ATX to Lab Bench Power Supply Conversion

Step 2: Planning

Planning is the most important step of any successful project. To plan this project, I created a few images. I am going to be using four binding posts, a power switch, a fuse holder, a power resistor, and two light emitting diodes (LED's) with current-limiting resistors. The first image details the circuit connections inside the power supply, where everything will be connected

When the power supply is connected to the wall socket, but not yet turned on, it provides a +5v standby signal, that can be used by the motherboard for things like wake-on-LAN functionality. We use this signal line to indicate when the power supply is plugged in with a red LED and a 330 ohm resistor. On my power supply, this signal line has a purple wire, and is labeled "+5VSB" on the circuit board.

When the power supply is first turned on, it must go through a start-up sequence, to ensure that everything is working, and that it is able to provide stable power to the computer. When the start-up sequence has completed, it signals the motherboard by providing +5v on the "Power Good/Steady" signal line. We will use another red LED and 330 ohm resistor to indicate when the power supply is running. On my power supply, this signal line has a gray wire, and is labeled "PGS" on the circuit board.

The power resistor is a 10 ohm, 10 Watt resistor, commonly called a "sandbar", because they are usually coated with a material that feels like sand. Most power supplies need a minimum load to keep them running, so this sandbar resistor provides a constant minimum load between the +5V rail and Ground. I've heard that newer power supplies also need a load on the 3.3v rail, your mileage may vary.

In the second image, you can see the diagram for the front of the power supply. Here I have marked where the components will go, including the LED's, the binding posts, the fuse holder, and the switch.

The third image is what the power supply looks like without any modifications. You can see the various voltages I am going to use along the front edge.

Comments

[hschmutz]

Hy there
I know this is an older post but i will try to make this power supply for me.
What fuse do you take ?

[matthewbeckler]

Hi, thanks for the comment. Choose your fuse based on your application (what you're going to power with your power supply). The fuse is primarily there to protect your circuit from mistakes in your wiring, not to protect the power supply from your mistakes (it doesn't need protection). If you are running small circuits like an Arduino that will draw less than 500mA, then a 1 amp fuse should be fine. If you are charging RC batteries from your power supply, then you'll need a 10 or 15 amp fuse. Basically get a fuse that can handle just a bit more than the maximum amount of current than you expect to need for your application. Good luck!

[Skarz88]

How do you use the -12v and -5v?

[matthewbeckler]

Hi Skarz88, not really sure what exactly you are asking? Those wires produce voltages at -12 volts and -5 volts relative to the ground (black) wires. They are generally not very useful to the electronics hobbyist since they have very limited current capacity (not the tens of amps available on the +5v and +12v lines). Does that answer your question?

[jmunoz3]

Matthew, great tutorial. I wanted to point out that on your schematic you have placed the 2, 330 ohm resistors on the cathode leg leads of the 2 LED lights. I do believe that they belong on the anode legs. Once again great tutorial.

[matthewbeckler]

Thanks, glad you like the tutorial. You can put the resistor on either side of the LED, either the anode or cathode side. I haven't really ever heard of a convention for where to put the resistor. Thanks for the comment!

[dcopeland1]

You can use the resistors on the anode side as well. Just as long as you do have a resistor in series with the LED's. Remeber an LED is a diode that will light up when forward Biased. So you can have a load resistor on the cathode to ground if you want and it will work the same. If wanted to try something fun, build a clamper or limiter circuit with a Diode, two resistors, a 1.5V battery and a 5 power supply. Now just a quick warning to others that are wanting build one of these. I can attest to the Capacitors on the 110V side of the PS. These bad boys hurt if you touch the outer shell. Trust me. So be careful with what you do...

[Vspec]

My psu needed the power resistor on the +12v instead of the +5v in order to turn on.

It's made by Delta electronics, manufacture date 2008.

[tomato1991]

Just wanted to say thank you for posting this.
Nice and easy to understand, and it was the exact thing I needed.

[harshesh]

Will a 10W 100Ohm Resistor work as the power resistor ?
When I use the 10W100Ohm Resistor and connect the green line with the black line, Some noise comes for a second and then stops .. Sometimes the noise comes for 5 seconds and then the Fan starts but then immediately stops ! .. Is there something wrong with the Power Supply I am using or is it the resistor ?

[matthewbeckler]

Hi! Somewhat counter-intuitively, the larger the resistance you have, the less current it was draw from the power supply. A 10 ohm resistor will draw 500 milliamps (mA), a 100 ohm resistor will draw 50 mA, and a 1000 ohm resistor will draw 5 mA. Presumably there's some minimum current draw that your power supply requires on the 5v line (and maybe the 3.3v line too?) and if you don't draw enough current (resistor is too large) then it won't stay on.

