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Is There a Size Cutt-Off for Filter caps?

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Myke

Thu Jul 17 2008, 06:34AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

teslacoolguy wrote ...

Those wall warts must have been very poorly made because every one i have opened up had a elec cap of 47uf or higher.

I think that he meant that there was also a .1uF cap in addition to the other cap. Not just a .1uF cap. I don't really see the need for a .1uF cap in the wallwort because the transformer should be fine with noise.

Dr. SSTC

Thu Jul 17 2008, 06:48AM

Registered Member #1407 Joined: Fri Mar 21 2008, 07:09AM
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Myke wrote ...

teslacoolguy wrote ...

Those wall warts must have been very poorly made because every one i have opened up had a elec cap of 47uf or higher.

ye thats hat i meant lol and it was uesed for a phone so i guess thats why

Proud Mary

Thu Jul 17 2008, 07:48AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
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Posts: 4992

aonomus wrote ...

I've been wondering, if you are making say a 12V 30A power supply which might have some ripple, can you put a bunch of 10,000 uF caps, some 1 uF poly caps, and some 0.1 uF caps, that way the large ripples are taken care of by the big caps and small ripples by the ceramics?

At first sight, placing your capacitors side-by-side in parallel, would seem to add their values together as though they were a single capacitor, thus: Ceq = C1 + C2 + C3 ...Cn etc.

But real-world, non-ideal, capacitors have another property called Equivalent Series Resistance (ESR) = Z(Ï‰) = R + j X(Ï‰), which can and often does become significant as frequency rises.

This is why you will sometimes see filter circuits with capacitors of widely different values connected in parallel - because at higher frequencies the smaller capacitors may be more efficient than the larger ones, and so will preferentially re-direct these higher frequencies to earth.

quicksilver

Thu Jul 17 2008, 02:33PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
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Harry wrote ...

......But real-world, non-ideal, capacitors have another property called Equivalent Series Resistance (ESR) = Z(Ï‰) = R + j X(Ï‰), which can and often does become significant as frequency rises.

This is why you will sometimes see filter circuits with capacitors of widely different values connected in parallel - because at higher frequencies the smaller capacitors may be more efficient than the larger ones, and so will preferentially re-direct these higher frequencies to earth.

This explains a great deal of things! I am going to do a search on this aspect - (ESR) - of design. Often, unfortunately, searching theory yield little "real world" examples.
However, to the best of your knowledge, would this concept allow the use of widely diverse frequencies or do the frequencies need to sequence themselves in a formula or pre-prescribed manner (& limitations)?

aonomus

Thu Jul 17 2008, 04:34PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
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Thats what I thought Harry! Good to know the stuff about ESR though, I've never taken that into major consideration although I should.

Proud Mary

Thu Jul 17 2008, 05:06PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

You will sometimes see a large value aluminium electrolytic in parallel with a much smaller tantalum type, the reason being that aluminium electrolytics have yards of aluminium foil in them, creating significant (undesirable) inductance, whilst the inductance of the tantalum is much lower, so will not present such a large inductive reactance to the passage of undesired signals - such as ripple - to earth.

And do make sure you earth all these ripple capacitors to the same point if you can, lads, to stop current loops forming in your filter.

How many - and what values of parallel by-pass capacitors you add to your filter/smoothing circuit depends on the application.

In a radio receiver, for example, you will want to make sure your extra by-pass capacitors have low reactance at the receiver frequencies, to reduce mains borne interference, but this isn't going to matter at all in a DC welder.

So look to your application first, see how 'clean' and stable your DC must be for it to work properly, and then choose the filter capacitors needed to get the results you want.

GeordieBoy

Thu Jul 17 2008, 06:08PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
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Four ceramic caps across the rectifier diodes are also quite common in low voltage DC supplies.

Dr. SSTC

Thu Jul 17 2008, 11:10PM

Registered Member #1407 Joined: Fri Mar 21 2008, 07:09AM
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GeordieBoy wrote ...

Four ceramic caps across the rectifier diodes are also quite common in low voltage DC supplies.

ye thats what i had

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