How to select a power supply

Description

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 http://incompetech.com

Transcript

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.

[Music]

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!

How to make a "super" extension cord (aka power distribution box)

Description

This is an overview of a recent project making a "super" extension cord. It's a multiple outlet box for "wall wart" power supplies, battery chargers and the like. It's intended for low amperage devices.

Transcript

Hi Makers, Builders and Do-It-Yourselfers. Harley here.

My wife has this pile of miscellaneous chargers and wall warts and things that need to be plugged in and she only has one outlet. It's kind of a mess to be changing things back and forth. It's a messy pile; she doesn't know what to do with it. She came to me the other day and asked if I could help her find a solution.

We went to some home improvement stores looking for power strips and found some. Umm, nothing that really worked… that we thought would work well with what we thought we needed in terms of count and spacing and that kind of stuff.

So, I had some materials left over, some outlets, some romex, some wire nuts, some wood, and uh, from previous projects, and decided I'd try to make something. I talked to her about all the things she wanted to plug into it, where she wanted to put it, kind of basic specifications, requirements, types of things and started to work.

And then I took some scrap wood and make this box. The sides are made out of particle board. The top and bottom are MDF. The back is plywood. The dimensions are based on what she needed and constrained by what I had on hand. It was looking a little bit rough, so I put a couple coats of black spray paint on it and to try to dress it up a bit.

I picked up some 3/4 by 3/4 by 1/8 inch angle aluminum and made some brackets for each row. I drilled holes where I wanted the outlets and switches. And did a test on these supposedly self-tapping screws on some scrap material. It worked fine. I started driving it into this material and it promptly broke off. I got a punch and punched it out and a tap and then tapped all the holes and everything worked fine then. I think this material was a little thicker than my test material and it was just too thick for the self-taping screws. We've got switches for each row to be able to switch them on and off independently of each other and a pilot light to tell you when it's on or off.

So the next step was to wire everything together. And we'll flip this over and take a look at how that works.

So these three lines come from the plug. The white one goes to the switch for the pilot light and then around to the white side for each of the outlets. The black line comes up to the unswitched side of the switch and another black line comes from the switched side to the black side of the outlets. We have the copper line that comes into all the green screws for the ground circuit.

Each outlet in the row is simply daisy chained to the one before it. White goes to white. And black goes to black. And ground is simply looped around the green screws. Starting at the switch and all the way down to the end.

Now that I've shown you the physical wiring, let's look at a schematic diagram. There are three items in this device: the plug, switches with pilot lights, and outlets. Starting with the plug, we have hot, neutral and ground lines. The hot is black. Neutral is white. And ground is uninsulated copper. The ground simply goes from one device to the next, connecting to the green screws. The neutral goes first to the pilot lights and then to the neutral side of each of the outlets. Next the hot goes to the switch. This switch with the pilot light can be setup in several different configurations. In a future video, I plan to show some of the different ways this switch can be used, but for the purposes of this project I want the light to come on when the switch is turned on so I'm going to use the default configuration here. The hot line goes to the unswitched side of the plug that's not connected to the light. Then I connect the switched hot side from the switch to each of the outlets in succession. This means there's no power coming from the hot side when the switch is off. When it's on, power flows to both the pilot light and the outlets.

After I wired everything together, I needed to make a cover for the front. I did this out of some 26 gauge sheet metal and just cut 10 square holes for the outlets and switches. In order to do this I used three cutting tools. First of all I used the standard, kind of scissors style aviation shears; typical of what you use most often for cutting metal. Then I also used some dual edged cutting nibblers, shears, it's gone by a couple different names I've heard. And finally I used some nibblers to really clean up the edges and get some precision cuts.

First, using a fine tipped permanent marker, a tape measure and combination square, I laid out lines for the holes.

Next I created a starter hole simply by using a large screwdriver as a punch.

Then I opened up the hole with the metal snips.

Now with the larger hole, I could use the dual edged snips to cut out the majority of the hole. I'd not heard of this type of cutter until several weeks ago. It worked well to cut out from the middle of the sheet. and there's no curl in the metal afterward like there is with standard scissor-style snips. However, they don't work well on an edge if there's not enough support. Also, they have a kerf of about an eighth of an inch that you have to take into consideration when doing your layout.

After cutting out the majority of the hole, I went back to snips for a bit of touch up.