It really sounds like the behavior you describe is due to insufficient load current, or perhaps some other problem. I would try a power resistor with a smaller resistance value to try and draw more current. You could try connecting two of your 10W 100 ohm resistors in parallel, producing an equivalent resistor of 20W 50 ohms that will draw 100 mA, and dissipate P = V * I = 5 * 0.1 = 0.5 watts shared across both power resistors. Good luck!

[cubemike99]

Awesome Instructable and explaination, but why does the resistor need to be such a large wattage? Can a higher resistance/wattage be used?

[matthewbeckler]

Mostly it was what I could get from radio shack easily without having to make an order from digikey/mouser/etc. I've never heard "official" numbers about the minimum load current required to keep a power supply, but 0.5A seems to be working for most people. You could use a resistor with higher resistance to try and reduce the load current (V = I * R), and then you could use a resistor rated for less power dissipation (P = V * I) if you want to experiment. You can always use a resistor with a higher wattage, that won't hurt anything, but if you get too low then the resistor will likely heat up and might burn you (funny story: The earlier makerbot 3d printer heating elements to melt the plastic filament for extrusion were actually just power resistors that were driven far beyond their designed power rating, so they get really hot).

[dcoates1]

Thank you! Out of all the instructables I found, yours was the most helpful with your wire diagram. Thank you! You can check out an image of my painted and modded power supply here if you're interested.. -> http://mixologicaldisaster.wordpress.com/2012/04/09/atx-benchtop-power-supply-v2/ :D I tried posting the image below..hope it works..

[matthewbeckler]

Wow, that looks really well done and nicely polished! I like the excess of power connectors in the front of the frame (power pole or something like that?) and the speaker-style wire terminals are a really good idea. Nice work, thanks for sharing your photos and experience!

[brusho150]

HI!
Brilliant instructable I am new in electronic engineering and wanted a lab power supply will you please tell me that how Can we add a potetiometer(voltage regulator),rheostat, voltmeter and ammeter to calculate and vary voltage and current?
Thanks

[matthewbeckler]

You don't want to use a rheostat or potentiometer to directly produce your output voltage. What you want to do is use something like a LM317 adjustable voltage regulator, which can use a potentiometer to output an adjustable voltage. There are probably higher-current versions of the LM317, but it depends on your applications.

You should be able to either modify an existing volt/ammeter to do what you need, or you could always whip up some sort of microcontroller + tiny current sampling resistor + 7-segment displays for your volt or ammeter. Good luck!

[brusho150]

Thank you mam.

[reddeth]

Well after spending an hour or two putting this together in an old ATX power supply, I think mine is actually dead :S

I get 5v to the purple (standby) wire, but when switched on the fan does not come on and I get no voltage whatsoever on the grey (power on) wire, or to any 3, 5 or 12v lines. I have a good ground, so I know it's not that. Is the power supply just dead? For whatever it's worth, I hear an odd clicking sound which sounds like it's coming from the area of the large capacitors. Bad sign I assume?

[crf250rider14]

Looks like your power supply has non standard wire colors, just like my first. Seems like it is shorting out (same as mine did) check your -12v wire and your 3.3v wires. I know my -12v was solid blue and my 3.3v were blue with a white strip running down it. I had them connected to the same terminal. Maybe this is the same in yours.
Scroll down on this page, and look at the pinout.
http://www.informit.com/articles/article.aspx?p=339053
Is this what yours looks like?
Hope this helps.

[hitachi8]

guy just a question , i made a Benchtop power supply ( almost finiched ) and i didn't put any resistor on the 5volt rail ...( i didn't use the 5volt ) the power supply run fine and everything seems to work ... is that normal ? Do i realy need to put a R on the 5volt ? ( i am using an old power supply : pentium 2 or lower

[matthewbeckler]

Hey there, thanks for the message. I think the resistor is only needed, well, if it's actually needed. Many people have said they needed it, but some haven't, and if it works for you without, excellent! One thing I've heard from some folks is that without the resistor, the output voltage on one or more voltages might be "unstable" or "wiggly" where it bounces around, instead of being steady at, say, 12v. This may or may not affect you, and it might not even be a problem if it does affect you. I guess, in conclusion, is that if it works, it works! Thanks for sharing your results, it's one more data point I can use when other people ask me questions for their project. Thanks, Matthew

[Gelfling6]

Your supply may have one built onto the main board inside, like mine. (when I unsoldered the extra wires, I spotted a 33-ohm 5V carbon film resistor soldered in-line with the +5V to GND.)

A good point to watch, when you turn on the supply, does the cooling fan power up to full? or partial, or just for a brief moment, then spin down, with absolutely NO load applied? If it spins up to full, there is some form of load already at work. If the later (slow, or spins down) there is no active load resistor, and will require the load resistor to maintain operation.

Here's my 400W supply, with one of the Drive power cables still installed. (which I can use to power external hard drives.)

