# Zener Votage Clamp - Having a blonde moment!

#### gdallas

Joined Apr 25, 2012
74
Wish to protect my circuit in line with intrinsically safe standards, specifically regarding the maximum total capacitance for any given supply voltage. At 3.3V my 2.2uF capacitor is IS, at 30V it is not. 30V would be an abnormal operation; fault condition. However I need to consider this and protect the circuit. My thoughts are to add a 1n5333 zenner and current limiting resistor as shown in yellow (previously this wired straight through)
The chip shown (ADUM421) provides a regulated 3.3V to both power itself and anything upstream as required. If my FET failed, and allowed 30V to be present at my VCC, I would expect an addition of my zener to clamp that an ensure only 3.3V as passed. My working is a follows, under open circuit conditions.
Vss = 30V, 1k current limiting resistor, and assuming a 1n5333 is suitable (reverse breakdown voltage of 3.3V) then I would drop 26.7V across the resistor and current limit to 26mA. If this is correct, unsure how the zener performs at normal conditions. i.e. assume 3.3V. any ideas??

thanks

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#### ericgibbs

Joined Jan 29, 2010
19,026
hi gd,
For explanation consider that the 3v3Z is not present but the 1k is..

So under normal conditions the 3.3V supply to the circuit will be only 3.3V -[ Icct *1k]

If the circuit normally draws 10mA say , thats 3.3v - [ .01mA*1k] = 3.3v -10V,!!!,

Can you see the problem.?
E