I'm an occasional reader of this forum, and I have a question. I want to use a Radio Shack 13.8VDC 15-Amp Power Supply Model: 22-508, or something similar, to power some devices in my backyard. I want to power some outdoor lights that I plan to make from car backup lights (they are cheap - $3.50 each at AutoZone) as well as a water pump also from a car (such as windshield washer pump or gas pump) for a small waterfall. In other words, everything made from car equipment, designed to work off a car battery, but running off a power supply that should be like a car battery. Will this work? If so, how many lights could I operate from it?
I would look for another pump, the windshield pump will fail quite fast if run continuous. Look for what is called a bilge pump at a boating shop. They run on 12 volt and will last longer than the car windshield pump. But yes on the power supply running it all. I assume you wanted 12 volt for safety reasons outside.
Actually, in addition to the safety, I want to do it because it seems cheaper and I may have more control over the items. I'm trying to do this on a low budget. I wonder if an electric gas pump would also work with water - they must run pretty continuously.
It should work fine. Here's the basic electronics stuff you need to know to work out if your chosen components are suitable, and how many lights you can hook up:
The number of lights you can run depends on how much power each one needs. It's possible your lights will only display a rating for power (Watts), so you need to convert that to current (Amps) so you know how many you can use before you reach the 15A limit of the power supply. For that the formula you need is:
Current = Power / Voltage
Once you know the current, just total up the current for all the devices (pump + lamps) and make sure it's under the maximum your supply can provide.
If you want to put several amps through a reasonable length of wire at a low voltage the resistance of the wire itself becomes be a concern. In order to work out how big a concern it is, you need to pick a number for the acceptable voltage drop, let's say 2 Volts (you could get away with a 3V drop, or even more - your lights won't be as bright and you pump won't be as fast, but they should still work). Let's say you have 20m of wire, and you stuff draws 10 Amps. Ohm's law is:
Voltage = Current * Resistance rearranging for our example, we get Resistance = Voltage Drop / Current so Resistance = 2 / 10 Resistance = 0.2 ohms
So we need to find the thickness of wire which has less than or equal to 0.2 ohms for 20m, or 0.01 ohms/metre, or 10 ohm/km. A quick search on the 'net for 'wire gauge resistance' finds a table which shows that copper wire 1.45mm in diameter (15 AWG) is suitable. Less current and/or shorter wires and/or greater allowable voltage drop means you can use thinner wire than in this example.
If I were an electronics teacher, I'd now ask you to use all the above to work out why long-distance power lines are run at really high voltages. But I'm not, so I won't.
They will not generate much flow. You say you want it for a waterfall type setting, so you will need it to create substantial flow depending on the size of falls you want. Still to get that from even a bilge pump may be pushing it some. The windshield pump would get more flow than a gas pump, but like I said it will burn out fast. Your only problem I see with what you want to do is the water pump. The lights may draw .5 amp at least so if you don't overload the supply to much, maybe keep it down to 10 amp for that 15 amp supply. How much is that supply going to cost you? 13.8 volt at 15 amp. It can't be cheap.
Radio Shack sells the 15 amp for $80 (180 watts) and a 25 amp for $100 (300 watts). There may be cheaper routes, such as building my own, but these seem competitive with the transformers that come with outdoor lighting kits, and I like the idea of being able to use car parts for it. The waterfall part is being built around a creek that runs through my yard, and I want to pump a (possibly) small amount of water up and have it come down over some rocks or other surface I will embed in the banks. It doesn't have to be a huge flow of water, just enough to be visible and make some nice watery sounds. It could be that the car pumps won't be a practical choice - maybe they draw too much current. Maybe the idea is too exotic to actually work well.