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ATX to Lab Bench Power Supply Conversion

In my sophomore year of college at the University of Minnesota, I started into my main

electronics classes, and needed a good power supply for working on lab projects at home. Myroommate Adam  told me about somebody online who had converted an AT computer powersupply into a lab bench power supply, so I decided to do the same thing. !ou can also check outthis link for a very similar guide by their user  Abizarl. I have also documented this project on mywebsite at http"##www.mbeckler.org#powersupply#  if you are interested.

Warning! There are several large capacitors in AT power supplies, that will store a dangerouscharge for a long time. $lease let your power supply discharge, completely unplugged from thewall outlet, for a few days before opening it up. !ou can probably be seriously hurt, so please bevery careful. I am not a lawyer, but I hereby release myself from as much liability as I can, for anysort of injury you sustain, or any trouble you get into.

Step 1: Bacgroun

%irst, a bit of background on a typical AT power supply"

&omputer power supplies are  'witch Mode $ower 'upplies ('M$'), which use high*fre+uencyswitching circuit elements to provide a high*+uality output voltage, with good energy eciency.-ne side eect of this technology is the minimum load re+uirement that each power supply has. Inorder to function properly, the power supply needs at least a very small electrical load connectedto it. In other words, AT power supplies will only work if you have something connected to it. /ewill be using a power resistor to provide this minimum load.

Also, modern power supplies do not simply have an -%%#-0 switch, they have what is known as a1soft1 power switch. This normally makes no dierence to the user, as the computer behaves thesame, but when you shutdown your computer, the motherboard can turn o the power supplywhen it has 2nished shutting down. This re+uires us to add our own power switch to the powersupply chassis.

 To protect our circuit from accidental (and careless3) short circuits, we will install some fuse*holdersand fuses, which will disconnect the circuit supply lines if too much current 4ows. The si5e of the

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fuses are up to you, but a 6 amp fuse will work just 2ne for most circuits. !ou really should putfuses on all supply lines."pate: /hile the diagrams show fuses on all voltage rails and no fuse on the ground line, when Iactually built my power supply, I was young and foolish and only put a fuse on the ground wire. It7smuch safer and a better idea to put fuses on all signal lines and not the ground line. Thanks tomany emails and messages on Instructables about this oversight.

Step #: Planning

$lanning is the most important step of any successful project. To plan this project, I created a fewimages. I am going to be using four binding posts, a power switch, a fuse holder, a power resistor,and two light emitting diodes (89:7s) with current*limiting resistors. The 2rst image details thecircuit connections inside the power supply, where everything will be connected.

/hen the power supply is connected to the wall socket, but not yet turned on, it provides a ;<vstandby signal, that can be used by the motherboard for things like wake*on*8A0 functionality. /euse this signal line to indicate when the power supply is plugged in with a red 89: and a ==> ohmresistor. -n my power supply, this signal line has a purple wire, and is labeled 1;<?'@1 on the

circuit board.

/hen the power supply is 2rst turned on, it must go through a start*up se+uence, to ensure thateverything is working, and that it is able to provide stable power to the computer. /hen the start*up se+uence has completed, it signals the motherboard by providing ;<v on the 1$owerood#'teady1 signal line. /e will use another red 89: and ==> ohm resistor to indicate when thepower supply is running. -n my power supply, this signal line has a gray wire, and is labeled 1$'1on the circuit board.

 The power resistor is a 6> ohm, 6> /att resistor, commonly called a 1sandbar1, because they areusually coated with a material that feels like sand. Most power supplies need a minimum load tokeep them running, so this sandbar resistor provides a constant minimum load between the ;<?rail and round. I7ve heard that newer power supplies also need a load on the =.=v rail, yourmileage may vary.

In the second image, you can see the diagram for the front of the power supply. Bere I havemarked where the components will go, including the 89:7s, the binding posts, the fuse holder, andthe switch.

