Mike,
There are advantages and disadvantages of both approaches, out of
personal preference and a costing perspective, based on your circuit
description, I would use one large cap (i.e. around 6600uF) for both
circuits.
Depending on your regulation methods though, I would suggest that you think
about your PSU - generally the more voltage you are dropping off your
supply, the more heat you generate and the higher rated your components need
to be. If I were building a supply with +5 and a variable 0-30 (for example)
supply, then you would need a 35V rated transformer, which means you need to
drop 30V off your main supply rail to achieve a +5V rail (with a 7805 reg or
similar), which means that you are going to be dissapating a LOT of heat
Just a few thoughts for the day ...
Regards,
Jon
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Maybe I misspoke in my posting...I wasn't asking whether or not to
use a large cap, but whether I needed two of them (one for each
subcircuit).
I have positive and ground rails and planned on having a single
2200uF cap across them to remove ripple from the mains. These lines
would then feed the two subcircuits (one for a variable supply and
one for a +5V supply).
My question is...Do I need a large cap at the input to each
subcircuit or will a single one at the positive/ground rails suffice?
-Mike
--- In Electronics_101@y..., "Jonathan Luthje" <jluthje@p...> wrote:
dear dot-dot
no it is not neccessary to run a capacitor on each of your "sub
circuits",
in fact, if you take it to the other extreme, you don't really need
to run a
capacitor at all! The supply will still work - although you may end
up with
quite a bit of noise on the line ... say around 60Hz?
The primary purpose of having a large-ish cap on the supply is to
remove the
ripple from the mains and to provide any surge currents that you or
your
project may require, rather than trashing your transformer /
rectifier
arrangement (yes, everyone I realise that there are other
advantages of
having capacitors on the supply line .. but for a basic power
supply, there
is no need to go into them). Smoothing is very handy when you are
working
with low voltage logic / mcu's / audio. Having a high surge
capability is
good when working with servo's, other capacitative loads, audio,
amplification etc.
Basically the bigger the better, personally I would go the whole
hog and
spend the few extra dollars on a bigger cap, or bigger set of caps
(if you
are suppling +/- rails you will probably need two anyway).
Regards,
J0n (aka dot-dash-dash-dash dash-dash-dash dash-dot)
----- Original Message -----
From: ". ." <dnixon9@h...>
To: <Electronics_101@y...>
Sent: Tuesday, August 28, 2001 1:45 PM
Subject: [Electronics_101] Capacitors in power supply circuits
I'm new to electonics and one of my first projects is a benchtop
power
supply. It's going to have variable and +5V outputs.
I've seen many circuit designs for multiple-output power supplies
and
nearly
all of them have a large (1000 or 2200 uF) cap at EACH of the
subcircuits.
Is this necessary?
If I'm using a single transformer for both subcircuits why can't
I have a
single 2200 uF cap after the bridge rectifier that feeds each of
the
subcircuits?
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