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Home >> Resources | Support >> Guides to PC Modification >> 

Using a Second Power Supply

by Nathanael Barbettini

You are encouraged to make links to this article from your website and tell your friends

The following advice is based on many years of experience. It is provided as a free service to our customers and visitors. However, Directron.com is not responsible for any damage as a result of following any of this advice. You are welcome to distribute these tips free to your friends and associates as long as it's not for commercial purposes.


Ask someone how many power supplies they have in their computer, and you'll get a look like you're from another planet. Most people are content with one power supply, and in most cases, that's all you'll ever need.

However, for some people, especially serious modders with countless fans, cathodes, and who-knows-what-else, one power supply doesn't provide enough juice. In this guide, we will explore putting a second power supply in your computer, and hooking it up to a switch. If you are a modder, enthusiast, or just want to tinker, keep reading!

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Power Supply Types

There are two basic types of power supplies. There are AT power supplies, which are older and in older computers, and ATX power supplies, which you will find in virtually every new computer you can buy.

There are two fundamental differences between AT and ATX power supplies. First, the switch mechanism is different. AT power supplies use a normal on-off switch, which directly turns the power supply on or off.

ATX power supplies use a momentary switch which does not directly control the power. Instead, the switch signals the motherboard, which performs one of three actions:

  • If the computer is off, the power supply is turned on (which turns the computer on)
  • If the computer is on, the computer goes into power-saving mode (standby)
  • If the switch is held for more than 4 seconds, the power is cut and the computer turns off.
Because of this difference, ATX power supplies are better for projects that require the second power supply to turn on automatically when the computer is turned on.

The second difference is in the motherboard connector: AT power supplies provide two 6-pin connectors (figure 1), which are easy to insert backwards. The ATX connector is a single 20-pin connector that only plugs in one way (figure 2).

Figures 1 and 2: The difference between AT (left) and ATX (right) motherboard connectors.

Both power supplies provide two types connectors for plugging devices into. These connectors are called Molex connectors, and they come in two sizes (see figure 3 and 4). A power supply will generally have a few of each size.

Figures 3 and 4: Large (left) and small (right) Molex connectors.

There is no difference between the two sizes other than the size itself. Both sizes provide the same amount of power to whatever device is plugged into it (12V and 5V). The small Molex connectors are generally used only for floppy drives. Large Molex connectors power hard drives, CD/DVD drives, and many fans and lights as well.

Note: You can purchase large and small Y-adapters if you run out of Molex connectors. Be careful when using the Y-adapters however, because if your power supply does not have enough power for all the devices attached (especially true for older, lower-wattage supplies), you can damage it.

Now that we've looked a little at the types of power supplies, let's look at how to put a second one into your computer.

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Adding an AT power supply

While this article will mostly focus on how to put a second ATX power supply into your computer, let's take a quick glance at doing the project with an AT-style supply.

Unlike an ATX power supply, an AT power supply does not usually need anything special to get it to turn on. All you need to do is to connect an on-off switch rated for 125VAC (in North America) and turn it on. If you recycle an old AT power supply from an old computer, the switch will most likely be already attached. See figure 5.

You can purchase AT Push-Button Switches from Directron if you need a new AT switch.

If there isn't a switch connected to the power supply already, you'll have to connect one yourself. There may be a diagram on the switch or the power supply itself showing how to connect the switch. Unfortunately, there is no set standard for AT switch wiring, so the connections will be different between manufacturers.

Warning: Be careful when you wire up the switch! Make sure it is connected properly before switching it on. Serious damage to your power supply, computer, or electrical wiring could result if the switch is wired incorrectly. If there is no diagram on the switch or power supply, look for some help at an electronics store or on the Internet.

Figure 5: An AT power supply all ready to be turned on.

After you've connected the switch and powered up the supply to make sure it works correctly, you can put the two 6-pin motherboard connectors somewhere out of the way and start plugging devices into the Molex connectors.

One disadvantage of recycling that old AT power supply and using it for a second power source is the on-off switch requirement. There isn't an easy way of making the power supply turn on when your primary supply does (as you can do with an ATX power supply). For some people, having to use a separate switch is fine. If you are not one of those people, read ahead and use an ATX power supply instead.

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Adding an ATX power supply

Using an ATX power supply as a second power source is not much harder than using an AT power supply. The only difference lies in the way you get it started.

Instead of an on-off switch like an AT power supply, ATX power supplies rely on a momentary switch, which is actually connected to a header on the motherboard. When the switch is pressed, the motherboard shorts the PS_ON pin (pin #14) on the 20-pin ATX motherboard connector (see figure 6). We can recreate this easily without the momentary switch.

