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.
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?
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.
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.
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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.
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.
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.
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 somewhere. If he had to buy a new battery charger then your solution would be a better option...
Also here is a link on how to convert a computer PSU into a general purpose power supply:
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