# Using a Power Supply

Um, so I'm building this power supply but I am somewhat new to a lot of actual electrical devices...

I have a precision power supply unit. There is an AC voltage option and a DC voltage option.

Within the AC voltage are there are three terminals. The outer two are yellow and marked 6.3VAC and the inner one is 2.6VAC.

For DC there are four terminals the first two being red, then a black one, then a yellow.
They are marked respectively, +5v/3A, +1.25-15V/1A, COM, and -1.25--15V,1A

What does the COM terminal do? And with this power supply, only one terminal will need to be used at a time, true?

This is making me feel stupid.

Thanks.

Oh, if you want to look at a picture,

berkeman
Mentor
Um, so I'm building this power supply but I am somewhat new to a lot of actual electrical devices...

I have a precision power supply unit. There is an AC voltage option and a DC voltage option.

Within the AC voltage are there are three terminals. The outer two are yellow and marked 6.3VAC and the inner one is 2.6VAC.

For DC there are four terminals the first two being red, then a black one, then a yellow.
They are marked respectively, +5v/3A, +1.25-15V/1A, COM, and -1.25--15V,1A

What does the COM terminal do? And with this power supply, only one terminal will need to be used at a time, true?

This is making me feel stupid.

Thanks.

Oh, if you want to look at a picture,

COM means the common terminal for the DC section. So to use the 5V supply, you connect your + wire to the +5V plug, and your - wire (your ground wire) to the COM plug.

The COM plug is typically floating with respect to earth ground, so you could stack several power supplies if you wanted to, by connecting the + output of one power supply to the COM of a second power supply. You should check with your DVM to see if that power supply's COM plug is connected to earth ground or not. Is there a separate earth ground terminal somewhere on the power supply (I couldn't see it in the picture)? Does the power supply use a 2-prong or 3-prong power plug to the AC Mains?

Hi Everyone,

Thank you for sharing your information. Really I am very appreciated here and this is the very useful information. Power supplies are generally rated by wattage. The more watts, the more money, Some offers more IDE drive connections. Some offer more SATA power connections. Some are specific for AMD vs Intel boards. Some are interchangeable between board brands. It is mostly a matter of preference, but mainly a question of how many watts you are getting yourself into. The total number of watts indicated is a total number of watts that it can support on demand. If you're buying a new power supply, it's okay to plan ahead, pretend like you are doing an upgrade to your computer and buy something that's actually ratted better than what you may need.

Thanks a lot
Dagwood Torres