Introduction: A Maker's Guide to ATX Power Supplies

There are tons of Instructables on how to hack a discarded ATX (computer) power supply unit (PSU) into a lab bench power supply. This is not one of them.

My observation is that most makers, hackers, and mad scientists don't follow instructions very well. We like to improve on what already exists and fit the product to our needs. My objective here is to give you all the information that you might need in order to hack an ATX PSU to meet your needs. This isn't so much an Instructable as it is an Informable. This is meant as a reference guide and so I've tried to avoid extensive explanations. If you want a greater depth of information, see Step 6.

Table of Contents

"Introduction" ~ <-- You are here.
"Step 1" ~ Some background information and suggestions for projects
"Step 2" ~ A list of standard wire-colors and functions in an ATX PSU
"Step 3" ~ A chart of wire-associations on ATX connectors
"Step 4" ~ A chart of wire-associations on other connectors (molex, floppy, PCI-e power, etc.)
"Step 5" ~ Tips, tricks, warnings, anomalies, and conspiracy theories
"Step 6" ~ Sources & further reading

Requisite Disclaimer: Electricity is scary (where fear is equal to voltage x amperage / [stupidity + EtOH]). If you're not afraid of electricity, then there's a good chance that you don't understand it and should do more research before messing with it. Everything in this Instructable is correct to the best of my knowledge, but please use your own good judgement. I am not a licensed electrician, professor of physics, PhD engineer, or any other remotely-credible title. You should clearly not trust anything that I have written. By using this information, you are agreeing that I am not responsible for any of your actions or their results, even if I totally am.

Step 1: Background Information & Suggestions

So, you want to hack an ATX PSU, but you don't really know where to start? There are basically two common hacks. Either,

A. Open the PSU; cut the connectors off the wires; drill holes in the existing housing; insert binding posts, switches, etc.; and attach the wires appropriately (See the photos for my version of this method).


B. Build a box with the requisite binding posts, switches, LEDs, etc.; wire them to ATX and Molex connectors; and plug your creation in to an unmodified PSU.

Method B is more work, but allows you to easily swap out the PSU when you burn it out doing something dumb. (Did you see that pretty power supply box in the photo? it's now a paperweight. With binding posts.) It also gives you more room to add functionality. And you get to build a box! Birdseye maple and dovetails, anyone?

One piece of vocabulary before we continue: A "rail" generally refers to a PSU's output of a particular voltage. In the context of ATX PSUs though, "rail" refers to each output that has a separate group of current-regulating circuits. This nuance is why it makes sense to talk about having multiple +12V rails in some PSUs.

Step 2: Wire Colors & Functions in the PSU

Upon opening a PSU, you will find that it is a jubilee of wires in the worst way possible. Thankfully, the colors are (typically) standardized as shown in the chart. Your PSU may not have all of the wires on the chart, depending on the age and output of your unit. See the notes in the chart.

Remember, you should probably only be messing with the wires that would be accessible without opening the PSU's case (i.e. wires that exit the PSU and end in connectors). If a wire is completely internal to the PSU (i.e. starts and ends its run inside the PSU's enclosure), you probably don't need or want to change it. The chart only shows the wires that leave the enclosure.

Whatever nefarious plans you have for these wires, realize that each pin in an ATX connector is rated to a max of 6 amps. It might be a prudent assumption that other portions of the circuit are similarly rated. So, for instance, if you plan to use all 20+ amps that the +5V rail can throw, you should probably connect as many of the red wires as practical to whatever you're powering. That might mean sticking a bundle of wires on your binding post (if you're using option A from Step 1), or it might mean taking connections from the ATX connector and a few molex connectors (if you're using option B). Either way, the more wires, the better.

