Wednesday 8 January 2020

Beware of this ATX PSU tester!

I bought an £8 ATX PSU tester from eBay recently.

I had the idea that I could use this as a cheap display panel for a home-built bench power supply which would be based on an ATX PSU plus a Boost/Buck variable voltage+constant current converter. This would save buying separate voltage displays for the PSU outputs (3.3V, 5V, 12V, -12V) and also provide a shunt resistor (which is inside the tester) plus an ATX PSU socket and a warning buzzer if any of the rails were outside of specification, and all for £8.

Note: The shunt resistor is only intended to be powered for a short time and it got too hot if I left it connected for more that 10 minutes.

However, when I tested it on 3 different ATX PSUs, I spotted an issue - can you see the problem in the picture below?

When I connected a Molex power plug which carries 12V and 5V power, instead of the 12V LED lighting up, the 3.3V LED lit up instead!

I contacted the Chinese seller and tried to explain the problem using the picture above and a video. He implored me not to give -ve feedback and I asked him to give me a refund or send me one that worked correctly.

I received the replacement unit - which of course was exactly the same!

I disassembled one unit found that the three LEDs are just wired to the Molex and SATA connector pins using in-line series resistors.

So this (knock-off) model seems to have mis-labelled LEDs - however units from other suppliers have LEDS which are labelled in the same order as this one, so the PCB must be a different design from the other units.

The simple fix is to just stick new labels over the "+12V" and "+3.3V" silk screen printed labels.

How to tell?

The picture on eBay did not show the same unit that I actually received!

The printed text was slightly different in that the 'Use correct connector' text on my unit did not extend beyond the connector block, also the 'POWER SUPPLY TESTER' text did not extend beyond the width of the LCD display, the unit that was sent also had straight sides and the LCD readings are displayed in a different arrangement - compare the picture above with the picture below which was used in their eBay advert.:

eBay picture

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