An Introduction to Accurate Voltage Measurements - Measuring PSU Rails
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Measuring PSU Rails
First, let’s talk about the basics. To start with I’d advocate testing your power supply unit’s (PSU) voltage rails. Once you’re familiar with sticking the probes of a multimeter to particular parts while your system is powered on, then and only then should you move on to measure your VCC. It’s not harder, but requires practice.
I will keep the guidelines short on PSU rails because a quick Google search will return more than a few dozen guides. Here are the representations of colors on different plugs. AUX connector: RED = 5v; ORANGE = 3.3v; BLACK = GND (ground). MOLEX connector: YELLOW = 12v; RED = 5v; BLACK = GND.
We’re going to start measuring the voltage rails. You can use either an analog VOM, if you can handle it, or a digital multimeter (DMM), because that’s easier. Always make sure that it is set on the appropriate voltage range (if it does not have auto-detect). However, to keep this article simple I’m going to use the term DMM for our multimeter.
(A really common Digital Multimeter.)
Start with the 4-pin Molex plug. Insert the red probe of your DMM into the yellow connector and the black probe to any black ground. By doing this, you’ll be measuring the 12v rail. Next, stick your DMM’s red probe into the red connector on the Molex plug and the black to any black ground. The results are for the 5v rail.
Now the 3.3v rail is the next. You can measure the 3.3v by inserting your DMM red probe into either one of the two orange connectors on the AUX, while the black probe is inserted into a black ground connector (it can be any ground: from the AUX or a 4-pin Molex).
As a side note, you can measure these 3 rails (3.3v, 5v, 12v) from the 20-pin plug too. The technique is the same. Just make sure you stick the probes into the correct connectors and that your DMM is on auto-detect or the appropriate voltage range. Once again: orange, red, yellow = 3.3v, 5v, 12v; respectively. Black is always ground (GND).
That covers the PSU rails. I’d advise monitoring your rails on both idle and load setups so that you could check their condition. Each PSU should be within the ATX specs (5% variance). They are the following: 3.14v – 3.47v; 4.75v – 5.25v; 11.4v – 12.6v. If these aren’t met then you should replace your power supply unit as soon as possible.
You’ve come to the end of the first segment of this two-part series. By now you should have learned exactly why we shouldn’t rely on internal voltage reports, and ultimately you’ve just become familiar with handling a multimeter to measure the voltages.
Keep practicing because the next segment is coming and it’s going to be “tough.” Stay tuned, you won’t want to miss it! We are going to cover techniques to measure your CPU core voltage (VCC) in real-time with a DMM while your computer is running!
Are you ready for that?
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