You should also know that a PSU (because it is a "Switched-Mode Power Supply", or SMPS) requires a minimum load in order to provide consistent voltage. Most people suggest taking one of the +5V (red) wires and wiring it to ground (black) through a 5 or 10 ohm, 10 watt resistor. For some PSUs, the fan is just enough load to get them to power up without the resistor, or they may have a resistor built in. Given that not having a large enough load will detract from a unit's reliability in unpredictable ways, I would strongly suggest using an additional resistor (or use your +5V to power a tiny USB hot-plate to keep your coffee within spilling-distance of your sensitive electronics--that's fine too). If you want to waste the minimum amount of power, or if your unit behaves erratically, you can usually look up your PSU's minimum loads for each rail on its spec sheet. From there, you can probably figure out which rails need resistors, and how big those resistors need to be.

Step 3: The ATX Connector

The two common ATX connectors are 20 and 24 pins. There are also 20-pin connectors with adjunct 4-pin connectors that can be mated together to function as a 24-pin connector. 24 pins is the newer standard.

Chances are that if you have a PSU with a 24-pin connector you will not have a -5V rail.

You can also buy adapters to use a PSU with 20 pins on a 24-pin motherboard connector or vice-versa. These adapters are also a good source of a pre-wired connector if you don't want to scavenge one off of a motherboard and then spend an eternity soldering wires to each pin. (Mmmmm, de-soldering and re-soldering 20+ connections. Have you gotten your California-recommended dose of lead today?)

If you are planning on connecting to your PSU through the connectors (i.e. "method B"), you will need to use more than just the ATX connector to safely use the PSU's full power. As mentioned in "step 2", a conservative estimate would be to use at least one supply wire per 6 amps carried (based on wire-gauge-to-amperage tables, as well as the max rated amperage of a molex connector).

Image Credit: Please note that the supplied graphical chart is not my work. It is reproduced here for informational purposes only from This site is also linked to in References & Further Reading (step 6).

Step 4: Other Connectors

In addition to the ATX connector, PSUs will have some or all of the following additional connectors:

~ 4-pin molex connectors (aka "Peripheral Connectors")
~ +12V2 connector (aka "P4 12V" connector)
~ PCI Express (PCI-e) power connector
~ SATA power connector
~ Floppy drive power connector
~ Aux power Connector(s)

I would suggest that you ignore the last three connectors listed, as they are small and fiddly to deal with.

To safely get the maximum power from your PSU, you are likely interested in the 4-pin molex, which will give you more wires coming from the +12V1 and +5V rails, allowing you to spread the load and lower the resistance. Many PSUs have a "daisy chain" of multiple 4-pin molex connectors that are connected in series on the same set of wires (see the last picture). Obviously, you only need to connect to one connector per set of wires.

The +12V2 connector is intended to be connected to some motherboards in order to supply additional power to newer, larger processors. It is labeled "+12V2" because it is usually a completely separate rail from "+12V1". On the PSU's label, it will give separate maximum loads for each +12V rail. You can use them separately, or wire them together to achieve a higher maximum load. Realize that the PSU may have a maximum total load as well, or that there may be maximum loads for groups of rails (e.g. +12V1 is rated to 16A and +12V2 is rated to 14A, but the side panel may say that maximum load for both +12V rails is 20A, so even if you wire them together, you're not going to get the 30A that you might have thought).

The PCI-e power connector is intended for graphics cards with power demands higher than 75W. It will likely only be present on fairly new PSUs that supply >450W. These connectors may have rails of their own ("+12V3" and up), or they may not.

Image Credit: Please note that the two supplied graphical charts are not my work. They are reproduced here for informational purposes only from This site is also linked to in References & Further Reading (step 6).

Step 5: Tips, Suggestions, & Anomalies

As I mentioned in a few of the previous "steps"; there will be maximum rated outputs for each rail individually and probably also for groups of rails. See the first image for an example of how a PSU label shows these limitations.

