Question about power supplies

[Craig]

since we're on the topic of power supplies, have any of you used a computer (AT or ATX) power supply as your bench power supply?

I have been using an old AT power supply I pulled from my old Pentium 166 computer. It has 12v and 5v leads. I recently measured the 5v output and it's at 5.26 volts. Since then, I have purchased a 7805 regulator and am using that with the 12v supply. The output is now at 5.06v. Do you guys think that this is an ok setup? How would you improve upon it?

[Janitor Tzap]

Craig,

The 5.26 volts from the AT Power Supply is just on the hairy edge.
But not a problem with most AT Power Supplies.

TJ Byers wrote a excellent article on AT Power Supply repair in the Sept 1996
issue.
You can down load the article from the Back Issue area of the Nuts&Volts Site.

I don't see any problem with using the AT Supply for bench testing.
Except you don't have variable voltage.
That and the 12Vdc may not provide enough current if your working on one of
the power hungry Car Amps, GPS's, Etc....


Signed: Janitor Tzap

[haklesup]

You most certainly can use one, especially for PC related projects. there are just 2 or 3 things to remember. AT supplies power up from the power cord or a simple power switch. ATX and later designs use a sense wire in addition to the power switch. They appear dead if you just plug them in. I I forget if you ground or tie the sense wire to 5V wire to get it to turn on.

The second point is that the + and - supply voltages on these supplies were generally not symmetric with respect to the amount of current they supply. Don't assume the -5V supply has the same power as the positive half. Just check the label to know what you have.

A final point, particularly on older AT supplies is that some supplies are not regulated at 0A, you must have a minimum current flowing for it to work properly. Again, the minimum current would be on the label specs.

[Rodney]

On many of the computer power supplies, you must load down the 5 volt buss to get the proper voltage on the 12 volt buss. See www.rcbatteryclinic.com for a good desertaion on using these for 12 volt power supplies.

[GoingFastTurningLeft]

I believe this was covered in the Q&A section of N&V a few issues ago, maybe last month.

[rshayes]

A computer supply would probably be adequate for digital circuits. These are switching supplies and the output will probably be too noisy for direct use with analog circuits, such as audio amplifiers.

Adding filters and a linear regulator would help this problem, but is probably too much trouble in general.

[jollyrgr]

A few points:

I posted a scan of a ATX power supply schematic and data sheet in this thread:

http://www.servomagazine.com/forum/view ... hlight=atx

BUT WAIT!!!!

AT and ATX power supplies should have a load on them; usually a power resistor on the 5V supply.

The data sheets on several supplies I've come across state that if there is NO LOAD on the 5V+ supply (i.e. Vih or V open circuit) is 5.25V+. See section 4.1 about the floating open circuit voltages in my previous posting. Thus the 5.26V+ you are seeing is expected. (And I would not bother with the 7805 for one quarter of a volt. Do realize this type of supply is DESIGNED for digital circuits and there is no need to run a circuit at 5.00000000VDC! Likely most digital cicuits would work from 4.0V to 6.0V. 5V is "accepted" value and NOT an absolute!)

If you are the type that could care less about schematics, data sheets, etc. and want a cookie cutter solution, here is what you do.

Find pin 14 of the multi pin header of the power supply. Usually pin one will have something different about it to show it is pin 1. But pin 1, 2, and 11 are all 3.3VDC and should be the same color. Pins 1, 2, 3 etc. in one column, pins 11, 12, 13 are in the other column. So if you can locate three wires the same color (USUALLY orange) on one corner of the connector, the two same wires in the one column will be pins 1 and 2. Likely pin 14 will have a GREEN wire but this is no guarantee!

A ATX power supply is never really off when plugged in! There is a 5VSB+ supply (or similar name) that goes to pin 9 to run the mother board when the computer is "off". This is for things like the Wake On LAN, power switch sensor, etc. If you plug in the supply but the fan does not start spinning, connect the black lead of your meter to one of the black COMMON wires. All should be exactly the same. Then probe the connector for a 5V source. When you find this, that will likely be pin 5. ON MOST SUPPLIES the red wires are +5VDC and are turned on and off. The "housekeeping" 5VSB supply will have a different color and will be very low current as compared to the regular 5V supply which can supply tens of amps.

Now what do you do? Simple. Find pin 14. Ground this pin to turn on the supply, hold it at +5V to shut down. See that 5VSB supply I wrote about in the previous paragraph? There is your 5V to shut down the other supplies. Per the posting this is a PURPLE wire on the one supply schematic. Any of the black wires can be used for the ground or POWER ON location. Use a pull up resistor and a short to ground for power on. What do I mean by this? Take something like a 1K resistor and connect pin 14 to pin 9. This will allow pin 14 to "see" +5VSB and shut the supply off. Connect a switch between pin 14 and one of the ground wires. When you close the switch (turn it on) the supply will turn on.