# Current limiting

Discussion in 'Electronic Basics' started by Nobody's Real, Sep 21, 2003.

1. ### Nobody's RealGuest

N00b Question:

I have an old 150 watt atx computer power supply which I would like to
use to power something. The label shows that at 12v the max output is
4.7A, but I would like to limit that to a max of 1A, what would be the
best way to do this?

Thx.

2. ### John GGuest

Why would you bother?
The device , if designed to run from a 12volt supply will use whatever it
needs.
The 4.7amp rating on the power supply is only what it can supply if required

Of course experimenters (professional or amature) use current limited power
supplies so as not to burn something when they get their design wrong. But
reading between the lines I do not think that is what you are asking for.

3. ### Owen LawrenceGuest

I have an old 150 watt atx computer power supply which I would like to
1A fuse.

- Owen -

4. ### happyhobitGuest

An LM317 in current limit mode with a 1-ohm resister (2 watt minimum) will
limit current to 1.25 amps or use a 1.25-ohm resister for 1 amp output. Don'
t forget a heat sink.

http://www.national.com/ds.cgi/LM/LM117.pdf

Jay

5. ### Bill BowdenGuest

The output will only be about 9 volts using a 317 with 12 input,
and there will be no voltage regulation using constant current mode.
The 317T will self limit to 1.5 amps, but you still need 2 volts
more going in than coming out.

-Bill

6. ### happyhobitGuest

Hi Bill,

I agree with everything you said. So? He wanted a current limit, not
voltage regulation.

Do you have a better suggestion?

Jay

7. ### CFoley1064Guest

I agree with everything you said. So? He wanted a current limit, not
If I absolutely had to have +12V regulated at a current limit of 1A from a PC
power supply, I guess I'd have to use a step-up switcher to boost to a higher
voltage (say, 18VDC or so), and then use a linear regulator to step down to 12
with a current limit.

It's possible that the "free" solution could be the most expensive one here.
If I wanted to do this on the cheap, I'd use a laptop +18VDC supply (just look
around, they seldom fail, and people can't bring themselves to throw 'em out
when the laptop dies) and then use a linear post-regulator on a perfboard to
get that [email protected]

Good luck.
Chris

8. ### Bill BowdenGuest

Maybe he could try a "Low dropout" regulator such
as LM2940 that only requires 0.5 volts overhead and
limits to 1 amp. That will give him 11.5 regulated
and limited to 1 amp.

But he may also need a minimum load of 5 or 10 watts
just to turn on the PC supply. They don't work
at 1 or 2 watts. Maybe use a few 12 volt automotive
tail lamps across the 5 volt output for a minimum load.

-Bill

9. ### John GGuest

Wouldn't it be nice if "Nobody" told us what he really wants to do instead
of people suggesting outlandish solutions to an undefined problem.
Or is it that he is so out of his depth that he cannot define the
requirement?