I Have the Power... Supply!" - How PSUs Work Inside Our Simulation Computers


Blog post by SimGHOSTS President Scott Crawford, MD

I Have the Power... Supply!" - How PSUs Work Inside Our Simulation Computers

Power supply units (PSUs) inside of a computer case have little influence on our daily lives. If they work for the computer when it was built, they rarely have problems. This article will discuss what can go wrong and what you need to know about them.

Power supply units are rated in watts, and most function on both 110V or 220V AC power supplies, thus making them universal across countries and power grids. Most standard desktop computers will require nothing more than a 300-400 watt power supply, and calculators such as this can help you determine your need. They will likely function for decades without problems. Other than replacing a cooling fan on occasion, they are unlikely to need repair. If the cooling fan does not begin rotating when the PSU is powered on, it will need to be replaced. Some fans will emit a sound as the bearings begin to fail, giving you time to plan this repair before the entire PSU overheats and needs replacing.

The power connector from current PSUs has been essentially standard since the mid-1990s, and is called an ATX power supply connector. It has 3.3V, 5V, and 12V outputs. These specific voltages and common (or ground) wires are arranged in a 2-row connector with 24 total pins (originally 20). This, much like the USB port, is almost taken for granted as one of the few truly universal connections available in the world of computers. As with all standards, however, there are exceptions.

ATX power supplies all allow for a unique trick that can be used to test the PSUs, or use them without needing an attached motherboard or computer. Having a reliable and high power regulated PSU can be useful for low-voltage light fixtures, USB charging equipment or in place of other power adapters. The ATX power supply does not turn on when plugged in, but instead will output power only when “switched on” by connecting pin 16 (usually the green wire) to a common wire (usually a black wire) terminal on the connector. Some PSUs even come with an adapter in the box to allow this connection for external operation. While this jumper wire is connected across the pins, the power supply will turn on and connected devices will operate (see diagram below by Collab, update by Sbmeirow (Own work) [CC BY-SA 3.0], via Wikimedia Commons)).

Dell has been producing proprietary motherboard and power supply units for a number of years that, although functional, will not allow the interchange of parts outside of the manufacturer or device for which they were produced. This problem is most common when upgrading to a “high-end” graphics card. These cards, whether from AMD or NVidia, will require additional dedicated power connections to support the massive processing demands on these devices. They may have either a 6-pin or 8-pin connecto,r depending on the power usage requirements.

With the explosion of virtual reality viewing devices, like Oculus Rift, Microsoft HoloLens, and HTC Vive, the need for high-end cards is increasing as well. Our center required a new graphics card to operate the Oculus Rift headset, but the new video card required more power than the included 365 watt PSU in the Dell Precision Tower 3620. The PSU looked standard in nearly every way until examining the connection to the motherboard. Instead of the customary 24 or even 20 pins, only 8 were present.The same green wire was there and could be used to turn on the device as described above, but the remaining pins only offered 12V (or very close to 12V) outputs. The most dangerous part of this motherboard connector is that the traditional ATX PSU plug would fit onto the eight pin motherboard header, but with incorrect power outputs on the connected pins.

Beyond just getting a system with a fast enough processor, enough RAM, the correct graphics chipset, and power supply with 8 pin power connectors for the graphics card, an additional adapter was required to install a standard PSU into the case.


To test the system, one power supply was connected to the motherboard, and a second to the graphics card. With the PSU that was connected only to the graphics card powered on using a jumper wire on the green pin 16, the computer system turned on and booted without issue. Although it looks like a physics experiment, the knowledge of PSU function created a solution while waiting for an adapter that would take a month to arrive.