If you are planning to modify a PSU ("method A"), be sure to test it for functionality before you put all the effort into modifying it. You can do this by plugging the PSU in and then using a small piece of wire to connect the green "PSU on" pin on the ATX connector to any of the black "ground" wires. This will turn it on so that you can check the outputs with a voltmeter. Don't forget to check for +5V on the grey "self-test OK" wire. Alternatively, you can use a power supply tester designed to test ATX PSUs, which you can typically buy for around $20 USD online. I would recommend against testing an unknown or salvaged PSU by installing it in a computer, both because it may damage the computer and because your "testing" may not be comprehensive.

If you plan to use fuses on your project, put your fuses on the outputs, NOT the ground (Yes, you will need quite a few fuse-holders). The PSU already has an internal fuse or other overload protection, so using fuses mostly enables you to protect whatever you're powering from the PSU's full wrath. You could also use small circuit-breakers, which are available at many electronics retailers.

In order to figure out which of a PSU's +12V lines are on different rails, you can (after unplugging it) use a multimeter to check resistance between the +12V pins on the different connectors. Any resistance greater than essentially zero is indicative that the two connectors that you are testing are on different rails.

If you are adding components inside the PSU's enclosure, be careful to leave enough space for airflow so that the PSU can still cool itself. if you need more space, you can always move the fan onto the exterior of the case, using the original screw holes.

If you drill any holes in the case, try to keep the metal filings out of the electronics!

If you would like to add a variable voltage feature to your PSU, there are several ways to do it (see some of the links in step 6). The simplest way is to use a potentiometer (a variable resistor), which will probably limit the amperage of your adjusted voltage to somewhere between 1 and 2 amps (unless you find a really monstrous potentiometer, in which case, please buy me one too). Another option is to use adjustable regulators (e.g. the Texas Instruments LM338). This approach would be more complex, but could allow for a higher maximum amperage on your variable output (see the comments for more discussion on this topic).

Step 6: Sources & Further Reading

Many thanks to everyone who has gone before me and made this type of information available on the Internet. I can't lay claim to very much original research, but I hope that you have found my aggregation and condensation of information to be useful. If you need more information or more detail, I'm happy to try to help you myself, but you many find the following references helpful as well. Also have a look at the comments section on this Instructable--I've answered a few questions there, when the entirety of the answer didn't seem to fit into the rest of the Instructable. I hope that you take your new knowledge and parlay it into some serious DC amperage. Remember, if knowledge is power, then knowledge of power is power squared.

General Information on ATX PSUs:
Wikipedia's page on Computer PSUs
Wikipedia's page on the ATX standard (link is pinned to the section on the PSU connector)
Great info on load-balancing and rails, as well as a nice page on connectors
Pin-outs for all common PSU connectors wants you to know how every connector ever is pinned!

Helpful Project Pages from Outside Instructables:
WikiHow has a page with some helpful photos and good tips
This page has good information, but I wouldn't follow his instruction to only use one wire per binding-post!
From the above page: From 1996-2000, Dell used non standard wire-colors! Gah!
An interesting project to create a variable-output benchtop power supply from an ATX PSU

Other good Instructables on PSU modification:
Excellent diagrams here, and good ideas about fuse-use
A very slick supply, built to allow PSUs to be easily exchanged (<-- this project is my personal favorite)
This hacker has created and sells an adapter to easily use a PSU's power without opening it up
(I would note that the product sold in the last link only uses an ATX connector, so you probably can't safely use the PSU's full power with such a product.)


misterxp made it! (author)2017-07-16

Thanks LynxSys! Very interesting ad very useful

NathanG107 made it! (author)2017-05-09

Hey i need to generate 12v 5a from my dell power supply, I am just trying to get enough power to run my laptop, in the end, i cut yellow and black to play around with from the position you have pictures of in step 4, the first pic with the 4 ports, 2 black 2 yellow, only one of each, but this is not working?

