# Spike protection

Thread Starter

#### apqo1

Joined Oct 5, 2008
52
I found a suggestion elsewhere online that a 10μF capacitor across the input of a device could protect against a "300V spike of a few millijoules delivered through a 50Ω source impedance."

Does it sound like the author is suggesting the circuit in the attached diagram? Is this really sufficient to absorb the contemplated 300V spike, without damage to the protected device or the capacitor?

Aren't there devices designed specifically for this purpose? If so, can anyone point me to a representative datasheet?

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#### crutschow

Joined Mar 14, 2008
34,392
The energy stored in a capacitor is E = 1/2 CV$$^{2}$$ thus the increase in voltage across a 10μF capacitor by adding, say 3mJ from the spike, is √(3mJ * 2/10μF) = 25V. This depends only upon the spike energy and is independent of the initial spike voltage (ignoring any stray inductance effects). Whether you consider a 25V increase is "protecting against the spike" depends upon the circuit you are trying to protect.

There is not special rating required for the capacitor to do this expect that it's voltage rating needs to be greater than the original voltage on the capacitor plus the added voltage from the spike energy.

#### MaxHeadRoom

Joined Jul 18, 2013
28,669
Normally in the real world on DC supplied device this is done with a reverse connected diode or rectifier.
If the supply is AC in nature then a R/C snubber series combination is used.
Max.

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#### #12

Joined Nov 30, 2010
18,224
While crutschow gave the exact answer to the question, I would like to say that spike protection takes many forms depending on the spikes and the equipment that needs protecting. Sometimes it requires more than one protective device.

#### Alec_t

Joined Sep 17, 2013
14,312
Aren't there devices designed specifically for this purpose?
Google 'TVS'.

#### MaxHeadRoom

Joined Jul 18, 2013
28,669
Or there's this.
Max.

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Thread Starter

#### apqo1

Joined Oct 5, 2008
52
Thanks to all. Much reading to do...

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