How to select a power supply

Friday, April 14, 2017

How to select a power supply


A contact recently asked "How do I select a power supply for my project?" Once a project moves past the prototyping state using a battery, picking the power supply is a critical element of a personal electronics project. In this episode of House of Hacks, Harley discusses the four items to consider when choosing a surplus power supply.

For a written transcript, go to How to select a power supply

Music under Creative Commons License By Attribution 3.0.
Intro/Exit: "Hot Swing" by Kevin MacLeod at


What do turkey basters and power supplies have to do with each other? And why am I in the kitchen? We're going to talk about all this today at the House of Hacks.


Hi Makers, Builders and Do-it-yourselfers. Harley here.

I was recently asked about selecting a power supply for a hacked together project. There are four things when selecting a power supply that you need to pay attention to.

The first two are simple. The last two are a little be more complex but not too bad.

First is the input, you need to make sure that your power supply is appropriate for what you're plugging it into. For the most part you're going to be using locally supplied power supplies, probably surplus stuff that you've scavenged, and in that case it's going to work because it's designed for your local environment. In the United States that's going to be 110 to 120 volts AC. Pretty much anywhere else in the world, with a few exceptions, it's 220-240 volts AC. So the first item, while it's there and you need to be aware of it, it's really simple.

The second item has to do with the output. Power supplies can either output volts AC, indicated by VAC or a squiggly line or it can output in volts DC, indicated by VDC or a straight line. And you need to select the type of current that's appropriate for your project. Most, if you're doing low-voltage stuff, most of those are going to be DC, but depending on what you're working on, AC may be appropriate for your case.

The last two items are volts and amps. And these are similar to properties of water systems so we'll look at that here in a minute with the turkey baster and the sink.

But in short, volts have to do with, kind of, the pressure that the electrons are pushing into your circuit. And you need to make sure that this is appropriately ranged for your circuit you're working with. Generally circuits have a minimum and maximum voltage. You need to make sure that the voltage coming from the power supply fits within those parameters.

And finally there's amperage. Amperage is more like capacity. So it has to do with, as long as your power supply meets minimum requirements for your circuit, you're good to go. Your power supply can provide more amps than you need, it just can't provide less. So, make sure you know what your circuit requires and your power supply at least meets that minimum.

For example, a circuit that requires 250 milliamps (ma) would work just fine with a power supply that supplies 250 ma, 500 ma or 100 amps. Any of those would work just fine. However, if the power supply says it's rated for 100 ma, that's going to be too little and your circuit won't work right.

So let's go look at the sink and see how water correlates to volts and amps.

OK. As I mentioned, volts have to do with the amount of pressure and amps have to do with the capacity.

If you think about a water system, there's a whole lot of capacity here. The city has probably thousands of acre-feet of water that are sitting behind these pipes. They can provide pretty much all the capacity that we need for our little simple demonstration here.

It also has a lot of pressure. We control the pressure by the knob here, the lever, and if we put this on here and we give it just a little bit. This would be like not enough volts where we have a really weak stream here and the circuit isn't going to work right because it just doesn't have enough oomph to make it work.

If we increase the pressure to just the right amount, we get a nice flow without overdoing things and we reach a point of equilibrium here where the equivalent of the circuit is going to work just fine because we have the right amount coming in, not too much, not too little and everything's going to work just fine. And this is kind of equivalent to the volts controlled by the lever here.

If we increase the voltage too much though, what we end up with is a lot of leaks. And when you have leakage in electronics, that's a really bad thing. Things tend to blow up, burn up, magic smoke escapes, all that kind of good stuff. So you really don't want to put too much voltage to your circuit. You want to have just the right amount of volts that you get a good flow like that without having too much.

But now in all these cases, regardless of how much voltage I had, how much pressure I had coming out of the circuit, I still had huge, huge, vast amounts of water sitting in reservoirs behind these pipes. And that's equivalent to your amps. Your circuit will only use the amount of amps that it needs, regardless of how much capacity your power supply has.

So in summary, there are four things to look at: the input voltage and current and the output current, volts and amps. Make sure that you have the sufficient volts within the range that the circuit is designed for and that you have at least the minimum number of amps that are required by the circuit and you're good to go.

Until next time, go make something.

Perfection's not required. Fun is!