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 The third image is what the power supply looks like without any modi2cations. !ou can see thevarious voltages I am going to use along the front edge.

Step $: %rilling holes

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Bere, I planned out and drilled the holes in the case. My power supply was a smaller form factor, (Itwas from a mini*tower case), so there wasn7t a lot of space to work with.

Step &: Connecting 'ront(panel ite)s

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Bere, I am connecting the appropriate wires to the binding posts, power switch, fuse holder, and89:7s.

In an AT power supply, there should be a wire that is used to turn on the power supply. !ou can

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see this wire (It7s green) in the second pictureC it is the green wire in the middle, where it says1-0#-%%1 on the $&@. I connected this to the switch, and the other pole of the switch went toground. The ;<, ;6D, and *6D are connected right to their wires on the $&@. The ground wire isconnected through the fuse holder before the binding post.Initially, I was going to use green 89:7s, but I reali5ed I had many more red 89:7s than green 89:7s,so I switched them over to reds. In the 2rst picture, you can see the holders I installed into thefront. I connected the 89:7s through a common resistor to ground. The 89: on the left (from the

front view) is a standby 89:. It is lit whenever I have the power supply plugged into the wall. It isconnected to the ;<? standby wire on the $&@. In my $', it7s purple. The other 89: is the 1$ower-n1 89:, and it is lit when I have the power supply turned on. It7s connected to the 1$ower -E1signal wire, which goes to ;<? when the power supply detects that it has stabili5ed the voltages.In my $', it7s the gray wire.

Step *: Power +esistor

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Most modern AT power supplies re+uire a small load to stay in the -0 mode. I added a 6> ohm,6> watt resistor between ;<? and ground to provide this small load. It is strapped to the back wallof the power supply, where it should get plenty of air 4ow. It doesn7t actually even get warm duringnormal operation so it7s not a big deal.

Step ,: -inishe Pro.ect

Bere you can see the 2nished project, both with and without the cover. If you have any +uestions,please leave a comment and I will try to check back often to answer them. Thanks for looking, andgood luck3

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Eeep in mind that while I built my power supply many years ago with only the ground line fused,you should put fuses on all your signal lines and leave the ground line directly connected.

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Loo online or at your local co)puter store 'or an ATX co)puter power supply/ or is)antlean ol co)puter an re)ove the power supply 'ro) the case0 

"nplug the power cable 'ro) the power supply an turn o the switch on the bac 2i' there isone30 Also, be sure you are not grounded so that remaining voltage doesn7t 4ow through you to ground.

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+e)ove the screws that attach the power supply to the co)puter case an re)ove thepower supply0 

Cut o the connectors 2leave a 'ew inches o' wire on the connectors so that you can usethe) later on 'or other pro.ects30 

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%ischarge the power supply by letting it sit unconnecte 'or a 'ew ays0  'ome people suggestattaching a 6> ohm resistor between a black and red wire (from the power cables on the output side),however this is only guaranteed to drain the low voltage capacitors on the output * which aren7tdangerous to begin with3 It could leave the high*voltage capacitors charged, resulting in a potentially

dangerous * or even lethal * situation.

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4ather the parts you nee: binding posts (terminals), a 89: with a current*limiting resistor, a switch(optional), a power resistor (6> ohm, 6>/ or greater wattage, see Tips), and heat shrink tubing.

5pen up the power supply unit by re)oving the screws connecting the top an the botto) o' the PS" case0 

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Bunle wires o' the sa)e colors together0  If you have wires not listed here (brown, etc), see the Tips. The color code for the wires is" Fed G ;<?, @lack G round (>?), /hite G *<?, !ellow G ;6D?, @lueG *6D?, -range G ;=.=?, $urple G ;<? 'tandby (not used), ray G power is on (output), and reen G$'H-0 (turn :& on by shorting to ground).

