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DIY Budget 12-volt Bench Supply

Cheap and easy - and great for testing.

by Julian Edgar

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This article was first published in 2003.
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There are plenty of times when you want to run a 12-volt automotive piece of equipment outside of a car. You could be developing an electrically run system (eg a water/air intercooler where you want to bench test the pump) or even running a car sound system component (with the price of used AM/FM CD players, taking this route can make for a very cheap workshop sound system!) Or maybe you just want to do some quick tests to confirm that something car electric works - an electric radiator fan for example.

That's all well and good, but the trouble has been finding a source of 12-volt power. Power supplies that run to more than a few amps also run to lots of money - over AUD$100 for 5 amps and AUD$170 for 10 amps. But what if we told you that you could have an 8-amp, 12-volt mains-powered supply for under AUD$20? Oh yes, and as a bonus we'll throw in a 20-amp 5-volt supply as well. It won't develop a running-car voltage of 13.8 volts, but for this money and this much current, that's a problem easily overlooked.

So how do you do it? Easy - use a new or used PC power supply. All the dangerous mains-powered electrics are insulated and the actual wiring will take only a few minutes. To make it as easy as possible, pick an (older) AT-style supply. These are available secondhand in basically any old computer (you see cases complete with a power supply being literally given away) or you can do as we did and buy a brand-newie for just AUD$13.75!

Warning!

This project deals with mains power voltages, which can kill you. If you are not competent with handling mains-powered voltages, you are strongly advised not to proceed.

Here's the step-by-step process.

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This was our starting point - a brand new 'Max Power' 200W AT power supply. It was bought from www.rockby.com.au and is available for that incredible AUD$13.75 while stocks last. Note that buying new is not required - nearly every discarded PC that you find will have this type of power supply in it - and PCs are available for peanuts at garage sales, secondhand stores, computer fairs and so on.

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This type of power supply has a pre-wired mains-power on/off switch, whereas more recent supplies (ie ATX units) require the PC to tell the power supply that all is well before switching on, even after the button is pushed. That makes the older AT supply much more suitable for this type of standalone application - plus you'll find a heap more cheap AT supplies around than ATX ones!

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One way of identifying the AT type of supply is to look for these two plugs - one's got P8 on it and the other, P9.

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Here's the label on the power supply we bought. Note the outputs and their respective wiring colour codes:

+5V 20A Red
+12V 8A Yellow
-12V 0.5A Blue
-5V 0.5A White

Most power supplies will use these same colour codes for the voltages, while some power supplies will be able to supply even more current than these figures. (But always check the voltage outputs with a multimeter to confirm things are as they should be.)

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One of the benefits of using a PC power supply (besides cost, that is!) is that it comes equipped with a built-in fan. This particular unit is labelled 'Smart Fan' so we assume that the fan is varied in speed as the conditions require it. Very few off-the-shelf non-PC power supplies are fan-cooled.

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If you buy a secondhand power supply you'll then need to come up with the cord that has the IEC plug at one end and a mains power plug at the other. Try to get it from the same cheap source as the power supply - if you need to go out and buy a new cord, 50 per cent might be added to the cost of the supply! The power supply we bought came complete with the power cord.

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The approach that we adopted was to make a new snout for the power supply box. This was folded up from a single piece of 2mm thick aluminium. As seen here the sheet is a fraction narrower than the power supply - but that is only because we had an offcut that size. Normally, you'd make the new section that same width as the power supply. We used a home-made sheet metal folder to do the bending but some blocks of wood and a vice could have been used to achieve the same end result. The top and bottom of the new piece should fit snugly against the top and bottom of the power supply. You may want to make the front extension a little longer than the 30mm that we used - in our case things got a little snug behind the new face.

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The binding posts that you use can be as humble as these...

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...or as elaborate as these! We started off with the ones above but found that their threaded portions weren't really long enough to go through the aluminium panel and also give room for the solder lugs. We then swapped to these gold-plated ones, picking them for the simple reason that they were around the place - originally having been obtained for use on speaker boxes.

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Like many binding posts, they need to be mounted on an insulated strip if there isn't to be a short-circuit between them and the box. A piece of scrap black plastic was used for this purpose. Because the current that can be carried is fairly large, we used two solder lugs on each binding post and soldered these together.

