Super Capacitor for Power Supply Filter

davidallancole · 18th December 2005, 05:22 PM

Has anybody tried using electric double layer capacitors for the filter capacitor in a power supply?

I found I can get a hold of ten 4.7 F, 2.3 V low resistance super caps for around $30. I figured I would try building a +-10 V power supply with these babys. If I put five of them in series for each rail, it would be an effective capacitance or around 1 F.

I got this idea searching through the old messages. Some guy built a power supply using many 1000 uF capacitors in parallel to achieve something like 50,000 uF. It looked like he had a hundred capacitors in there.

poobah · 18th December 2005, 06:24 PM

Probably not a good idea... supercaps are an oddball. They are really more like a battery than a cap. Read the specs on them... you will see that ther are limits on charge current etc... Another way of saying it: "The ESR is VERY high".

All caps have ESR... Equivalent Series Resistance. So, every time you plop a cap in a circuit you are actually adding a resistor too. For filtering, you want ESR as low as possible... the opposite of a "super-cap".

These things are finding new applications all the time... mostly where people are trying to replace lithium batteries to keep chip memories alive etc...

For audio PS filtering, look for "low ESR". Then bypass close to the individual circuits with film/foil.

davidallancole · 18th December 2005, 06:52 PM

What is the purpose of the by-pass capacitor?

poobah · 18th December 2005, 07:03 PM

The local bypass caps stamp out the last little bits of noise right at the point use.

From your PS to your individual circuit blocks is a bit of inductance and resistance and also the potential for noise pickup. So, adding a bypass close to the circuit block is like adding another L/C or R/C stage to the PS. Not always neccesary, by any means.

The "secret" is that certain types of caps only behave truely like caps at certain frequencies... high frequencies skip right over electrolytics.

As an example... you might have 100uf (low ESR) at the PS... and then put a 0.1 - 1.0 uF film foil, or even ceramic, RIGHT across the power pins of an op-amp. The op-amp gets a supply of power free from inductance and behaves better...

davidallancole · 18th December 2005, 08:51 PM

Thanks man!

poobah · 18th December 2005, 08:55 PM

You betcha,

Every time you draw a cap, draw an inductor and a resistor in series (kinda the same with a wire). You don't have to do all the aweful math... but you will see what is happening.

Cheers -

Steve Eddy · 18th December 2005, 10:00 PM

The new carbon areogel supercaps have very low ESR.

se

poobah · 18th December 2005, 10:13 PM

Thats right Steve,

I have 4, 44 Farad caps, WITH low ESR, on my bench. I can't tell you where I got them, or why. They are not that big.

Let's just say, there are new things on the way...

18th December 2005, 05:22 PM	#1
davidallancole diyAudio Member Join Date: Nov 2005	Super Capacitor for Power Supply Filter Has anybody tried using electric double layer capacitors for the filter capacitor in a power supply? I found I can get a hold of ten 4.7 F, 2.3 V low resistance super caps for around $30. I figured I would try building a +-10 V power supply with these babys. If I put five of them in series for each rail, it would be an effective capacitance or around 1 F. I got this idea searching through the old messages. Some guy built a power supply using many 1000 uF capacitors in parallel to achieve something like 50,000 uF. It looked like he had a hundred capacitors in there.

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18th December 2005, 06:24 PM	#2
poobah diyAudio Member Join Date: Nov 2005	Probably not a good idea... supercaps are an oddball. They are really more like a battery than a cap. Read the specs on them... you will see that ther are limits on charge current etc... Another way of saying it: "The ESR is VERY high". All caps have ESR... Equivalent Series Resistance. So, every time you plop a cap in a circuit you are actually adding a resistor too. For filtering, you want ESR as low as possible... the opposite of a "super-cap". These things are finding new applications all the time... mostly where people are trying to replace lithium batteries to keep chip memories alive etc... For audio PS filtering, look for "low ESR". Then bypass close to the individual circuits with film/foil.

18th December 2005, 06:52 PM	#3
davidallancole diyAudio Member Join Date: Nov 2005	What is the purpose of the by-pass capacitor?

18th December 2005, 07:03 PM	#4
poobah diyAudio Member Join Date: Nov 2005	The local bypass caps stamp out the last little bits of noise right at the point use. From your PS to your individual circuit blocks is a bit of inductance and resistance and also the potential for noise pickup. So, adding a bypass close to the circuit block is like adding another L/C or R/C stage to the PS. Not always neccesary, by any means. The "secret" is that certain types of caps only behave truely like caps at certain frequencies... high frequencies skip right over electrolytics. As an example... you might have 100uf (low ESR) at the PS... and then put a 0.1 - 1.0 uF film foil, or even ceramic, RIGHT across the power pins of an op-amp. The op-amp gets a supply of power free from inductance and behaves better...

18th December 2005, 08:51 PM	#5
davidallancole diyAudio Member Join Date: Nov 2005	Thanks man!

18th December 2005, 08:55 PM	#6
poobah diyAudio Member Join Date: Nov 2005	You betcha, Every time you draw a cap, draw an inductor and a resistor in series (kinda the same with a wire). You don't have to do all the aweful math... but you will see what is happening. Cheers -

18th December 2005, 10:00 PM	#7
Steve Eddy diyAudio Member Join Date: Sep 2002 Location: Sacramento, CA	The new carbon areogel supercaps have very low ESR. se

18th December 2005, 10:13 PM	#8
poobah diyAudio Member Join Date: Nov 2005	Thats right Steve, I have 4, 44 Farad caps, WITH low ESR, on my bench. I can't tell you where I got them, or why. They are not that big. Let's just say, there are new things on the way...

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