This was actually my very first real project in the world of electronics. It was completed over a year ago, I just never took any good pictures or finished the write up.
Now, since I have been using it and would like to revise the project, I figured it would be appropriate to talk about the first revision and have it documented before it’s changed forever. After all, it was my first 🙂
Bench-top ATX Power Supply
Proper bench power supplies are expensive, and foodling around with wall warts or USB power is just not very convenient. What is convenient however, is a standard ATX computer power supply. These units are very common, and they’re all very similar. All ATX supplies have +12v, +5v, +3.3v, and -12v with some pretty decent current capabilities. If you find a good one, it might have plenty of current for even more power hungry projects. Just about any standard supply will be fine for simple uses.
One step further, using a LM317 voltage regulator on one of the +12v rails will give you instant adjustable voltage to fill in the gaps between all the fixed voltages (like +6v and +9v).
This is by no means a revolutionary discovery in the electronics’ hobbyist world – there are many projects just like this all over the internet. I simply wanted one of my own, and of course the experience of putting a useful project together.
I picked this is as my first project because first of all it’s easy and simple. Second of all, this would be an invaluable tool to use for all future projects. There’s something awesome about building a tool and using it on a daily basis.
First of all, this project was inspired by this article on TG’s Electronic Exploration Blog. Like I said above, many people have build this project before, but TG really added some extras and made it look super nice. The blue panel meters and clean aluminium made me want one. If you compare our projects, mine is similar, and I would say it’s the more beginner’s version. I don’t have the extra features like the Ammeter or the LDO regulator, just the voltmeter and the normal LM317. These features might be added in my next revision however.
Finding an enclosure was the first challenge. A lot of projects online simply use the original case for the power supply, and throw some circutry in there. Well, I like breathing room and I had so many of these freaking ATX supplies around my lab that why not just gut one and use it’s enclosure?
So I grabbed most capable supply I had on hand, which (apparently) turned out to be a Dell 250W with about 14A on the +12v rail. I grabbed another supply that – may or may not have been functional – and gutted it for it’s steel case. Like I said, they weren’t in short supply around my place (no pun intended).
The second supply’s steel case was where I’d build my project in.
I decided to use banana jacks for my leads, so I installed one banana jack for each separate voltage from the power supply along with ground and a variable output. Even a year later, I never got around to labeling all the outputs, and quite frankly I never even used anything besides the 12v or variable out. It was easier just to dial in 5V with the VAR than it was to relocate the lead to the 5v jack.
Next, was to route the wires from the real supply to the case so it looked clean, and hidden. I decided to stack the two cases, and the whole end project would just look like two power supplies stacked. I did this for simplicity, as I didn’t have the resources at the time to cut and modify steel. (at least cleanly).
Each rail has numerous wires for connections to different devices within the computer, I really didn’t need 6 12V lines, so for each output, I cut most of the wires, and heat-shrunk them as close to the PCB as possible as to not get in the way. This was to also make sure they all fit through the hole that I’d drill between the two cases so the wires can “go upstairs”.
I made sure to bring out Purple, Gray and Green so that I can have control over these lines and not have to open the downstairs supply again.
- Purple is +5VSB “standby” which i’m not using in this revision, but brought it up just in case.
- Green is Power On. Of course, I wanted to be able to switch this on and off. I didn’t think this through, but the downstairs supply isn’t one that had a mains switch, but the upsairs one was. So what I needed to do was route the green from the bottom to the top and use the already present switch to turn the supply on. Easy. All you do is ground Green and power is on.
- Gray is Power OK. This is used to tell the computer that everything is okay and to turn on. For now, I just decided to hook this up to an LED.
After getting everything routed to the right place, I used rivets to permanently attach the cases together. I did it in a way such that It was still possible to remove the lid from the bottom supply to service it if needed. I made a mistake here, as I didn’t record the specs for the ‘live’ supply before I affixed the case over it. I didn’t want to undo the rivets so I just went with it. Over a year later (at the time of writing this) I had the bright Idea of checking the photos that I took… and one high-res photo later, I had the specs.
