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Bad capacitor, BAD!

by dnyberg2
Tags: capacitor
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dnyberg2
#1
Dec6-12, 05:45 PM
P: 66
Heres a good one for the group... I once had a design that had an 0603 ceramic 1000 pF 25VDC capacitor in the return leg of an RF circuit operating at 50 Mhz. Some guy changed it to an 0402 50 volt cap because using an O-Scope across this node you can measure 20 VPP on one side of the cap. Turns out the cap is really a 25 volt 0402 and I am concerned that this will reduce the reliability of this part. After all, it is the RF return for this CKT.

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the_emi_guy
#2
Dec6-12, 06:04 PM
P: 585
Assuming this is a DC block on a 50 ohm line you have about 1W of RF power.

Xc is about 3 ohms, so the voltage *across* the cap should be only about 1Vp-p. If this is right you should be o.k.

Can you measure the p-p across the cap?
yungman
#3
Dec6-12, 06:09 PM
P: 3,883
If power is not a concern, the two cap has the same voltage rating, I don't see any problem. I just don't understand, do you mean someone solder a 0402 into the 0603 footprint?!!!

MATLABdude
#4
Dec6-12, 07:36 PM
Sci Advisor
P: 1,724
Bad capacitor, BAD!

You should probably look it up at your specific manufacturer, but you can guesstimate the derated capacitor lifetime using the Arrhenius equation:
http://www.illinoiscapacitor.com/tec...lculators.aspx


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