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A slot was cut into the aluminium faceplate so that the rear of the binding posts stayed insulated from the box - there needs to be clearance between the terminals and the box, as well as between each terminal. At this stage four more holes were also drilled - one for a pilot LED and another large one for the switch. The two smaller ones are for the screws that hold the switch in place. Oh yes - and see those other two holes at the back? They're so that the upper part of the plate can be attached to the power supply, using the two screws that are already there.

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The next step is to do the wiring. This is the power switch, which should now have its cable neatly bundled up so that it will fit behind the faceplate - there should be no need to cut this cord. Note that the back of the switch is at mains voltage - the insulation around here (arrowed) should be improved with additional heatshrink and great care should be taken that this area is inaccessible and that these terminals do not touch the case when the power supply is finished. The earth lead (green with the eye terminal on it) will be connected later - so don't bundle it away.

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At this stage you shouldn't have touched any of this wiring - the stuff that feeds out those voltages that you want. But cutting is about to begin...

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... as you can see here! Cut all the plugs off then gather all the 'like' colour wires into separate bundles - so all the blacks together, all the reds together, etc.

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Solder all of the wires of each colour together, joining them to just two wires of the same colour. This step will reduce the number of wires from about 30 to six! (Two wires - rather than one - are used on each output to give better current carrying capacity. If you expect to be using the power supply near to its rated maximum a lot, you could use three or four wires going to each binding post, rather than two.) Note the blue (-12V), white (-5V) and orange ('power good') wires are not used in this design. However, if you want the -5 and -12V supplies, just add more binding posts and bring these supplies out to the faceplate.

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It's a good idea to place fuses in the supplies. This is as easy as wiring in-line fuse-holders like these into the 12 and 5 volt supply cables that you've just constructed. However, the power supply itself is protected by a fuse (and some supplies also have auto shutdown protection), so I didn't bother - I'll just be careful when I am using the supply!

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We decided to use a 10mm red LED as the 'power on' indicator. Why such a large LED? Only cos we had it around and we like large LEDs! You could use a 12V pilot lamp if you want, but if you use a LED make sure that you put a 560 ohm resistor in series with it then wire it to the 12V supply, with the long lead of the LED going to the positive.

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This is what your project should look like at this stage - the mains power cord tidied up; the 12V, 5V and earth leads brought down in number to just a pair for each; the white, orange and blue leads insulated and bundled away; and the LED wired up.

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The next step is to bring the faceplate up close to the power supply and solder the 12V, 5V and earth leads to their appropriate binding posts. Push the LED through the front panel and mount the mains power switch. Make absolutely sure that the rear of the mains power switch remains well-insulated, with no possibility of the switch terminals touching the case or being accessible to fingers.

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This is what the supply should look like now. However, with mine the switch had an immediate and unfortunate accident - the protruding plastic actuator broke off. Hmmm.

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However, I had another double pole, single throw mains power switch handy so I installed that instead. Don't change the switch unless you know exactly what you are doing with mains power. High voltages like these can kill you!

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Remember the earth lead that we mentioned earlier? Well, here's where it goes - under the nut on the screw that secures one of the front feet. What front feet? Oh well, I had some around the place...and their screws also attach the new snout to the underside of the power supply through the mounting lugs which are already there.

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The openings at each end of the new snout were filled with plastic pieces cut to size and push-fit into the openings - some glue will make sure that they stay in place. If the power supply is to be worked hard, drill some small holes in these pieces so that the fan can draw air in through the front vents in the power supply box. In my case there were enough small gaps around the case that the fan still flowed sufficient air.

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Depending on the PC power supply that you have selected, there may be a few tricks in actually using it. The one we had wasn't happy if you switched it on when a large load was applied - but it was as happy as Larry if you applied that same load after it was turned on. Others that we have heard of are the opposite - they need to have a small load happening before they'll start to work properly. So if you switch on and nothing much happens, keep these aspects in mind.

We won't try to kid you that this power supply is the answer to all your dreams. If you run audio equipment you may get some hum, and the voltage regulation on 12V is about plus/minus 10 per cent. And hell, if it cost a hundred bucks all-up that simply wouldn't be good enough. But when you try out the price and the ease of making it, well, things start to look damn good!

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