A lot of other projects online have a power resistor somewhere in the project to put a load on the PSU to keep it on. I think I got lucky, and this PSU doesn’t need that nonsense, or maybe everyone else is just crazy. If you read this and have a good reason as to why I still need one, let me know in the comments – I’ve been using this for over a year and had no problems with it staying on.
Next is the Regulator Circuit. At the time, I was an extreme novice at this, so It took a few tries to get this basic circuit to work. I’m not going to go into detail on how this circuit is designed, as it’s pretty common. I think it’s even an example in the LM317’s datasheet so just google it. Here’s a good starting point that’s very similar to what I did on Ladyada’s Site.
My project compared to TG’s project – he uses a more complex Low Drop Out regulator which gives him the ability to vary his 12v line between about one volt to just barely almost 12v. My circuit can go to about 10.1v on the VAR line, and that’s good enough for me. If i need exactly 12V, I’ll switch to the 12V rail.
The heat sink is probably a bit overkill, but it was what I had on-hand. I pulled it from a very old AT motherboard.
One of the big things I try to do with my projects is to make them takeapartable. That is, to make them modular so that I can take them apart and change parts easily without a lot of soldering. I designed this project to use connectors that I found in computers so that I can remove parts in the future. You can see from the photos, there’s a 12v rail Molex, a CD Audio connector, a 3-pin fan connector (never actually got used, product of bad planning), and later you will see an old AT PSU connector linking the supply controls with the regulator board.
Above is the regulator circuit installed in the ‘upstairs’ enclosure, along with all the connectors in place. The panel meter (see below), LEDs and potentiometer are also mounted and ready to go.
I cut out the standoffs from an old modem’s plastic case and used them to standoff the regulator board. It came out sloppy, but I’ll make up for it in the next revision.
Like TG’s version of this, I used one of those awesome 3-digit blue digital panel voltmeters to display the output of the VAR line. I didn’t include an ammeter in this revision, but plan to in the future. These panel meters are best found on ebay from a store called “asia-engineer”. It takes a few days to get to you, but they’re about $10 each and work nice. The plus with these meters is that they can use the same supply they are reading so there’s no need for a shunt or any nonsense like that. It measures between 0v-20v.
The red jacks in the middle are the +5V, +3.3V, +12V, and -12V outs. Green is the variable output.
For some reason, I decided to give the variable output it’s own switch. I think I just wanted to use the cool flippy switch or something because this is a pretty silly feature. I never used it.
Also, like mentioned above, I never used the 5V, 3.3V or -12V for anything but could be useful if you need to have two different voltages at the same time in the same project. I might keep these in the future revision.
The probes are from Radioshack, with alligator clips affixed to the ends.
What I learned
I don’t know what I was thinking when I picked out the potentiometer. It’s completely wrong. after it’s turned about 1/3rd the way, it sets the regulator to max voltage and the other 2/3rds of rotation is nothing. This makes it very difficult to dial in a specific voltage as it’s really touchy. Next time I’ll pay more attention to this.
All my holes I drilled are crooked. I’ll do this better next time.
Once again, planning is key. I wasted time soldering in a fan connector thinking that I would connect the case fan to this. Well, it turns out that since the board and fan are on the same part of the shell, there was no need to separate them. I simply wired the fan to the main harness.
Next time I’ll remember to record the specs for the supply before permanently covering it with up by riveting the other case over the label.
Future Revision [KR-1.2]
I found an awesome Aluminium case at Skycraft Orlando a couple weekends ago and It’s perfect for the future revision of this project. It has enough space were I can relocate the original power supply’s board into it, and also have room for the regulator board and possibly more panel meters (ammeter). I’m pretty excited, and I have started the planning process. I’ll update this section with a link in the future.