Power Source From Computer Into a Bench Lab Supply

Power Source From Computer Into a Bench Lab Supply

Here's a quick and easy way to turn an old ATX computer power supply into a bench lab power supply.

Below is a typical power supply from a computer. Some power supplies require a load on them to operate. Some have enough of a load to operate with just the built-in fan running once you turn it on. For a supply that needs a load you'd have to add a resistor. My supply was purchased for $3 at a resale shop and doesn't need a resistor.

So, how can we get this power supply up and running when it's out of a computer? Easy! Just place a jumper wire from the 4th wire to a black wire. Usually the 4th wire is green. It will be the only green wire in the big plug. There will be many black wires. I chose to jump the green wire to the black wire that was to the right of it in position 3 from the right (with the little lock on the connector facing you).

With this setup my supply was able to turn on and its fan started running. Success! I did not need any resistors apparently. 

Theoretically I could have stopped here and just used the plug as a power output. But that wouldn't be much fun. So I kept going.

I unplugged the unit from the wall again and cut all the many connectors off. Cut as close to the connectors as possible to leave the wires longer.

I opened up the power supply at this point to see inside. BEWARE: the capacitors inside this (and any) device will stay charged for months until one of two things happens: either they are bled off by a resistor; or they shock and kill/harm you! 

A capacitor is very much like a battery. It holds energy. When you stick a battery up to a light bulb it will slowly discharge energy to light the bulb for hours. When you stick a capacitor up to a light bulb it will discharge all of its energy instantly: like a camera flashbulb or a stun gun. I wouldn't be doing this if I didn't know what a capacitor is and what they looked like. 

It always amazes me when people take out a flyback transformer from an old CRT television and then ask around as to the "pin-out" of which of the zillion connectors they need to do certain things. Duh! Just take a photo of the item in question before you rip it out of the device! I did this, but it's not really needed because everything on this power supply is color coded.

Here you can see that all the orange wires are +5 volts DC.

Here's a photo showing that the single blue wire is -12 volts and all the yellow wires are +12 volts.

This is the important one: the 4th wire which is usually green! Okay, as you can see the green wire is labeled "PS/ON" for "Power Supply ON". This is the other end of the green wire that we connector to a black wire for the unit to power on. Now you know why this had to be done. I could put an on/off switch between the green and black wire, but my supply already has an an on/off switch.

Here is a photo showing the black wires. They're labeled "GND" for ground / common.

So, when I cut the all the connectors off I took only one of the black wires and kept it tied to the green wire. All the other black wires will be used in a moment.

Loosely tie any black wire to the green wire.

All the colors get separated out and will joined together.

Pencil out a line or two for hole drilling (with the cover off the actual supply). Make sure that you don't end up putting the output holes right over the large heat sinks inside the unit or else the wires will be pinched on their way out of the unit. 

Okay. Here we are: all the wires (except 4) are exiting the unit. I could have stopped here and soldered all the different colors to themselves and used the supply.

bought some INSULATED binding posts. Actually they're banana plug females. Where they touch the case they are PLASTIC and the little metal rings that tighten them to the back of the metal casing doesn't make contact with the center connection of each jack. If you used a plug or post that has a metal thread that touches the metal case YOU WILL DESTROY YOUR SUPPLY AND BE SHOCKED/ELECTROCUTED. 

Shocked means that electricity traveled through part of your body. Electrocuted means to be executed via electricity. It's a subtle difference I suppose. This unit is capable of up to 35 Amps. Enough to destroy most multimeters if you tried to measure the amps without a high voltage divider probe. I thought, "hmmm...that's weird, let me try my cheap meter on its 10 Amp setting." The result was that the meter wires heated up instantly and I pulled it away. I was lucky, but forget about measuring the amps. I had no issues with the voltage measurements and my multimeter wires plugged into their normal sockes: the black common/ground and the normal red plug (not the 10 Amp red plug).

Here are the banana plugs installed. Much nicer than bare wires sticking out of the case.

Here are some power leads plugged into my power supply.

Here are the results with no load other than the multimeter and the internal fan going. I printed a copy and pasted it to the top of the unit.

A recap of what wires did what including the 4 wires that didn't get connected to banana outputs.

Yellow +12V

Blue -12V

Red +5V

White -5V

Orange 3.3V

Black Ground/Common

Purple +5V always on "SB" Stand By mode.

Green PS/ON

Grey +5 PG feedback "Power Good"

Okay, so one of the black wires was soldered to the green wire. All the rest of the black wires were soldered to the COM/GRD output.

The purple wire always has 5 volts running to it, even when the power switch is turned off on the supply. If the supply is plugged into the wall it has power. You can hook a red LED up to the wire and then you'll know when the power supply is plugged in but not turned on. I didn't want to waste a black wire for that setup.

The grey wire is the "Power Good" feedback wire. If the power supply is running properly this wire will give out 5 volts. You can hook this up to a green LED and then you'll know when the power supply's on/off switch is set to on and everything is functioning properly. Again, you'd have to pair it to a black wire, which I didn't feel like doing.