[hitachi8]

My output voltage move ( when i plug or unplug something ), but just a little bit . like .4volt ... but for my need , i dont realy need a constant 12volt so ... Also , i put my fuse( 220volt 1amp) on one of the 110volt Input Wire . is that O.K ? Thank you for helping people , it is realy nice .

[matthewbeckler]

Putting a fuse on the AC side is a good idea, since it protects all DC voltages from over-current situations (like a short circuit to ground). Certainly easier to add than to put fuses on all DC voltage lines (5v, 12v, etc). Thanks!

[purpulhaze]

Possibly would be better to add separate fuses for use with different projects needing more or less current.

[Archive555]

Yeah, especially since most of the different power lines (3.3V, 5V, 12V) each have different load ratings. I know, for example, that mine has a maximum of 18A output for the +12V line, but yours might be different, although it will probably say on the side of the PSU. If not, look up the specs of yours online.

[dnutman]

My power supply is rated for 17A at +12V... what size and type of fuse do I need for this?

[hitachi8]

depending on the load you will be using it ...
you need to do some math , because AC amp and DC amp is not the same , and you will be using current on the 12V so the AMP ratting will be different on your 120V AC .

i will suggest that you put a 1Amp at fist , if you have problem switch to a 2Amp or more .
normaly 1amp on the 120VAC is enough for most DIY project .

1Amp X 120Volt = 120Watt . this is alot of power .
??? X 12Volt = 120Watt = 10Amp
so
you will need between 8-10Amp on the 12Volt for your 1Amp fuse to burn .
so , if you want to draw 17Amp , i sugest you a 1.5Amp fuse on your 120VAC .

[dnutman]

Alright, thanks for your help.

[lamikam]

Need Help!. I got the thing built today. I was testing it for a while, and all the outputs were good. Now, when I power on, There are no voltages on the outputs. The +5V standby power (purple wire) is on as 5V, but the "Power On" wire ( Gray) shows no activity, and all the outputs (5/12/-12/3.3) are 0 volts.
The fans are not running either. Any easy way to figure out what is going on?
I don't want to mess with the high voltage in the board.

Thanks!

[matthewbeckler]

Do you have any sort of load on the power supply, like the power resistor mentioned or whatever it is you're going to power with this supply? I've heard that some PSU units won't stay on unless there is some minimum load attached.

Does the fan 'twitch' when you plug it in, or when you flip the power switch?

[lamikam]

FYI, I found a cheap and small replacement for fuses.

http://www.circuitprotection.com/catalog/fundamentals/PSWFundamentals.pdf

[mozartripper]

Can I use one or two 6V LED instead of a 10Ω/10W resistor between the +5V and the ground ? And also if I use 6V LED for the power on and standby will I need the 330Ω resistor ?

[matthewbeckler]

6V LED, eh? Never heard of those before...maybe it's two standard LEDs connected in series? Strange.

The 10 ohm resistor is used to provide a small, constant load on the 5v line, which is frequently required to get the power supply's internal circuitry to work properly. With the 10 ohm resistor across 5v, it draws 500 mA, which is most likely way more current draw than is necessary. I would guess that 100 mA would suffice. As long as you draw more than some minimum current from 5v (maybe 100 mA?) you should be fine.

The resistor in-line with the LEDs are used to control how much current goes through the LED. Once the voltage across an LED exceeds its "forward voltage" (usually between 1.5 and 3v, depending on color) the LED will pretty much conduct as much current as it can. We use a series resistor as a simple method to restrict how much current can flow through the resistor and LED. Since you're talking about 6v leds, I don't think they would even work for the power on and standby LEDs, as those are only 5v signals, which isn't large enough to exceed the 6v LED forward voltage.

I could be wrong, however, as I've never seen or worked with 6v LEDs. Do you have a link to their product page on Digikey/Mouser/etc?

Good luck!

[mozartripper]

wow sorry i mean 6v incandescent light (like the small ones in a Christmas three). I don't use them a lot so I always call them led

and I make mine work by shorting p-on and ground wire + the incandescent light on 5v + 80mm fan on 12v (because the internal fan connector was broken)

[purpulhaze]

Can anyone suggest fuse ratings for the output lines?

[matthewbeckler]

Keep in mind that fuses are mostly for protecting your circuit, not your power supply. The power supply should shut itself down if you try to draw too much current, but given the large current capacity of most power supplies, you could easily turn your breadboard / printed-circuit board traces into very hot heating elements without a fuse. Somewhere between 1 and 5 amps should be fine for most general-purpose electronics work.

[jimtran93]

some people say 1 Amp is ok, i would just go with a 5 Amp though. it really does depend on your power supply limitations and what your going to use it for

[maclover201]

 This tutorial worked awesome for me. I didn't need the huge resistor though.