From: Richard Prosser <rhprosser@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Thu, 2 Jul 2020 14:24:47 -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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my 2c worth.

Tmax =3D 95C
What's Tmin? The ESR can increase significantly at low temperatures.

I'd also be concerned re the use of an electrolytic for an RC timer.
Tantalum or possibly even a high value MLC if a digital counter /
oscillator is not acceptable. Don't go overboard with the voltage safety
rating, a 450V cap on a low voltage circuit may not retain it's capacity.
Go for ~double if possible.

If an RC timer is required, you can get longer delays using a buffer stage
provided you protect against leakage paths (Guard rings etc). But you're
still at the mercy of the components, which is mainly the capacitor.

Select quality manufacturers & use the industrial (or military) grade rated
caps.

RP

On Fri, 3 Jul 2020 at 08:14, Alan Pearce <kiwiantipodean@googlemail.com>
wrote:

> 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 t=
o
> > 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 ho=
ur
> > > for
> > > the motor and timing capacitor.
> > >
> > > The capacitors will spend most of their life exposed to something lik=
e
> the
> > > following: power applied for 1 hour with 5 seconds of operation
> somewhere
> > > during, 1 hour with power removed. That cycle would be repeated
> something
> > > like 88000 times to get 10 years of power applied.
> > >
> > > On Thursday, July 2, 2020, Jason White <
> whitewaterssoftwareinfo@gmail.com>
> > > 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
> from
> > >> a
> > >> reputable brand (perhaps Nichicon or Panasonic?) with a high hour an=
d
> > >> 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 tw=
o
> > >> "large value" electrolytic capacitors. Wide temperature range, very
> > >> loose
> > >> tolerance and performance expectations from the capacitors. I'd like
> the
> > >> 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
> the
> > >> motor rail is high. I'm concerned about this capacitor's ESR and
> > >> capacitance staying in spec over temperature and time. Ripple curren=
t
> 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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