Tempco of electrolytic capacitors?

[Michael A. Covington]

How much does the capacitance of an electrolytic capacitor vary over a range
of, say, 0 to 30 C?

I'm wanting to throw together a quick and dirty "egg timer" to time 3
minutes and am wondering if I could get acceptable reproducibility with a
TLC555 monostable. It's not for eggs...it's a photographic application. I
know I can build a much better one with a microcontroller! But 5% accuracy
is quite sufficient in this application.

Thanks!

 

[Spehro Pefhany]

On Wed, 15 Dec 2004 16:11:30 -0500, the renowned "Michael A.
Covington" <look@www.covingtoninnovations.com.for.address> wrote:

How much does the capacitance of an electrolytic capacitor vary over a range
of, say, 0 to 30 C?

I'm wanting to throw together a quick and dirty "egg timer" to time 3
minutes and am wondering if I could get acceptable reproducibility with a
TLC555 monostable. It's not for eggs...it's a photographic application. I
know I can build a much better one with a microcontroller! But 5% accuracy
is quite sufficient in this application.

Thanks!

Probably less than a few percent for a low-leakage aluminum
electrolytic over that range.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Graham W]

Spehro Pefhany wrote:

On Wed, 15 Dec 2004 16:11:30 -0500, the renowned "Michael A.
Covington" <look@www.covingtoninnovations.com.for.address> wrote:

How much does the capacitance of an electrolytic capacitor vary over
a range of, say, 0 to 30 C?

I'm wanting to throw together a quick and dirty "egg timer" to time 3
minutes and am wondering if I could get acceptable reproducibility
with a TLC555 monostable. It's not for eggs...it's a photographic
application. I know I can build a much better one with a
microcontroller! But 5% accuracy is quite sufficient in this
application.

Thanks!

Probably less than a few percent for a low-leakage aluminum
electrolytic over that range.

My favorite approach is to use an audio frequency oscillator
followed by a divider. This has the advantage of small stable osc
components and the abililty to use an oscilloscope on the osc.
o/p to calibrate the timer. I use two gates from a CMOS 4001
for the osc and then something like a couple of 4024 seven
stage binary ripple counters to get a longer period output.

So a 91 Hz osc should get you a 180 second output.


--
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[Arthur W.]

My suggestion is to use a tantalum capacitor instead. Electrolytics not
only have much higher TCs (temperature coefficients) than Tantalums but also
have much wider tolerances and higher leakage. Because of the higher
leakage you cant use high resistor values in the 555 timing circuit whereas
you can if you use Tantalum capacitors. You may never get to 3 minutes with
an electrolytic!!

Please feel free to contact me at abw0628@optonline.net if you want to
discuss this more.


"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:ll3ds01tna1vpiroqpqpi17rqjtf7nqnr4@4ax.com...

On Wed, 15 Dec 2004 16:11:30 -0500, the renowned "Michael A.
Covington" <look@www.covingtoninnovations.com.for.address> wrote:

How much does the capacitance of an electrolytic capacitor vary over a
range
of, say, 0 to 30 C?

I'm wanting to throw together a quick and dirty "egg timer" to time 3
minutes and am wondering if I could get acceptable reproducibility with a
TLC555 monostable. It's not for eggs...it's a photographic application.
I
know I can build a much better one with a microcontroller! But 5%
accuracy
is quite sufficient in this application.

Thanks!

Probably less than a few percent for a low-leakage aluminum
electrolytic over that range.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers:
http://www.trexon.com
Embedded software/hardware/analog Info for designers:
http://www.speff.com