From: Alan Pearce <kiwiantipodean@googlemail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Thu, 2 Jul 2020 13:13:34 -0700
Subject: Re: [EE] Specifying the right electrolytic capacitors for long (10
	year) operating life?
Thread-Topic: [EE] Specifying the right electrolytic capacitors for long (10
	year) operating life?
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I would also be worried about using a wet electrolytic for an R/C
timer. If an R/C timer is needed then a tantalum would be a must.

>From my time on the space industry we were required to derate
capacitors to 50% of max working voltage, so on a 12V power supply
line one would use 25v capacitors. I would recommend that you use a
similar derating factor, but you could reduce the derating factor by
using Tantalum-polymer capacitors. These have two advantages, firstly
they can be run in reliability conscious applications at a lower
derating than standard caps and secondly they do not have the nasty
failure mode of going short circuit.

But coming back to the timing, maxim and the like make ICs that are
configurable timers without using R/C circuits, and OI would have
thought would be a more reliable way of doing it, after all you are
looking for a 10 year design life.

On Thu, 2 Jul 2020 at 20:29, Harold Hallikainen
<harold@mai.hallikainen.org> wrote:
>
> I'd be concerned about using an electrolytic in an RC timer. Capacity
> tolerance and leakage current could be issues. Is there any way to go to
> another capacitor technology, perhaps running at a higher frequency
> followed by a digital counter? I've seen electrolytic-based RC timers
> never time out due to leakage current.
>
> Harold
>
> > Addendum, by low duty cycle I mean about 5 seconds of activity per hour
> > for
> > the motor and timing capacitor.
> >
> > The capacitors will spend most of their life exposed to something like =
the
> > following: power applied for 1 hour with 5 seconds of operation somewhe=
re
> > during, 1 hour with power removed. That cycle would be repeated somethi=
ng
> > like 88000 times to get 10 years of power applied.
> >
> > On Thursday, July 2, 2020, Jason White <whitewaterssoftwareinfo@gmail.c=
om>
> > wrote:
> >
> >> Hi everyone,
> >>
> >> I'm looking to learn about the critical parameters in specifying the
> >> "correct" electrolytic capacitor to last a long time (10 years).
> >>
> >> I suspect it comes down to selecting a "long life" series capacitor fr=
om
> >> a
> >> reputable brand (perhaps Nichicon or Panasonic?) with a high hour and
> >> temperature rating, low ESR, and significantly higher than needed
> >> voltage
> >> rating.
> >>
> >> I have a safety related circuit where I have no choice but to use two
> >> "large value" electrolytic capacitors. Wide temperature range, very
> >> loose
> >> tolerance and performance expectations from the capacitors. I'd like t=
he
> >> two capacitors to "last" 10 years of having power applied.
> >>
> >> The first capacitor is being used in a RC timing delay. Consequently,
> >> the
> >> "ripple" current would be in the single digit microamp range. I'm
> >> concerned
> >> about this capacitor's capacitance and leakage staying in spec over
> >> temperature and time. Low duty cycle.
> >>
> >> The second capacitor is being used to provide bulk "high-ish voltage"
> >> decoupling capacitance to an electric motor. The supply impedance to t=
he
> >> motor rail is high. I'm concerned about this capacitor's ESR and
> >> capacitance staying in spec over temperature and time. Ripple current =
is
> >> assumed to be similar to motor supply current of ~3 amps. Low duty
> >> cycle.
> >>
> >> Tambient-max=3D95C
> >>
> >> Any advice, anecdotes, or links to relevant literature would be
> >> appreciated.
> >>
> >> --
> >> Jason White
> >>
> >
> >
> > --
> > Jason White
> > --
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