# Help with picking a power supply

Discussion in 'Electronic Basics' started by Rod, May 23, 2005.

1. ### RodGuest

It has been a while since I was in school working with this stuff so maybe
someone out there can help me in this area. I have a piece of equipment that
has 12 VDC input, and is rated 170mA (max), and 2.25 Watts (max). What I'm
trying to do is find a off-the-shelf power supply that will allow 1 unit to
be powered or, up to 16 simultaneously to be powered. For the life of me, I
can't remember how to calculate this out. Can someone help?

-Rod

2. ### kellGuest

To run sixteen units, your power supply would have to supply nearly
three amps (at twelve volts); but it would also have to supply twelve
volts on the very light 170 mA load that a single unit presents. The
voltage of an unregulated supply (wall wart) will vary quite a bit over
such a wide range of loads. I would suggest you get a reasonably
priced regulated switching power supply from one of the surplus supply
houses.
http://www.allelectronics.com/cgi-bin/category.cgi?category=480&item=PS-1242&type=store

3. ### RodGuest

Thanks Kell, this helps a lot!

5. ### James T. WhiteGuest

If you decide to go this way, you'll likely have to put a load resistor on the
+5 output to get the multi-output PS to regulate properly. Most PC supplies
expect to have a load of 10% or more of the rated output current on the +5.

6. ### Nikolas BrittonGuest

There is nothing complicated about it, it's just standard
multiplication and division.
170mA * 16 = 2720mA. 1000mA = 1A so the total amperage is 2.72A, always
round amps up not down. you need 3 Amps. To calculate wattage then you
want volts * amps.... 12V * 2.72A = 32.64 watts. If you know only the
wattage and volts it is you can use that to find the amps, vise versa.
32.64W / 12V = 2.72A.... 32.64W / 2.72A = 12V.

My first pick for a 12Vdc power supply (that can do 3+ Amps) would be a
car battery charger. If you want you could add a 12V regulator stage
and/or some large (10,000+ uF) filter capacitors for cleaner power.
http://www.sears.com/sr/javasr/prod...2871221000&subcat=Battery+Chargers+&+Boosters

Also what james said about computer PSUs is true. (only the new ones,
i.e. ATX???) They require a minimum load for it to work correctly. Also
ATX and newer style PSUs do not have a power switch. You will have to
short 2 pins on the 20 pin block to turn the unit on and off, I don't
remember the pin numbers. If you are going to go with a computer PSU
then you want to use the old AT style PSU. With computer PSUs the
yellow wire is always +12, red is always +5 and black is always ground.
Go to a local mom & pop computer shop an ask if they have an old AT
style PSU, I bet they would give it to you for 5 bucks or less.

7. ### James T. WhiteGuest

I've seen the problem on older AT style PSUs, too. Wattage doesn't seem to
matter as I've seen it on both 400W and 85W AT supplies.

8. ### John SmithGuest

Wouldn't a car battery charger perhaps also require a minimum current? I'd
go for a standard lab bench PSU.

9. ### kellGuest

Doesn't a car battery charger allow the voltage to vary quite a bit? I
thought they adjusted for current going into the battery. Present it
with a variable 170 mA to 2.7 A load like the OP has -- what will a car
battery charger do? Deliver a steady 12 volts? Maybe you can tell me.
Plus the one in the link costs 45 bucks or so. And what about ripple?
No need for filtering on a car battery charger. Might not have a
single filter cap on the output -- I know my bike battery charger
doesn't.

10. ### kellGuest

Okay I just noticed Nikolas addressed the ripple issue in his post.
But I still think the OP should go with a small, light, well regulated
switching \$20 power supply than a bulky battery charger that he would
need big extra caps for, or something else that costs more than
necessary or requires a lot of extra work. Unless he's an inveterate
scrounger/serious hobbyist like some of us, with a big junkbox, and
enjoys messing about with such stuff.

11. ### Nikolas BrittonGuest

The reason I picked the battery charger is because it can deliver high
amps and most people already have one, I think I have 3 around here