# Spike protection

Discussion in 'General Electronics Chat' started by apqo1, Nov 24, 2013.

1. ### apqo1 Thread Starter Well-Known Member

Oct 5, 2008
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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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2. ### crutschow Expert

Mar 14, 2008
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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.

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Jul 18, 2013
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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.

• ###### snubber.jpg
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4. ### #12 Expert

Nov 30, 2010
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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.

5. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Jul 18, 2013
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Or there's this.
Max.

• ###### SnubberDesign.pdf
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7. ### apqo1 Thread Starter Well-Known Member

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