So, why didn't I just use one each of all the colors instead of bundling them all together with solder? The thicker the wire (or the more little wires soldered together) the more amps it can handle. They were there and it was fun so why not build it to withstand anything it can theoretically put out.



So there you have it. My project in a nutshell:

1. Jumper the green to black to see if it'll start up without adding resistors.
2. Cut the connectors off.
3. Bundle all the same colored wires together and solder them (all the reds together, etc.).
4. Solder the green to a black.
5. Cut and tuck away the grey and purple wires.


That's it really. The other steps were drilling holes and making it look pretty with the banana output connectors. Then testing voltages and labeling the outputs.

After that I just slapped a "Logusz Particle Physics Lab" sticker on it...because it was laying around.

The power supply hooked up to just a multimeter is very stable. The voltage changes just .01 volts over time. The -4.72 voltage changes to -4.71 volts and back repeatedly. Not bad, but then again this is a regulated power supply! 

A regulated power supply is designed to provide stable voltages. They can typically keep the voltage (or current) stable with a narrow range. They're perfect for delicate electronics such as computers.

This is also a switching power supply which means it flicks on and off to manage the power and stability issues. This makes it lightweight, which is good for carrying around and it consumes less input energy too.

I paid $3 for the power supply at Disabled American Vets (DAV), which is a resale shop just like the Salvation Army. The banana connectors were super expensive: a whole bag for $12. These were the cheapest insulated for use on metal boxes ones I could find at the time.

Uninsulated ones are around $4 for a few, but they're only for plastic cases: you'd die if you used them on a metal box like this. I had a bunch of Amazon points so I basically got theses free anyhow.

It was fun and thus worth it to me.

Bench Top Laboratory Power Supply

Bench Top Laboratory Power Supply

I was going to buy a bench top power supply, but I wanted to build one on my own to learn about them first. So I soldered together an Elenco XP-720 AC/DC full-wave rectified 15v negative or positive DC 12.6v AC center-tapped 1 Amp supply with 3 Amp at 5v DC power supply.

That's a heck of a long name, and I wanted to learn what all (at least some) of that meant. I learn by doing. I'm not one of those people who go around saying, "you should..." or "somebody should..." do whatever. I do what I do.

One thing I don't do so often is solder. It's been a while, so I soldered up my last project to practice. I still have the soldering iron my father gave me when I was ten years old. Although more and more solder is the horrible lead-free type. 

And now a brief rant about solder that you can skip:

Lead-free solder is poisonous to the person using it.

Leaded solder is not poisonous.

Lead vaporizes at 1100° F, which is way higher than the temperature it melts at. You will encounter zero lead fumes while soldering. This is a scientific fact you can easily research yourself. Weller is a company that sells fumes extraction safety devices and even they say the lead isn't the issue, it's the flux (more on flux below).

Lead-free melts at a higher temperature (making work less well), spits hot droplets more to burn your arms, and gives off terrible fumes. It also is porous, meaning even if used properly if will fail way sooner than lead solder. It also is more brittle. You will risk your health to build an inferior item for the benefit of...?

The governments of the world want to protect theoretical people who don't exist from drinking well water on garbage dumps from having old electronics in land fills somehow leach lead into the water...but wait, they wanted to ban plastic bags because no water or air gets to them in landfills to decompose them?

They are poisoning solderers to protect people who drink water from garbage dumps.

The safest choice for solder in your home is lead.

The most dangerous part of any solder is the flux in it. It's made from pine tree sap and breathing the fumes (or even having the fumes contact your skin) can cause a long list of dermal and breathing problems because those fumes contain formaldehyde, hydrochloric acid, all sorts of benzines and toulines and phenols that are just bad, bad, bad. Yes, the trees are killing trying to kill us!

Any way, the future of solder is MesoGlue!  MesGlue is as nanotechnology were there is a jagged piece of Indium. When this is mated with a fitting jagged piece of Gallium it liquifies without heat and creates a solder joint. I love Gallium, it's my second favorite metal, next to Bismuth. Gallium feels like lead, but if you hold it in your hand it will melt!!! Just like liquid Mercury. I have some, and it really melts in my hand. Magicians make spoons out of Gallium, then dip then in hit water to make then disappear (disolve).

Your reward for reading that rant: a nicely laid-out soldering station design:

Here is my messy soldering on a circuit board. I solder wires together all the time, but that's easy: weave the wires together and heat. Apply the solder to the wires, not the iron. You have to be quick for diodes and such-heat can damage them and they're really, really tiny!

You'll notice my 40 year old soldering iron tip is grungy. After this I cleaned it the way you're not supposed to: sand down to the copper, plunge it while hot into plumbers flux (which shouldn't be used with electronics) and add high tin content solder. Tip gets a mirror finish in just a few seconds! All over the internet they say not to expose the copper of the tip, however if you talk with people who use soldering irons for a living they say that if they run out of tips they just jam a piece of thick copper wire into their iron and get to work!

Clipping the excess is easy with diagonal cutters (some call them "dikes" for DIagonal CutterS. I got these Italian made Hakko CHP-170 ones off Amazon for $3 with free shipping!  I wish I had these 30 years ago, they're so good I can't tell the difference between closing them empty or actually cutting excess circuit legs/posts/leads off after soldering.

