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'[EE] Specifying the right electrolytic capacitors '
2020\07\02@123843 by Jason White

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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 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 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 current is
assumed to be similar to motor supply current of ~3 amps. Low duty cycle.

Tambient-max=95C

Any advice, anecdotes, or links to relevant literature would be appreciated..

-- Jason White
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2020\07\02@125755 by Jason White

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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 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 <spam_OUTwhitewaterssoftwareinfoTakeThisOuTspamgmail.com>
wrote:

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2020\07\02@142818 by Harold Hallikainen

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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

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2020\07\02@160239 by Dwayne Reid

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Hi there, Jason.

I use Tantalum capacitors for RC time delays.  Good quality electrolytic caps might give you good results - or not.

Both bead-type and hermetically-sealed Tantalum caps have been reliable for me for decades.  I do not have reliability data for SMT Tantalum caps - I just haven't been using them long enough.

However, do NOT ever use a Tantalum cap on a high-current power rail.  They fail if exposed to even a momentary over-voltage transient.  The end result is often a hole in the PCB where the capacitor used to be.

dwayne


At 10:38 AM 7/2/2020, Jason White wrote:
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2020\07\02@161251 by Alan Pearce

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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
<.....haroldKILLspamspam.....mai.hallikainen.org> wrote:
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2020\07\02@171204 by Denny Esterline

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> , perhaps running at a higher frequency followed by a digital counter
>

May I suggest the CD4541B (and it's variants)
https://www.ti.com/lit/ds/symlink/cd4541b.pdf

Effectively a 555 timer with a couple counter stages added on.
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2020\07\02@172500 by Richard Prosser

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my 2c worth.

Tmax = 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 <kiwiantipodeanspamspam_OUTgooglemail.com>
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2020\07\03@121756 by Jason White

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Tmin= -55C (very cold)

Thanks everyone for the RC timer advice. I will use an alternate chemistry.
What about the motor capacitor?

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