The LCD Power Supply Tester is designed for field use to diagnose possible power supply issues. Similar in function to a voltmeter but less accurate, it will tell you if the voltages coming out of each cable are within specifications. The advantage of this tester is that the power supply does not have to be removed from the computer, nor do the ground and power on pins have to be shorted. Also, the unit requires no batteries, no test leads, and is very compact in size.  Several companies sell the exact same tester under different names, but from the pictures you can see they are all the same. 

COOLMAX PS-224 LCD Power Supply Tester @ Newegg.com

Features of the Tester: 

ATX 24 pin test
8 pin and 4 pin Molex test (motherboard)
6 pin Molex test(pci-e)
4 pin Molex test(IDE power cable)
SATA power connector test
Floppy drive power header test
P.G Test (Power Good)*

*Many people wonder what the Power Good Value means. It’s simply the time in milliseconds from when the supply is turned on, to when the actual voltage is applied. Motherboards also have a Power Good rating that tells them how long of a delay to wait before checking for the proper voltages.  Both values should be similar, and usually are.

Use:
1. Plug the 24 pin ATX connector into the Power Supply Tester. This powers the unit for all tests that use the LCD
2. The LCD screen should turn blue, and you will probably hear a beeping from the test unit. This is normal. The beeping is because the +12V2 cable is not connected yet. Any flashing voltages, or the letters “LL” or “HH” displayed indicate an error.
3. To stop the unit from beeping, connect either the 8 or 4 pin motherboard Molex connector, or the pci-e 6 pin Molex connector. You will now see the “LL” under 12V2 replaced with a proper voltage.
4. The SATA and IDE power tester connections are located on the sides of the unit. There are 3 green lights corresponding to the +12v, +3.3v, and +5v that will light up if the proper voltage is detected.  You should disconnect the 24 pin ATX connector before conducting this test.

Accuracy: The claimed accuracy of the Power Supply Tester is +/- .1 v. In practice the actual measured voltage may vary depending on the power supply model. Below is a comparison of 2 power supplies, the Power Supply Tester Voltage, and voltage as measured with a voltmeter. 

As you can see, voltages differ at most +/- .41 from the voltmeter readings, except for the 4.5 vs 5.11 reading on the Thermaltake 500w +5v rail.  Obviously, if you get a bad reading, further checking with a voltmeter may be in order.  I checked a few other Thermaltakes 500w and got similar readings.  I believe the discrepancy is cause by the +5v having a load on it used to check the P.G value, which the volt meter doesn’t add.  For general use I think this tool can be a good quick power supply checker for building new systems and examining old systems.