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Old 27th April 2005, 06:01 PM   #1
jeremym is offline jeremym  Canada
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Default Big cap in parallel with SLA battery supply?

Any thoughts on putting a large cap in parallel with the battery power supply? Some claim it maintians the Voltage during higher current transients. Would it be an improvement? I'm running a stock SI with a 6AH, 12V SLA battery.

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Jeremy
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Old 27th April 2005, 07:11 PM   #2
rha61 is offline rha61  France
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you need to put a big and low-ESR electrolytic cap after the battery to reduce the psu impedance

alain
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Old 28th April 2005, 06:55 AM   #3
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I'm actually looking in the same direction. The EE store on campus sells parts at low prices and I'm going to wonder in there tomorrow. The online site says they have 10,000uF 16V caps for $2.80. Given I there aren't too many good electronics stores imeadiately nearby I was thinking I'd give them a shot. First though I'd like to know what sort of ESR I should be shooting for.

(oh yeah, I've gotten some lower valued caps there before, and they were Fu Lee brand.)
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Old 28th April 2005, 05:56 PM   #4
ohenry is offline ohenry  United States
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Quote:
Originally posted by velmeran42
First though I'd like to know what sort of ESR I should be shooting for.

(oh yeah, I've gotten some lower valued caps there before, and they were Fu Lee brand.)
I was wondering the same thing regarding ESR. Is 0.02 ohms for a 20k uF cap a reasonably good value?
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Old 29th April 2005, 01:47 AM   #5
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I ran into the EE store on campus, turns out the things are only $0.70 for the 10,000uF 16V electrolytics so I snagged one. It's labeled "SC" with a a little lightning bolt logo. The guy working the counter today had no clue what brand it was, or what any of the specs are, but for $0.70 I can take a chance.
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Old 29th April 2005, 02:23 AM   #6
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If you want a low ESR, you might want to consider paralleling several caps, or several smaller caps. You might also consider a small film or ceramic capacitor in parallel to maintain a low ESR at high frequencies.
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Old 29th April 2005, 07:22 AM   #7
rha61 is offline rha61  France
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Quote:
Originally posted by ohenry


I was wondering the same thing regarding ESR. Is 0.02 ohms for a 20k uF cap a reasonably good value?

battery have a high ESR
any cap will do the job , but it's better to have low ESR in the audio bandwidth , you can use especially caps for audio ( Elna Silmic or Cerafine , Blackgate , Nichicon Muse ... which have vibration-softening electrolyte and foils )

alain
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Old 29th April 2005, 12:06 PM   #8
ohenry is offline ohenry  United States
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Quote:
Originally posted by Dr. Photon
If you want a low ESR, you might want to consider paralleling several caps, or several smaller caps. You might also consider a small film or ceramic capacitor in parallel to maintain a low ESR at high frequencies.
Thanks for the advice. I'm still a bit confused regarding the "low ESR at higher frequencies" statement. This isn't a signal cap application, but a power supply cap. I saw graphs depicting ESR slopes across frequencies, but wonder if that's a concern in this application. BTW, the cap I have is 0.02 ohms at 120Hz if that matters.

I hope you guys don't mind schooling me (old dog learning new tricks).

Thanks again...
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Old 29th April 2005, 12:30 PM   #9
rha61 is offline rha61  France
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it's not a signal cap but to simplify , the voltage V is constant but the current I varies with the signal ( in the low-ESR cap (and battery too) )

alain
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Old 29th April 2005, 11:26 PM   #10
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Quote:
Originally posted by ohenry

I'm still a bit confused regarding the "low ESR at higher frequencies" statement.
You're right to be confused here. ESR is frequency independent. Impedance isn't.

Simplifying slightly, an electrolytic cap can be modelled as a series circuit of an ideal capacitor, a resistance and an inductance. The resistance is partly in the electrolyte and partly in the aluminium foil. The capacitance is formed on one side by electrolyte, on the other side by aluminium and a thin layer of aluminium oxide serving as the dielectric.
The inductance is largely determined by the size of the cap, and corresponds roughly to a U shaped wire loop with the legs as far apart as the leads and about half as high as the can.

This shows that "low ESR ar higher frequencies" is not quite correct. You can have low impedance at higher frequencies but this is the inductance which is at play. The ESR is dominant at intermediate frequencies only.

Both for ESR and inductance, a parallel combination of many smaller caps is better than one large one.

"Audiophile" caps have a lower metal resistance compared to the electrolyte (=thicker aluminium foil) and have better damping of mechanical resonances. And of course a nice story.
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