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Date: Mon, 14 May 2007 09:53:14 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Tantlum capacitors
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Ed Anderson wrote:
> I must admit, I've yet to see a blazing cell phone and there are a 
> bunch out there.  However, I think there could be a possible reason 
> for that.  The "energy" available from a 12-16 Volt aircraft 
> electrical system is a bit higher than from a small 3.3 volt battery 
> in a cell phone.  So the failure mode would likely tend to be more 
> "explosive" with higher energy source - more energy to feed the "bang!".
>
> I can not say it was the fault of a Tant capacitor for certain, but I 
> recently had two boards fry.  One clearly resulted from a short that 
> caused the voltage regulator to overheat and  actually char the board 
> around it.  I caught the other one before it got to that stage.  In 
> any case, I've never had a ceramic capacitor fail and have redesigned 
> my boards to use ceramic. The only down side I could see if that you 
> might need to use two ceramic in parallel if you need over 10-20 uf.
> So, while Al's expert viewpoint certainly appears valid to me, since 
> my board will be airborne and I can use ceramic, I will err on the 
> side of caution.
>
> Ed
> ----- Original Message ----- From: "Al Gietzen" <ALVentures@cox.net>
>>  All of the
>> failures I've seen (2) occurred in the first few seconds of testing
>> (1) or because the electronic assembler installed the capacitor
>> backwards (reverse polarity) (1).

This may be the most important point, Ed.  My experience was on a test 
fixture at the end of the assembly line.  It was the first time the caps 
had seen a biasing voltage.  Think of it as dramatic crib death.  It was 
stated at the time that the insulating oxide formed on the tantalums 
when the voltage was applied.  Don't know if it was true, but the 
solution is to bring the voltage up rather slowly to give the chemistry 
a chance to work.