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Compatibility Issues

Power Supply 101: A Reference Of Specifications
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Backward and Forward Compatibility

If you have reached this point, I’m sure you have some questions. For example, what happens if you purchase a new power supply that has a 24-pin main power connector but your motherboard has only a 20-pin main power socket? Likewise, what if you purchase a new motherboard that has a 24-pin main power socket but your power supply has only a 20-pin main power connector? The answers to these questions are surprising to say the least.

There are adapters that can convert a 24-pin connector to a 20-pin type, and the other way around, but surprisingly these adapters are not usually necessary. The truth is that compatibility has been engineered into the connectors, power supplies, and motherboards from the start.

If you look at the 24-pin main power connector diagram and compare it to the previous 20-pin design, you’ll see that the extra four pins are all placed on one end of the connector and all the other pins are defined the same as they were previously. The design of these connectors is such that it allows an interesting bit of backward compatibility. The result is that you can plug a 24-pin main connector directly into a motherboard that has a 20-pin socket (and vice versa) without using an adapter! The trick is to position the connector such that the four extra pins are empty. Depending on the latch design, the latch on the side might not engage, but the connector will otherwise plug in and operate properly.

The next figure shows how you would connect a new power supply with a 24-pin connector to a motherboard that has only a 20-pin socket. The terminals on the 24-pin connector that are highlighted in gray would plug directly into the 20-pin socket, whereas the white highlighted terminals would remain free and unconnected.

Connecting a 24-pin main power connector to a 20-pin motherboard socket.Connecting a 24-pin main power connector to a 20-pin motherboard socket.

Logically, this works because the first 20 pins of the 24-pin connector that match the 20-pin motherboard socket contain the correct signals in the correct positions. The only problem that might arise is if there are some components on the motherboard directly adjacent to the end of the 20-pin power socket that physically interfere with the four extra unused terminals on the 24-pin connector.

What about the opposite condition, in which you have a new motherboard with a 24-pin socket but your power supply has only a 20-pin connector? In this case, four terminals at the end of the motherboard socket are not connected. This also works because the 20-pin portion of both the connector and socket are the same. But this example raises another question: Will the motherboard operate properly without the extra power pins? Because the extra signals are merely additional voltage pins that are already present in the remaining part of the connector, the answer should be yes, but if the motherboard draws a lot of power, it can overload the remaining pins. After all, preventing overloads is the reason the extra pins were added in the first place.

Some motherboards sold from 2004 through 2010 that use a 24-pin main power connector also have an additional peripheral (that is, disk drive) power connector onboard designed to provide the extra power that would be missing if you connected a 20-pin main power connector from your power supply. The documentation for these motherboards refers to this as an alternate or auxiliary power connector. Some boards included both standard and SATA style drive connectors to supply this extra power.

If you plug a 24-pin main power connector into the 24-pin socket on the motherboard, the alternate or auxiliary power connection is probably unnecessary. However, if you plug a 20-pin main power connector into the 24-pin main power socket on the motherboard, and that board has one of these alternate or auxiliary power connectors on board, then you should probably use it. In that case simply select a spare peripheral (disk drive) power connector from the power supply and plug it into the alternate or auxiliary power connector. Most power supplies have several extra peripheral or SATA power connectors for supporting additional drives. Using a 20-pin main and the alternate or auxiliary power connector satisfies the power requirements for the motherboard and any PCI Express x16 video cards drawing up to 75 watts.

As a side note, you should be careful when plugging in the mismatched connectors so that they are offset properly. The main, +12 V, and PCI Express graphics connectors are Molex Mini-Fit Jr.–type connectors that are keyed by virtue of a series of differently shaped plastic protrusions used around the terminals, which fit similarly shaped holes in the mating connectors. This keying is designed to prevent backward or improper off-center insertion, but I have found two problems with the keying that should be noted. One is that some alternate low-quality connector brands are built to looser tolerances than the original high-quality Molex versions, and the sloppier fit of the low-quality versions sometimes allows improper insertion. The other problem is that, with sufficient force, the keying on even the high-quality versions can be overcome. Because plugging a 20-pin connector into a 24-pin socket—or a 24-pin connector into a 20-pin socket—is designed to work even though they don’t fully match up, you need to make sure you have the offsets correct or you risk damaging the board when you power it up.

