ATX Power Supply Tester: check all voltages （1）

Published:2011/8/14 22:08:00 Author:Li xiao na From:SeekIC

By Ton Giesberts

PC power supplies can often be bought cheaply at places such as computer fairs. But it isn’t that easy to check if such a (second hand) power supply. This dedicated tester makes that job quick and straight forward.

Apart from the power supply and this tester, you’ll only need a mains cable (and socket!). All outputs from the power supply can be tested under load and any deviations from the nominal values are shown on 6 LEDs.

Although the power supply in a PC has little bearing on its overall speed, there are times when it needs to be replaced. This may be because the old power supply has simply given up the ghost, and sometimes the internal fan has become too noisy, or an upgrade of the PC has increased the power requirements above that what the old power supply can deliver.

ATX power supplies are available from virtually every computer shop. When you buy a new power supply it is obviously safe to assume it will be in perfect working order. But when you buy a (used) power supply at a computer fair or boot fair you want to be sure that it works before you fit it into the case and connect it to the motherboard. A quick test would be very useful then. The true hobbyists may also want to investigate the exact fault in a broken power supply. But it isn’t a straightforward job to test a PC power supply with a multimeter.

The power supply tester described here is a very useful and compact tool. We have to admit that you probably won’t need it very often. But once you have acquired one. word will spread amongst your circle of friends and you shouldn’t be surprised when you’re called to ’quickly’ check a PC power supply for them.

What is measured?

Our tester doesn’t require a separate power supply, as it takes its power from the PC power supply under test. All you need to do is plug the power supply into the tester and then use a mains lead to connect it to the mains. A rotary switch is then be used to quickly check all the out-put voltages. The percentage deviation of a selected output is shown on 6 LEDs. Two of these LEDs show whether the deviation is positive or negative and the other four indicate the percentage difference from the required output voltage.

For output voltages that are connected to more than one pin only the first pin is tested. (A power supply generates only a single -5 V supply, even though it is made available on several pins.)

There is a 26-pin header (K2) on the PCB that can be used to test each pin individually. The outputs are connected through 1 kΩ resistors to protect them against short circuits. If you connect an extension lead to this header you can use a multimeter to take measurements from any pin.

New ATX 2.2 specification

This tester was designed for recent ATX power supplies, but it is also ready for use with new power supplies described in version 2.2 of the ATX specification. These have a main connector with 24 pins instead of 20 (75 Watt extra for use by PCI Express cards).

There is a curiosity in the new specification regarding the -5 V connection. According to version 2.2 of the specification it is no longer used and the pin in question (20) is marked as NC (not connected). However, according to the manuals of several motherboards with a new 24-pin connector the -5 V is still present. So keep in mind that when you test a power supply with a 24-pin connector the -5 V output may or may not exist. The -5 V should always be present on a 20-way connector.

The change from 20 to 24-pin connectors is compatible with the older 20-pin connectors, with an extra +3.3 V, +5 V, +12 V and ground added to one end. An older ATX power supply with a 20-pin connector fits in a 24-pin socket and can only be inserted one way, so mistakes aren’t possible.

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