Re: How does it affect power supply operation if filter caps are doubled in size?
From: at (poista.taatii_at_poista.welho.com)
Date: 08/30/04
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Date: Mon, 30 Aug 2004 18:13:14 +0300
"CFoley1064" <cfoley1064@aol.com> wrote in message
news:20040830094122.23098.00004595@mb-m07.aol.com...
> >Subject: How does it affect power supply operation if filter caps are
doubled
> >in size?
> >From: "at" poista.taatii@poista.welho.com
> >Date: 8/30/2004 4:41 AM Central Daylight Time
> >Message-id: <cgush7$kla$1@nyytiset.pp.htv.fi>
> >
> >I've got a power supply that has 2 470uF 400V filter caps in series. The
> >power supply puts out 400VDC.
> >
> >Now if I change the caps to 1000uF 400V, what will happen? Will the
output
> >voltage of the supply change?
> >
> >(this is just a basic supply that fullwave rectifies AC with diodes, then
> >comes the filter stage between the supply voltage and ground)
> >
> >-at
>
> Good morning. You didn't say if you've got 120VAC or 240 VAC input.
Here's
> one way you can get "400VDC" from 120VAC using two caps in series (view in
> fixed font or M$ Notepad):
>
> 1N4006
> o-----o--->|----o-----o+
> | +|
> 120VAC | ---
> | C ---
> | |
> o-----|---------o "400VDC"
> | +|
> | ---
> | C ---
> | |
> '---|<----o-----o-
> 1N4006
> created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
>
> As was mentioned in another post, putting 2 470uF caps in series will give
you
> a net capacitance of 470/2 = 235uF. Doubling up the caps to 1000uF will
get
> you a net capacitance of 1000/2 = 500uF.
>
> In an unregulated supply, your DC voltage will be dependent on ripple
voltage
> (each of the 2 caps is recharged 50 or 60 times a second). Doubling the
> capacitance will cut the ripple voltage in half, which will result in a
higher
> DC voltage, and that could cause problems. Look at the load and evaluate
> whether it can handle the higher voltage. Sometimes it makes a
difference.
>
> Oh, yes. Do be careful here. In this type of unregulated supply, both +
and -
> are at line voltage potential, which could be dangerous.
>
> Good luck
> Chris
Thank you for your reply.
It's good to know how it works. If there's a problem with the higher DC
voltage, I'll increase the resistor size in the power supply. That will drop
the voltage some. Or is there a better way?
Don't the filter stages also drop voltage? Or do they increase it? Is it the
resistors between the filter stages that make the voltage drop, and not the
caps?
If I put resistors parallel with the caps (as is sometimes used), the
resistors will also drop some voltage, is this correct?
-at
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