Many PSUs come with modular cables, and it would be easy to reuse your old cables when swapping out your PSU, right? Except doing so can fry your build. Here’s why.
Modular Cables Aren’t Universal
Many cables inside your PC are standardized. Fan headers, Molex drive connectors, the 24-pin Molex header for motherboard power connections, and so on all use a common standard. The way individual wires are laid out in the cable and the connector is referred to as the “pinout.”
If you’ve ever seen a diagram of any sort of plug that labels the individual pins, sockets, or contacts as “+5V” or “GND” you’re looking at a pinout diagram telling you which part of the connector does what.
Surprisingly, however, pinouts are not standardized on modular PSUs except to whatever internal standard the manufacturer chooses to use—and without testing the pins you’d never know.
While a particular +5v pin is always in the same location on a standard ATX power motherboard connector—known as the “device side” of the cable—it might not be in the same location on the port located on the PSU unit itself.
What does that mean exactly, and why is it problematic? It means that the modular cables that come with one PSU are “pinned” to match the pinout of that PSU and not every modular PSU.
If you simply unplug the PSU-side of cables from the PSU (while leaving them attached to your motherboard, GPU, and other components) and drop in the new PSU, you can effectively “scramble” the pinout pattern sending 12v of power to a pin that the hardware expects 5v or the power on the wire to a pin the motherboard expects to be a ground, and so on.
Every 24-pin ATX power cable will have three +12v pins and a -12v pin, for instance, but depending on the manufacturer, those pins could be anywhere in the 2×12 grid of pinouts in the connector.
In the best-case scenario, the pinouts on the modular cables from your old PSU and the new PSU match, and everything works great.
In the worse, but not catastrophic, scenario, the computer simply fails to boot or operate as expected, but there is no permanent damage.
This can be particularly frustrating because it is the start of a long troubleshooting process where the cause of your problem isn’t immediately apparent. A particular component might turn on because it has power but function erratically because it is undervolted.
And in the worst-case scenario, you boot up your computer with the pin-swapped configuration and fry one or more of your components.
This is exactly why you’ll often find warnings printed right on the back of the PSU with text like “Do not use modular cables from other power supplies” or “Designed for use with genuine Corsair Type 4 cable sets only.”
How to Avoid Modular PSU Cable Catastrophe
If reading the prior section has you a bit nervous, here’s how to avoid a crossed-wire situation altogether—whether you’re looking to prevent it from happening in the future or deal with a mess you have right now.
When In Doubt, Replace the Cables
The easiest way to avoid problems is simply to take the time to replace the power cables when you are replacing the PSU. In fact, any time this is an option, you should do it—no further reading is required. Use the new cables that came with the new PSU.
You save yourself an enormous potential headache by just spending five extra minutes wrangling with your cable management and swapping the cables.
Keep Your Unused Cables Together
When you buy a new PSU, clearly label the cables you have. When I do a new build, I usually don’t keep all the boxes for all the components, but I do keep the PSU box.
It’s decently sized, sturdy enough to store random accessory bits from the rest of the build, and, most importantly, I can keep all the unused modular cables in a box clearly marked with the PSU they belong to.
Even just putting the PSU manual and the cables in a gallon Zip-Loc bag is an improvement over tossing them in a drawer or random box.
This way, should you reuse the PSU in a different build or pass it on to a friend, all the cables for that PSU are in one place.
Identify the Pinout of Your Cables by Brand
While using the cables that came with the PSU is the most foolproof way to approach the problem, let’s say you’re trying to figure out which cables are compatible. Perhaps you’re sorting through the bin of miscellaneous cables in your workshop or or you bought a new GPU and now you need that cable you put to the side—but it needs to be the right one.
Some modular cables have the name and function printed on or stamped into the connector’s plastic plug like “Type 3” to refer to Corsair Type 3 cable pinouts. That’s fairly rare though, and often times if there is any labeling it is more like “PCI-E” or “CPU” which tells you what the cable is for but nothing about the pinout pattern.
If you know, the model number of your old PSU and you are certain that the cables in question came with that PSU (either because they’re currently attached to it still or the only spares you have could only belong to that unit) you can always look up the model with a Google search and find out what the pinout is. You can also contact the manufacturer, and they’ll send you a pinout list or diagram.
Identifying the Cables by Pinout Testing
Actually testing your PSU and your cable pinouts isn’t difficult. You can do it the easy-peasy way with a cable tester, or you can do it the relatively easy (but more certainly more hands-on way) with a multimeter.
Fuhengli ATX Power Supply Unit Tester
This simple all-in-one unit tests ATX power 20 and 24-pin connectors as well as PCI-e, MOLEX, and SATA power connectors, too.
If you have an unlabeled modular cable in your parts bin that you plan on using, you really must test it to ensure the cable relays the correct voltages and pinouts to the device-end of the cable. Anything else is just a dice roll and not worth the potential damage to your hardware.
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