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Topic: ATX breakout Board Modification suggestion! (Read 1426 times) previous topic - next topic

ATX breakout Board Modification suggestion!

Hello everyone,

I have built this ATX breakout board. Basically it's the same design as this made by Dangerous prototypes, but I used only though hole components, not smd. My strong suggestion is that beside the dummy load resistor on the +5V rail you include the dummy load resistor on +3.3V rail! With some ATX supply designs it's very important to do this, or the power supply will be damaged.

I have made the tryout with the ATX power by the manufacturer called ANS. It a cheap variety 400W ATX supply with +3.3V,+5V,+12 and -12V outputs. It's a half bridge switch mode design with PWM IC SC6105. When I turned on it for the first time with only +5V rail loaded with 10 Omh 11W resistor , after couple of seconds the power supply turned it's self off. Ok, so I thought that protection circuitry must have switched off the power supply because over-voltage on some of the other rails. So I loaded +5V,+12V and 3.3V with power resistors for the second test. But hey, strange thing happened, the power supply still didn't want to power itself, the power supply fan moved for a second and then nothing. I started to suspect that something in the power supply went broken the first time I plunged it in. I suspected the secondary side of the supply, especially protection circuit, sensing circuit and feedback circuit. After testing couple of component on the secondary side, I found that the PNP transistor (marking A1015) in the +3.3V sensing circuit developed open B-E junction. This is the direct consequence of the fact that the +3.3V rail was not loaded,causing abnormally high voltage to appear on the the B-E junction. I replaced it, loaded the +5V (15 Ohm 11W resistor) and +3.3V rail (8.2 Ohm 11W resistor), and the power supply is now working with no problems at all.

So beware, some ATX designs have this "flaw" if you can call it like this, because I guess the designers didn't count on the scenario that someone will power up the supply without any load on 3.3V rail.

Hope this info helps!