Dell Proprietary (Nonstandard) ATX Design

Most of these systems are no longer in use, but if you upgrade or repair any Dell desktop systems made between 1996 and 2000, you should be aware that they used a non-standard design, and upgrading either the motherboard or power supply can result in the destruction of the motherboard, power supply, or both!

When Dell converted to the ATX motherboard form factor in mid-1996, it unfortunately defected from the newly released standard and began using specially modified Intel-supplied ATX motherboards with custom-wired power connectors. Inevitably, it also had custom power supplies made that duplicated the nonstandard pinout of the motherboard power connectors.

An even bigger crime than simply using nonstandard power connectors is that only the pinout is nonstandard; the connectors look like and are keyed the same as is dictated by true ATX. Therefore, nothing prevents you from plugging one of these Dell nonstandard power supplies into a new industry-standard ATX motherboard you installed in the Dell case as an upgrade, or even plugging a new upgraded industry-standard ATX power supply into the existing Dell motherboard. But mixing either a new ATX board with the non-standard Dell supply or a new ATX supply with the non-standard Dell motherboard is a recipe for silicon toast.

The following tables show the nonstandard Dell main and auxiliary power supply connections. This nonstandard wiring is used on some of Dell’s early pseudo-ATX systems.

Dell Proprietary (Non-standard) 20-Pin ATX Main Power Connector Pinout (Wire End View)
Color
Signal
Pin
Pin
Signal
Color
Grey
PS_On
11
1
+5 V
Red
Black
GND
12
2
GND
Black
BlackGND
13
3
+5 V
Red
BlackGND
14
4
GND
Black
White
-5 V
155
Power_Good
Orange
Red
+5 V
16
6
+5 VSB (Standby)
Purple
Red
+5 V
17
7
+12 V
Yellow
Red +5 V18
8
–12 VBlue
KEY (blank)-
19
9
GNDBlack
Red +5 V20
10
GNDBlack
Dell Proprietary (Non-standard) ATX Auxiliary Power Connector Pinout
Pin
Signal
Color
Pin
Signal
Color
1
GndBlack4
+3.3 VBlue/White
2
GndBlack5
+3.3 VBlue/White
3
GndBlack6
+3.3 VBlue/White


If you study the Dell main and auxiliary connector pinouts I’ve listed here and compare them to the industry-standard ATX pinouts listed earlier, you’ll see that not only are the voltage and signal positions changed, but the number of terminals carrying specific voltages and grounds has changed as well. You could possibly modify a Dell supply to work with a standard ATX board or modify a standard ATX supply to work with a Dell board, but you’d have to do some cutting and splicing in addition to swapping some terminals around. Usually, it isn’t worth the time and effort.

Systems known to have this nonstandard connector wiring include the following Dell models:

  • Dimension 2100, 4100, B1000R, L Series, V350, V400, XPS B Series, XPS Dxxx, XPS Mxxx, XPS P133c MT, XPS Pro 180n, XPS Rxxx, XPS Txxx
  • OptiPlex G1, GX1, GX110, GX115, GX300, GXa, GXi
  • Power Edge 2100, 2200
  • Precision Workstation 210, 400


If you do decide to upgrade the motherboard in any of these nonstandard Dell systems, just be sure you replace both the motherboard and power supply with industry-standard ATX components at the same time. That way nothing gets fried, and you’ll be back to having a true industry-standard ATX system. If you want to replace just the Dell motherboard, you’re out of luck unless you get your replacement board from Dell. On the other hand, if you wantto replace just the power supply, you have several alternatives. Both PC Power and Cooling (www.pcpower.com) and ATXPowerSupplies.com sell replacement power supplies with the modified Dell wiring.

Fortunately, starting in 2000, Dell switched to using industry-standard ATX power connections in its systems.

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