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Subject: Re: [FlyRotary] Re: EC2 question
From: Chad Robinson <crj@lucubration.com>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
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Date: Sat, 12 May 2007 09:36:54 -0400
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On Fri, 2007-05-11 at 20:31 -0400, Ed Anderson wrote:
> Thanks Perry, I erred on the side of caution in selecting voltage
> ratings, so am probably OK, but I think I am going to redesign my
> boards to avoid use of Tant capacitors.  I've never had one destruct
> (yet) and would certain like to avoid it.  It sounds like a wise
> decision not to use them in Avionics designs..  I did a goggle and
> found a number of articles and studies on that problem with Tant
> capacitors - so clearly a real potential problem.

The failure mode is real, but that doesn't mean they should never be
used. Tantalums have provide excellent ESR and size characteristics for
their cost. Many power supply support ICs specifically call for them on
the output side to provide maximum stability in the voltage produced.
They're entirely appropriate for use there, since the voltage regulator
is going to prevent the over-voltage condition that will cause the Tant
to fail (or your whole device is going to go up in smoke anyway). They
also last longer than electrolytics (which actually have a shelf life)
and have better stability over a wide temperature range (not a bad idea
in an aircraft that might be kept outdoors).

You can also achieve a low ESR by paralleling two caps. Electrolytics
aren't always great for this (at a reasonable cost) but they provide
excellent capacity-to-cost ratios. Parallel them with a ceramic (which
does have a low ESR, often even better than a Tant) chip capacitor. It
won't be quite as good as the original thing, but how much ripple can
you tolerate anyway? As long as it meets your design specs it's fine.

It's an extra part to solder to the board though.

Regards,
Chad