Figure 6: ATX connector diagram.

To start an ATX power supply, we need to short the PS_ON pin. This is easily done by connecting a wire between the PS_ON pin (pin 14, usually the green wire) and any black Ground pin. This is shown in figure 7:

Figure 7: Shorting the PS_ON pin.

Unless you want your second power supply to stay on all the time, you'll want to hook up a permanent toggle switch, or connect it to your primary power supply so they both turn on at the same time. We will look at connecting a switch first.

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Starting an ATX power supply with a switch

First, you'll need to obtain a switch. A SPDT (single-pole, double-throw) switch is a good choice. You can get SPDT switches at any electronics store.

Second, unless the switch comes with wires pre-attached, you'll have to solder or otherwise connect some wire to the switch (see figure 8). The wire doesn't have to be anything special since there won't be much, if any, voltage running through it.

Figure 8: Connecting wires to the SPDT switch.

Once you have the switch ready with the wires connected to it, stick one wire into the PS_ON pin (remember, pin #14, usually green) and another into any black Ground pin. It would be a good idea to solder or tape the wires in place so they don't fall out and cause a short.

Alternatively, if you don't mind cutting some wires, you could splice the switch tail wires directly onto the PS_ON and ground wires. This method is somewhat permanent, however.

Tip: It's a good idea to plug something into the power supply to make sure it's working, since some power supplies have awfully quiet internal fans. A LED or fan with a Molex connector will do the job. This step is not necessary, but is helpful so you can see visually whether or not the power supply is on.

Once everything is plugged in, flip the switch and see if it starts. If it does, good job! Turn it off and start plugging your devices in.

If the power supply doesn't start, make sure it is plugged into the wall and the switch is securely connected to the PS_ON and Ground wires. The switch might also be on backwards, if so, try reversing the wires (put the one that was plugged into the PS_ON wire onto the Ground wire, and the one that was connected to Ground on PS_ON) and see if it works.

Some power supplies also have a switch in the back, so make sure that it's in the ON position if there is one on yours.

If the power supply still won't start, it may need a "dummy load" to get it going. A dummy load device is a resistor with a Molex connector plug that simulates a device on the line. Skip down to the section titled "Starting a stubborn power supply", later in this article.

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Starting an ATX power supply automatically

Depending on your project, you might want your second power supply to have a switch as described in the above section. For example, you could connect some extra fans to your second power supply and flip the switch on a hot day when your computer needs extra cooling power.

For most projects, however, the extra switch can be a hassle. You can ditch the switch altogether and connect your second power supply to your primary power supply, so the power button on your computer affects both power supplies.

To do this, you need a relay, which you can pick up at any electronics store. Make sure the relay you choose has 4 pins on the bottom and is rated for 12V DC or higher. Then wire it up according to figure 9.

Figure 9: Wiring diagram for the relay.

A relay works like a switch, except it is completely electronic. In this case, when the primary power supply is powered on and a 12V current is put into the relay via the yellow and black Molex pins, it shorts the green and black pins on the second power supply, turning it on. If you power on your computer and the second power supply doesn't turn on, check your connections. If all the wires are connected correctly, you may have a broken power supply, or your power supply may require a dummy load (see below).

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Starting a stubborn power supply

In some cases, a power supply may not start at all, no matter how much you coax it. In those cases, you will need to obtain a dummy load that simulates a device attached to the power supply. You can purchase a 1A resistor dummy load device with a Molex tail, all ready to be plugged in. You could also plug some fans or lights into the power supply to try and power it.

If a dummy load won't even start it, try plugging in a big device such as an extra hard drive or CD drive. If the power supply still won't start, it is probably broken, or some of the wiring (for the switch or short) is not making full contact.

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If you are a normal computer user, in most cases you do not need a second power supply. On the other hand, modders and PC enthusiasts can obtain great benefits by performing this simple addition.

I hope that this article can guide you through all the steps needed to add a second power supply to your PC. If you have trouble or just questions, post a topic on the Help Desk forums and someone will assist you.

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Related Links: | Switches | Power Supplies | Everything AT | Mid-Tower ATX Cases |

This information has been provided as a reference. Directron.com is not responsible for any problem as a result of properly or improperly following the advice above.

Last Updated: 6/26/03

(c)Directron.com, All rights Reserved.

If you find this article useful, please create a link to it from your website or tell a friend about it. If you have any comments or suggestions about this article, please email information@directron.us

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