From: "Christopher R. Carlen"
Subject: Re: Power Supply Basic Design Req
Date: Tue, 15 Oct 2002 08:19:16 -0700
Organization: Sandia National Laboratories, Albuquerque, NM USA
NNTP-Posting-Date: Tue, 15 Oct 2002 15:18:12 +0000 (UTC)
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George Shaw wrote:
> Can anyone suguest a design and the values or fomular needed for a
> simple DC power supply.
> Input voltage is 230V @ 50Hz
> using a full wave rectifier (which one?) to give 20V out @ 20mA
> current with less than 0.5V peak-to-peak ripple.
> I would like to model this in MultiSim so need the values etc.
> Please email me as I dont often get to check this group. As many
> suguestions, formula would be helpful.
You seem to indicate that you aren't considering a switching supply, but
rather a rectifier-type. Do you need regulation? Does it have to be
For the current you specify, you can get the smallest line voltage
step-down transformer commonly available, about 1.2VA or so with 24VAC
output, and rectify it with a small bridge rectifier or 4 1N4001 diodes,
then stick it into a filter cap. I don't have off hand a simple formula
for the ripple, and can't spend the time to come up with it right now.
I'm sure someone will provide you with that.
Alternatively, you can simulate it and just fiddle with the output cap
until you get the ripple you want.
But if you need the 20V output to stay constant, then you need a
regulator. For this low current, a simple LM317 adjustable regulator
set to 20V with a couple resistors will do. Check out www.national.com
and search for LM317.
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA