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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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 Posted: Mon Apr 16, 2012 3:56 pm Post subject: Regarding Capacitor Filters |
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I have watched the conversation regarding the big capacitor for filtering the generator/alternator/dynamo/etc. and want to make a few comments:
1) Usually a capacitor doesn't get charged to a higher voltage that was applied. But this is not true in all cases. Capacitors can and do "space charge" (all by themselves) to extremely high voltages. Really big, efficient, very high ESR (Equivalent Series Resistance--usually high voltage) caps are stored and shipped with a shorting strap for this reason. And yes, this doesn't have much to do with OUR situation. ...but just sayin'....don't learn stuff that ain't true.
2) Capacitors are almost always used with a "bleeder" resistor that guarantees that any retained charge will be dissipated in a short time after the power is removed. A capacitor is really a battery that should be disconnected or discharged if you are going to crash your airplane. You can calculate this (Google "capacitor bleed resistor calculator") but for general purposes a 200 ohm 2W resistor across the capacitor terminals will lower 15V to 5V in 5 seconds for a 22000 uF cap). Should you add one? Consider that the resistor improves the filtering all by itself and only add 75 mA to the load. So YES!
2) An electrolytic capacitor can be subjected to any number of insults. Some of these can easily be prevented. Let’s look at a typical part: A CDE Capacitor about the size of a D-cell battery: SLPX223M025H5P3—30 X 40 mm. 25 Vdc (32 Vdc Surge)
—High applied voltages damage the foil plates. These plates are self-healing usually, but they steadily degrade. Use a parallel Zener with a Vz below the withstand V of the capacitor for a longer, happier life.
—Temperature. Keep the cap far away from major heat sources (easy). Keep the cap from extreme cold (harder). Watch those environmental specs. Capacitor actually perform better HOT, but have shorter lifetimes. At -40C cold (brrrrrr…) the capacitance is about 1/10 of what it is when hot—but they warm up fast.
—Common Electrolytic caps have a limited lifetime—in this example 3000 hours. So you should perhaps replace it with at engine TBO, or if permanency is what you want, choose a different part.
For filtering, a capacitor is only part of the answer. Any decent filter uses a resistor (R), an inductor (L), in addition to a capacitor (C). These are called RLC filters, and usually have a small ceramic cap and a Zener too. Standard automotive filters are made like this and work fine in 14.5V aircraft applications.
See: http://www.jmkfilters.com/autofilterssc1.htm
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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net |
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bob.verwey(at)gmail.com
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| Posted: Tue Apr 17, 2012 4:20 am Post subject: Regarding Capacitor Filters |
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Thanks Eric!
As always, fascinating and informative for us techno 'heat seekers' out there!
Bob Verwey
On 17/04/2012, Eric M. Jones <emjones(at)charter.net> wrote:
| Quote: |
<emjones(at)charter.net>
I have watched the conversation regarding the big capacitor for filtering
the generator/alternator/dynamo/etc. and want to make a few comments:
1) Usually a capacitor doesn't get charged to a higher voltage that was
applied. But this is not true in all cases. Capacitors can and do "space
charge" (all by themselves) to extremely high voltages. Really big,
efficient, very high ESR (Equivalent Series Resistance--usually high
voltage) caps are stored and shipped with a shorting strap for this reason.
And yes, this doesn't have much to do with OUR situation. ...but just
sayin'....don't learn stuff that ain't true.
2) Capacitors are almost always used with a "bleeder" resistor that
guarantees that any retained charge will be dissipated in a short time after
the power is removed. A capacitor is really a battery that should be
disconnected or discharged if you are going to crash your airplane. You can
calculate this (Google "capacitor bleed resistor calculator") but for
general purposes a 200 ohm 2W resistor across the capacitor terminals will
lower 15V to 5V in 5 seconds for a 22000 uF cap). Should you add one?
Consider that the resistor improves the filtering all by itself and only add
75 mA to the load. So YES!
2) An electrolytic capacitor can be subjected to any number of insults. Some
of these can easily be prevented. Let’s look at a typical part: A CDE
Capacitor about the size of a D-cell battery: SLPX223M025H5P3—30 X 40 mm. 25
Vdc (32 Vdc Surge)
—High applied voltages damage the foil plates. These plates are self-healing
usually, but they steadily degrade. Use a parallel Zener with a Vz below the
withstand V of the capacitor for a longer, happier life.
—Temperature. Keep the cap far away from major heat sources (easy). Keep the
cap from extreme cold (harder). Watch those environmental specs. Capacitor
actually perform better HOT, but have shorter lifetimes. At -40C cold
(brrrrrr…) the capacitance is about 1/10 of what it is when hot—but they
warm up fast.
—Common Electrolytic caps have a limited lifetime—in this example 3000
hours. So you should perhaps replace it with at engine TBO, or if permanency
is what you want, choose a different part.
For filtering, a capacitor is only part of the answer. Any decent filter
uses a resistor (R), an inductor (L), in addition to a capacitor (C). These
are called RLC filters, and usually have a small ceramic cap and a Zener
too. Standard automotive filters are made like this and work fine in 14.5V
aircraft applications.
See: http://www.jmkfilters.com/autofilterssc1.htm
--------
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=371002#371002
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