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  Circuit Diagnosis[Issue 1, Sept 2009]

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Exploding Capacitors
I hardly got to finish my first cup of coffee when my phone rang.  It was one of my old clients. They were in a panic.  Capacitors on a circuit board used in a product I had designed for them years earlier were exploding.  They didn’t understand why.  I asked the usual questions.  Which caps?  Were there any changes to voltage or temperature?  Were they paying attention to the polarity of the caps? 
They assured me that there were no changes and yes they were soldering the capacitors on the board with the black stripe on the capacitor pointing to the + marking on the circuit board.  They said that only the larger capacitors were exploding and only the “round” ones.  I asked “round ones?”  They told me that their component supplier persuaded them into phasing out some of the square tantalum capacitors in favor of round aluminum electrolytic caps, since they were a lot cheaper.  “Oh,” I said. “I know the problem.”

Surface mounted tantalum capacitors have a stripe printed on the positive side of the part.  I prefer to use these parts in my designs, unless product cost is more important than performance.  I have found that tantalum caps tend to last much longer.  But, aluminum electrolytic capacitors, although cheaper, have the polarity stripe printed on the negative side of the part.  My client was soldering the newer parts in backwards.  Once they flipped the parts around, everything worked fine.

Tantalum Cap with Positive Stripe Aluminum Cap with Negative Stripe

Issue #1:

September 2009  First Issue

Page 1 Back
Envelope
Circuit
Diagnosis
Experimenter's
Corner
Good Idea
gone Badly
New Products Rants &
Raves
What the World
needs Now
Wily Widget

 


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