ATX in AT's clothing

I was going to use a very old AT power supply and convert it into a bench power supply, the reason being that it has a much larger case than an ATX which means there would be ample space for the seven binding posts I intend to install (five voltages and two ground). Moreover, it has an integral AC power switch right on the case--something you no longer get in current power supplies (and in some AT's the switch is not on the case but on the front panel of the computer). So this enclosure is perfect.

Alas, a check of the voltage outputs revealed that while the +5VDC line was fine, its +12VDC was only delivering 10.8V even when two hard drives were plugged in and whirring away. This was rather disappointing since a 10% deviation is simply unacceptable. But I really want this enclosure.

To cut to the chase I found an ATX power supply from a discarded Pentium 3 computer which showed output (positive) voltages within 5% of their stated values. After mulling it over I decided to disembowel the AT and transplant the ATX board into it.

Testing the ATX I discovered that while even without a load the PS starts (fan spins) and outputs were live, without any load the outputs were not too good: +5.25VDC and +11.5VDC. These two voltages are the most important to me so they're the ones I use as benchmark. I further discovered that loading the 12VDC line did not improve its output. But loading the 5VDC line not only pushed that output down toward 5.0VDC, but also pulled up the 12VDC line toward 12.0V.

My main reference for this project is How to Convert a Computer ATX Power Supply to a Lab Power Supply. In that article it advises using a power resistor to load the output. I thought, Why waste all that good power? Why not put it to some use? As in lighting up a bank of LEDs. I initially thought of using white LEDs that would be configured into some sort of a lamp to illuminate whatever circuit the PS was powering. However, I decided to illuminate the interior of the enclosure instead, using colored LEDs to bathe the components inside in theatrical lighting.

The AT has a 110/220VAC switch. Right above that is the socket into which the computer monitor's power cable is plugged. This area would be perfect and large enough for a small prototyping board with all the LEDs mounted on it. So I removed both the switch and socket. I cut a board down to a size that would fit that space. I then populated it with a total of 8 red and 10 blue LEDs. Supply is 5VDC and resistors were chosen such that current is about 15mA per branch. Measured forward voltage of the red LEDs were 1.8 and so two reds are in series per branch with a 100-ohm resistor as current limiter. Blues have a forward voltage of 3.0V and so there's only one LED per branch in line a 150-ohm resistor. There are 14 branches and so total current is around 210mA, for a total power dissipation of around 1000mW. Test shows that with this load, output voltages are as follows:

Ideal output (volts)	Actual output (volts)
-12	-10.76
-5	-4.77
+3.3	+3.32
+5	+5.17
+12	+11.96

The green and a black wire are connected to a switch at the back. When green is not connected to ground PS is virtually off including the cooling fan. When green wire is grounded PS supplies power to the outputs. The AC power switch has to be closed of course. Purple and gray wires ("standby" and "power-good", respectively) are not used and not connected to anything.



Here's the ATX board and its fan already installed in the AT enclosure. The connectors to the motherboard and drives have all been snipped off and I've bundled and temporarily taped the wires by color (voltage). The bunch of red-black-yellow wires on the right is for the LEDs. I initially was going to use the +12V (yellow) but then as I said above tests showed that using it does not contribute to +12V output regulation. This yellow wire eventually joined its siblings.




Here it is after the case cover had been drilled and the binding posts installed. I couldn't find any other colors besides red and black so I used red binding posts for the negative voltages as well. Notice there are two posts for ground.




Back view showing the AC power switch and the PS-ON switch. The reason for the latter's location is that there already was a 1/8" hole on that spot. I just made it larger. The "AMAX" plastic sticker nameplate is from the ATX. You can see that the power supply was made in 2000. The neat thing about this PS is that it actually lists the wire color codes.




A look inside the connections to the binding posts. All wires were terminated with ring lugs. Even if the nuts come loose the wires won't fall onto the board and possibly cause disaster.




A view from the fan side. You can see the rectangular holes where the the monitor power socket and 220/110V selector switch once were. Because the copper side of the prototyping board will be exposed and accessible through these holes, I mounted an aluminum sheet to seal them. The sheet is sandwiched between the casing the two standoffs for the board.




The prototyping board is clearly visible. The top two rows are red LEDs. The rest are blue. I ran out of diffused epoxy blue LEDs so most of them are the clear type.




And the LEDs are shining!



The following were taken in complete darkness. Even with 18 LEDs the total output is nowhere close to bright.