises too high it might become too high for the circuit it is intended to protect.

But, basically, it is a great way of protecting against ESD surges. Not so great with indirect lightning but good for ESD level energies.