Re-use a computer power supply for DC voltage
By Duncan Cairncross
There are times when you need DC (Direct Current) voltage for some project or other; it is mostly small applications—has to change the 230v AC mains into something else first, normally a low-voltage DC.
There are a number of ways of getting a DC voltage:
• A battery (i.e. a 12v car battery);
• A purchased power supply;
• A power supply from some piece of old equipment.
A battery is good but it tends to be heavy and mucky. A purchased power supply is very good but could cost more than $100. Most old equipment power supplies are basically a small transformer and a rectifier. This can give you a DC voltage but the voltage will change with different loads. They are very inefficient and—most importantly—most are very low power, less than two amps.
By comparison, a computer power supply is actually a really nice piece of equipment. It is a “switched mode” power supply. Even a small computer power supply will give four amps at 12v and more at 5v or 3.3v. Better, it is a proper power supply; the voltage will not change as you apply a load. Even better, every old thrown-away computer has one so they are effectively free. You don’t even need to open the case.
If you don’t have an old computer ask around friends, companies, schools, dumps. Somebody will know of one you can have and be pleased to see it go. A good computer power supply will be 300 watts, a not-so-good one 90 watts. Both are usable. The big one can supply 15 amps at 12v; the little one can only supply four amps at 12v.
Cut off all of the cable ties and connectors. Gather all of the cables of the same colour up together and you end up with bundles of:
Black - Ground
And single wires
The 300-watt power supply is the same apart from the fact there are more wires in the bundles and we have two extra white wires:
The next question is what voltage do you want? The basic 3.3v, 5v and 12v are simply between their respective colours and black:
|Black to Orange is 3.3v
Black to Red is 5v
Black to Yellow is 12v.
You can get more choices by using say Blue-to-Yellow which will give 24v. But these other options are much lower current than the first three. The Blue-to-Yellow is restricted to 0.3 amps for the 90 watt unit and 1 amp for the 300 watt unit. I don’t normally bother using these other options:
Blue - 12v
White - 5v
Purple + 5v (used for “standby” in the Personal Computer and not much use in a power supply.)
The last three are used by the power supply for sensing power On (green); power check line (grey); and sense line (brown).
I took a 12-way connector block (we used to call these chocolate blocks), screwed it to a piece of ply and stuck it on the power supply using double-sided tape.
First the black wires. There were about 20 of them. Each has about five amps capacity; we need two for connecting and about five to carry the maximum current. I put a piece of heat shrink tube over the bundle and cut off the surplus. Strip the wires and put the five into the first terminal of the connector, leaving the other two for later.
Do the same with the orange wires, leaving one for later (second terminal). Do the same with the red wires, leaving two for later (third terminal).
Do the same with the yellow wires. You don’t need any of these spare so put them all in the connector.
Now we need to convince the power supply that it is still connected to a computer. Twist together one of the “spare” black wires and the green wire and put them into the next terminal. You can connect these through a switch, but this power supply has a switch and anyway I normally just switch the unit on/off at the socket.
Twist together the “spare” orange wire and the brown wire and put them into the next terminal. With some power supplies, the brown may need to be connected to a red wire instead. I normally connect the orange first and if it doesn’t work, try connecting a red wire. That is what the second spare red wire is for.
Take the remaining black wire and put it into the next bloc. Take the one of the remaining red wires and put it into the next bloc. These two are where we need to connect the “load.” The remaining purple, grey, white and blue wires can be connected to their blocs.
The “load” should be connected to the red and black wires. I have used a spare computer fan, very useful if you are using an old computer case and could use some cooling.
The power supply needs a load in order to function, a load that is enough to operate the power supply, between ten (10) Ohms and 500 Ohms. The actual amount of the load is not important so long as the resistance is low enough. The power resistor in the photo is 470 Ohms. If you are using a resistance of less than 100 Ohms then it needs to be a power resistor, otherwise it will just melt. Try the “smoke test”—switch it on and see what smokes.
The old headlamp bulb is connected between the black (common) and the yellow wire (+12v) and lights up in the test. The “load,” or computer fan in this case, is spinning. The 90 Watt power supply is only good for 4 amps, not enough power to operate the headlamp bulb. This power supply is quite close to the minimum voltage, showing 11.48v on the meter. It should be between 11.4v and 12.6v.
Stick some labels on and we have two handy power supplies. The cost: two chocolate blocks or plastic terminal connector blocks, a power resistor and some heat-shrink tube (insulation tape would have worked just as well).
AC/DC—the nerdy bit
A transformer works by using the mains electricity to create an alternating magnetic field which in turn creates electricity in another coil. In the old days, if you wanted to convert mains AC to a different voltage and/or DC, you would use a transformer to get the correct voltage and then a rectifier to convert AC to DC, then finally a regulator that would effectively waste the surplus power to control the voltage to your needs.
Mains is 50 cycles per second, meaning the magnetic field is built and reversed 50 times per second. The important thing about this field is not its strength but how fast it is changing. At 50 cycles per second, if you want a lot of current you will need a large transformer. One big enough to provide 10 amps normally requires both hands to lift. The old-style transformer system is a dumb analogue system.
Then some smart cookies came up with a different way. A Switch Mode Power Supply is a “smart” system using a microprocessor to control the ON/OFF; it can control the voltage and current without wasting any energy. First, the AC is rectified to DC, then the DC is switched ON and OFF very fast using transistors.
This rapid switching along with some capacitors and an inductor (effectively the core of a very small transformer) provides the required voltage and current.
The ratio of ON-to-OFF (“mark space ratio”) controls the output. It could be ON 95% OFF 5% or ON 50% OFF 50% or ON 5% OFF 95%, or anything in between. The frequency used is much higher than the mains frequency of 50 cycles/seconds—a Switch Mode Power Supply operates at more than 50,000 cycles/second. It is the speed of the change in magnetic field that is important.
The difference between an old-fashioned transformer power supply and a switch mode power supply is like the difference between one of the old “automatic” lathes that were operated by a series of cams and a CNC (Computer Numerical Control) lathe run by a computer. The inductor (transformer) can be much smaller, theoretically even 1/1000th the size. Inverter arc welders and microwaves also operate on higher frequencies, allowing them to use much smaller transformers. Therefore, you will find that an inverter arc welder weighs a lot less than an old transformer arc welder.