Here are some lovely diodes that only work when the input power is positive (AC current alternates). One diode gives half-wave rectification so there are two per output. 2 x 60 cycles in US household AC equals 120 cycle output. Full-wave rectified.

The big cylinders are capacitors. They're kind of like batteries. They store current and release it smoothly: making crazy alternating current (AC) into pleasantly stable direct current (DC). There are many here for the various choices of output.

Dead center is a little black half cylinder that is an integrated circuit. It's part of the regulation for the 5v 3a output. It's keeps things really stable: if there's a voltsge drop this drives it back up to 5v. On the Amp side of things, if more than 3a is coming in the grey tube at bottom center shunts the excess to another integrated circuit which bounces in a look to the first one. This will overheat and shut down, which is better than overheating and catching on fire (but I got close, more about that later).

Speaking of outputs: here is a view of the binding posts from inside the box. Black is common/ground/true-earth ground/floating ground/negative depending on how you set things up (if you want to start an argument, just ask what these colors "officially" stand for in AC and/or DC applications). Red is positive output. Yellow is more positive outputs of AC, except one choice here is for negative -15v DC, so I have no idea what that qualifies as when operating in that mode.

The wires are for routing all the lovely electrons through this machine (yes machine: it can change AC to DC and/or vary Amps and Volts, and double the cycle rate of regular household electricity). The big white thing is just the transformer, which we've discussed in previous posts.

Although this transformer is center-tapped: instead of two wires coming out of it, it has seven! This allows you to connect one part of the coil and get half AC output, or connect in to the black post and get full 12.6v at 1a AC output (which might be why it's binding posts are yellow, like the negative DC). Simple, yet fancy.

17.8 Volts at 1 amp. Enough to kill. 

It's the Amps that kill. 1 is enough. 

17.8v x 1A = 17.8 Watts.

There is another output that outputs 5 Volts at 3 Amps. Even though that's only 15 Watts, the 3 Amps is enough to kill you three fingers over!


Here I have the positive (+) DC knob turned all the way up, giving me 17.8v at 1a DC.

On the left is my nicer multimeter that I bought to protect my 40 year old one that I love. The one on the right was only $3 and works great. I bought a few to use in situations where the multimeter might be damaged.

The expensive one has a thermometer! That drink isn't cold enough...

...but the $3 one has a built in transistor testing jack!

Most importantly, the expensive one can measure capacitance.

The expensive one also is auto-ranging, while the cheap one is manual. You have to pick the Volt or Amp range of the circuit your testing. Pick the wrong one and zap! Everything in electronics is crazy decimal places that are all abbreviated with k and m and M so it's easy to confuse ten volts (10v) with ten-thousands volts (10kv) when squinting at a circuit board.


Speaking of damage: below is a 10uF 25v capacitor which I blew up, literally! Pop!

You see, firecrackers are rolls of paper with gunpowder sprinkled inside. Capacitors are rolls of paper with oil sprinkled inside. Can you tell where I'm going with this?

To get full 12.6v power you connect a yellow post to the black. I connected yellow to yellow on the wrong (DC) side and bang! Shreds of paper all over the inside of the box. It was very loud. So I spent $5 and got 100 capacitors off Amazon as backup. It could have been a deadly mistake: wrong color and wrong side. This is why I don't go mushroom picking.

Now I have a ton of new knowledge about electricity and my very own power supply for further experimentation.

So, what's the first thing I did with my power supply besides testing? Lighting up an old vacuum tube. I put red marker on the two heater pins, which I found by using a multimeter on Ohm/resistance setting. These were the only two that had an actual reading. These two pins were, by the way, totally different than the two different schematics I found online!

I ran two power cables from my power supply. Since this is a Tung-Sol vacuum tube with a part number "6K6GT" I knew the heater takes 6 volts. The first number 6 is the one that tells you the voltage. Other tubes use 12v and thus have a 12 as their first number. I first tried 6v of DC power, but it only glowed with a barely visible pin point. I raised the voltage to 8v and it glowed more, but I was afraid of blowing the tube.

It wasn't super bright, like some tubes, but I did glow better when I hooked it up to my power supply's fixed 6.3v AC outputs. AC worked much better than DC.

Power supply update: I did buy a fancy looking store bought supply... the results were not good:

Oh dearest brand new Fling Dung Industries PS-305d 30 volt 5 Amp power supply, you died as you lived: somehow blasting out 50.2 volts at 6.66 Amps uncontrollably for about sixty seconds before your capacitor exploded.

Hopefully I'll get a return authorization and lug you to the post office. Seriously, this is I think the first thing I've ever returned to Amazon in twenty years of shopping there. [Update: return shipping from UPS and USPS was more than the refund would be. Do not buy this power source, which is sold under dozens of names. After re-recontacting the seller they sent a free shipping label, I sent it, someone there signed for it and...still no refund].

What! That loud noise wasn't microwave popcorn? Do popped capacitors have delicious salt and butter to lick off? No?!? Meow!