My Crazy World

My Crazy World

The Life and Musings of Andrew Almquist

Building a PC with Two Power Supplies

I recently decided to purchase a new graphics card to replace the on-board chipset that came along with the computer I bought a couple of years ago. Now I have experience with electrical engineering. I’ve been building systems for almost 15 years. I know exactly how to do what I wanted to do, but I made a mistake. In my excitement, I forgot that I needed to order a more powerful power supply to handle the huge load of the new video card.

Before I go any further, I am an expert. I have been working with electronics pretty much since I could walk. There are many things that I know that I can’t be sure to communicate when I write an article like this, so do not try this! I will not be held responsible for any damages that happen to you, your computer or anything else should you attempt this.

With that said, I received a decent graphics card on a Thursday, and wouldn’t be receiving the power supply to handle it until the next Monday. Being as impatient as I am, I couldn’t wait. Knowing that the graphics card would pull at least 65 watts of power at full load, I was able to calculate that the power supply that came with my computer wouldn’t be able to handle the load. I just added up the maximum wattage of all of my 12V components, divided the total by 12, and compared that to the 12V amperage rating of my power supply. So my power supply would definitely burn up under the load, what could I do?

I started checking spare power supplies I had around the house. Knowing that the video card would need at least 65 watts based on its processor, I knew I needed something with more than 6A available on the 12v rail. I didn’t know for sure that the card wouldn’t pull a heck of a lot more since the specifications for the card itself only specified that it required a “400 watt power supply”. In the end I decided I’d try a 300W power supply I had laying around, and use it to just power the video card. It provides 15A on the 12V rail which should be perfect. I also happened to know that it was one of the safest power supplies of its time, providing overdraw protection and other safeties that might be useful.

At this point I had to make a temporary modification in order to make the power supply work. Every ATX power supply has a 20 or 24 pin connector which connects to the motherboard. Within this connector is a pin, leading to a green wire, that disables the power supply when the circuit to ground is open. In order for the power supply to work, this circuit has to be closed. Usually this is done on the motherboard, however, the motherboard only has one plug. How was I going to make this work with two power supplies. Well, I had two choices. I could either run a jumper from the green wire to a black wire, or, run a jumper from the green wire on one plug, to the green wire on the other, and from a black wire to a black wire on the other. I chose to just jump the green and black together on the secondary power supply since I was only going to be using this for a couple days.

There is a disadvantage to the way I jumped the connectors. Before I can start up my system, I have to power on my secondary power supply. Had I bridged the two connectors together, both would have been powered by the power button.

And so I hooked it all up like this, hooked up the power wires to the graphics card, and got ready to start the system up. I powered up the secondary power supply, hooked to my graphics card first. Next I powered up the system. After it had started, I started feeling the secondary power supply and the air it was exhaling. As I expected, it was staying cooler than the air from the primary power supply. Had the air coming out of the power supply been hot, I would have shut down the experiment and waited for the new power supply to arrive. Instead I continued to monitor the temperature and continued to find that the system was working fine.

Three days later, it is still functioning fine. This is only because the maximum draw of the card is well below what my secondary power supply can handle. This also raises a difficult and somewhat disconcerting question. Why didn’t my video card manufacturer provide more information about the power requirements of my card? In order to find a sufficient power supply it is important to know the maximum current draws at each voltage. Exceeding them at any voltage can be disastrous. Newer atx12v power supplies focus the 12v rail, providing the most power where it is needed, however older ATX power supplies didn’t. I could conceivably have a 400 or 500w ATX power supply that wouldn’t handle it. I could also have drives and other devices drawing power from the 12V rail, and not have enough left for the video card. It’s important to understand how much power (W) and current (A) each rail is capable of as well as which devices use which to properly plan a modification like this. Especially when the modification requires more power than anything else.


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