Final clean-up was done with a nibbler. These are like tiny shears that punch out a small strip about a sixteenth of an inch by an eighth of an inch. They can be very precise, but since they don't take off much material, it take awhile to make cuts of any length.

The plan was to put some pop rivets in here on each rail in between each of the outlets in order to hold the sheet metal to the assembly underneath. But after putting in the cover plates, I find it's not going to go anywhere. It's really solid. And, if I just leave it the way it is, it's a whole lot easier to assemble, and if I ever need to take it apart in the future, it'll be much easier to, take apart and make any changes if I need to.

One thing I found after I put the cover plates on, there are a couple places where either I mis-measured or I cut too, too wide of the line, or something, or there's enough variation in the manufacturing of the cover plates and switches and all, that I have a couple places where I have a little bit of the sheet metal line coming through. I'm not real wild about that, but such is it in hacks.

So I drilled a hole here for the, cord to come through and vacuumed everything out and put a wire tie on the cord to act as a strain relief so it doesn't pull back out and put pressure on the ends of the wire.

If you don't have one of these and you do anything at all with wire ties, they're really handy. They're called a "zip tie gun." And they're like I think less than $10 at Lowe's or Home Depot and they work really well. You put the zip tie in here and just kind of pull, it cranks down, pulls everything tight, and there's also a little cutter in there so while you have it pulled tight, you can kind of twist, and it cuts it all off. It works really well if you have to do anything of any real significance with zip ties. One thing I have found though is with these really large zip ties, the cutter cuts really kind of too close and they have a tendency to pop off. So on this particular one I just used a pair of diagonal cutters and cut it, left about an eighth of an inch here on the end.

The next step is to kind of wire tie everything together with wire nuts and then we'll assemble the top and put on… we'll assemble the top and attach it permanently to the box.

Ok. So I wire nutted the solid core wire coming from the rest of the box to the stranded core wire coming from the plug. And just kind of wire tied all three of these together. The green to the copper. The white to the white and the black to the black. A couple things to keep in mind when using wire ties, particularly with stranded and solid core wire; the stranded wire wants to wrap around the solid core and so you need to strip off more length on the stranded than you have on the solid core. And then as you're tightening them down, you want to be tugging on each wire individually and the cap as a whole to make sure you have a good solid connection in there and that nothings going to come apart on you. Once it's put together though, I've never had these things work their way loose. They stay tight until you intentionally take them apart.

Ok. To hold the top on I got some angle aluminum to… that will go along the edges. One side of this is one half inch and the other side is three-quarter inch. I'd gotten some equal sided aluminum before and found that I didn't have enough clearance on the front with the face plate on here. So I went back and got some that had different lengths on the edges. But I think that… I'll put some screws in on the sides on the long side and I think that'll hold that on there nicely.

Got the edges on to hold the front on; got some trim pieces on it; eased the edges with a file so they're not quite so sharp; added a handle, some feet. I think we're ready to give this a try.

I have it plugged in. There's no smoke. The lights are still on. I think we're good. Although the switches are all off. I don't have any initial shorts anyway. So I'm going to give this thing a test.

I've got this little device here. I don't know what they're called but you can pick them up at home improvement stores and I'd be surprised if they're more than twenty bucks. They're really cheap. But they're really handy when you're working with 110 electrical outlets. They've got three lights on them and they have one pattern that shows up when it's correct and five different patterns for five different error conditions like open ground, open neutral, hot ground reverse, those types of things. Very handy if you're buying a house and you want to check, you should check your outlets before you buy, so when you make an offer you can put conditions on it if there's anything wrong, you can have it fixed. If you're doing your own electrical wiring you can check it. That type of thing. For this I'll use it to check my wiring in the box.

It's real simple, you just kind of plug it in and turn it on and viola, we have two lights that indicates correct. And to kind of verify I didn't get anything reversed as I went down the line, I'll plug it in to the last one and we still have to green, uh, orange ones indicating correct. Awesome. And we'll try the bottom one and turn it on. And correct. And turn this one on and it's correct. So, I'm assuming everything else is probably ok. I mean I could have something reversed and then reversed back, but, eh, I'm not going to worry with it. The pilot lights all come on. If we turn everything off, we should not have anything, and we don't. So I… hey… I think everything's good, this is looking great. Time to go install it.

There we go. We've got room for battery chargers. Other battery chargers. Wall warts. Expansion room. Places to put other things that aren't right here. Overall I think it'll work well. So until next time, go make something.