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%rill holes in a 'ree area o' the power supply case by )aring the center o' the holes with anail an a tap 'ro) the ha))er0 Use a :remel to drill the starting holes followed by a hand reamer toenlarge the holes until they are the right si5e by test 2tting the binding posts. Also, drill holes for thepower -0 89: and a $ower switch (optional).

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Screw the bining posts into their corresponing holes an attach the nut on the bac0  

Connect all the pieces together0 

o &onnect one of the red wires to the power resistor, all the remaining red wires to thered binding postsC

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o &onnect one of the black wires to the other end of the power resistor, one black wire tothe cathode (shorter lead) of the 89:, one black wire to the :&*-n switch, all the remaining black wiresto the black binding postC

o &onnect the white to the *<? binding post, yellow to the ;6D? binding post, the blue tothe *6D? binding post, the gray to a resistor (==> ohm) and attach it to the anode (longer lead) of the89:C

o

0ote that some power supplies may have either a gray or brown wire to represent 1power good1#1powerok1. (Most $'U7s have a smaller orange wire that is used for sensing** =.=?* and this wire is usually pairedat the connector to another orange wire. Make sure this wire is connected to the other orange wires,otherwise your lab power supply won7t stay on.) This wire should be connected to either an orange wire(;=.=?) or a red wire (;<?) for the power supply to function. /hen in doubt, try the lower voltage 2rst(;=.=?). If a power supply is non AT or AT compliant, it may have its own color scheme. If yours looksdierent that the pictures shown here, make sure you reference the position of the wires attached to theAT#AT connector rather than the colors.

o &onnect the green wire to the other terminal on the switch.o Make sure that the soldered ends are insulated in heat shrink tubing.o -rgani5e the wires with an electrical tape or 5ip*ties.

Chec 'or loose connections by gently tugging on the)0 Inspect for bare wire, and cover it to

prevent a short circuit. $ut a drop of super*glue to stick the 89: to its hole. $ut the cover back on.

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Plug the power cable into the bac o' the power supply an into an AC socet0  %lip the maincuto switch on the $'U if there is one. &heck to see if the 89: light comes on. If it has not, then powerup by 4ipping the switch you placed on the front. $lug in a 6D? bulb into the dierent sockets to see if the $'U works, also check with a digital voltmeter. Make sure you do not short any wires out. It should

look good and work like a charm3

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6ow to A 7ariable 7oltage to 8our ATX BaseBench Power Supply

If you7ve built a bench power supply using an old AT computer powersupply, you may feel limited by the choice of voltages ; =.=?, ; <?, and ;#*

6D? :&. 'uppose you7re breadboarding a circuit that7s meant to be run o aJ? batteryK This is how to build an add*on variable*voltage 1module1 foryour power supply.

•  The hand drawn circuit is the same circuit drawn in a bit more pcbboard friendly way using a 8M=6L regulator. There is one caveat of using;6D? and *6D? to achieve D? as we will be doing" the ;6D? can typicallysupply lots of current * NA minimum for a really small supply, often double ortriple that. The *6D? line, however, can often only drain a fraction of that. Mysupply is rated for .=A on the *6D? line, for eOample. @efore you add thismodule, you7ll have to make sure that your *6D? line is rated for 6.<A at aMI0IMUM. If you7re drawing for your project much less than the 6.<A maO of the regulator, you may be 2ne, but you could very easily run into problemslater.

14ather the )aterials re9uire an construct the circuit 'ro) thecircuit iagra)0  It would be a good idea to get the datasheet for theregulator from the manufacturers website.

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4et so)e banana leas an connect the 1#7 an (1#7 outputs'ro) your )oe ATX supply an connect it to the input o' yourvariable )oule0 Use a multimeter to measure the output voltage.

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5nce you have built the circuit test it care'ully an )easure theoutput voltage0 !ou should be able to vary the voltage from about 6.<? upto DD? by turning the variable resistor. If you are using the 8M=6L the output

current will be restricted to 6.<A, if using the 8M==PE it should be slightlyhigher check the datasheet for eOact information.