Please help :) thanks in advance

LynxSys made it! (author)LynxSys2017-05-10

Can you be more specific about what's not working? Am I correct that you're using wires from the +12V2 connector, but they're not giving you any power at all? Are you getting power on other connectors? Is the PSU plugged in and switched on? Is the green "PSU on" wire attached to a ground wire? Is the PSU's fan spinning?

Also, a side note: laptops are easy to fry and hard to fix. If the laptop is valuable to you and you're at all unsure about what you're doing, consider buying a laptop adapter instead of jury-rigging a PSU.

theGiantMidget2 made it! (author)2017-01-04

does anyone know why all my leads are reading double the proper voltage?

LynxSys made it! (author)LynxSys2017-01-05

It's hard to answer that without knowing more about your setup. Could you explain a bit more about your configuration or post some pictures?

theGiantMidget2 made it! (author)theGiantMidget22017-01-05

Hey thanks for the reply,

I actually figured it out, ya ready for this: I was using the AC section on my multi-meter instead of DC. Very simply solution :)

I'm no noob to transformers, power supplies, magnetrons and the like either I just made a dumb mistake :)

javier_orellana made it! (author)2016-04-11

I have a problem, there`s not green cable.
And i got blue/white +3.3V and a blue +3.3VS, ¨S¨ means what?.
I think the blue/white +3.3V instead the orange?. Please Help!!!

DaveZ9 made it! (author)DaveZ92016-10-15


zanef1 made it! (author)zanef12016-04-25

pin 14 and black ground not always green

javier_orellana made it! (author)javier_orellana2016-04-11

This is the Power supplie especs:

LynxSys made it! (author)LynxSys2016-04-17

That is very strange. Dell has used non-standard wire colors in the past, and it looks like they've done it again. I am hoping that the pin-placement is mostly the same, so that you can use the chart from "Step 3" (the one that shows the ATX connector from the pin-side) to figure out what your colors mean. Your pictures look like some of the pins might be different, though, so I would suggest that you also look to see if the wires have anything written next to the contact points where they are attached to the circuit board. Additionally, you should use a multi-meter to check all of the voltages and confirm your suspicions.

About the "+3.3VS" wire: I think the "S" probably stands for "sense." The 3.3 volt sense wire is normally either brown, or a similar color to the 3.3V wires (typically orange). If I'm right, then it needs to be connected to the +3.3V wires so that the PSU can properly regulate its 3.3 volt output.

Mark1207 made it! (author)2016-07-08

Hey i just wanna ask i can add another post for a 5volt with a 4amp or 5amp current? Because im gonna use that to charge 18650 batteries..

Is there a possibility to do that?

And also all of 3.3, 5 and 12 volts are all 16 amps, where should solder the brown wire?

My psu is Delta Electronics DPS-220U B-5 A if you wanna check it out..

I really need you held right now man..

LynxSys made it! (author)LynxSys2016-07-09

See my reply to your other comment.

mspencer8 made it! (author)2016-07-08

Hey i just wanna ask i can add another post for a 5volt with a 4amp or 5amp current? Because im gonna use that to charge 18650 batteries..

Is there a possibility to do that?

And also all of 3.3, 5 and 12 volts are all 16 amps, where should solder the brown wire?

My psu is Delta Electronics DPS-220U B-5 A if you wanna check it out..

I really need you held right now man..
LynxSys made it! (author)LynxSys2016-07-09

I will answer your questions below, but I want to start with a warning: DO NOT try to charge an 18650 or any lithium-ion battery using a homemade charger. Charging a lithium-ion battery requires specific voltages and a charger capable of monitoring and supervising the charging process. If you incorrectly charge a lithium-ion battery (including overcharging a cell, trying to charge an over-discharged cell, or supplying the wrong voltage to a cell), it will likely catch fire, explode, or both. I hate to discourage anyone from working on a project that they think is interesting, but charging lithium-ion batteries is dangerous, and building a charger for them should be considered an advanced electronics project.

I would suggest that you buy a well-reviewed, commercially-available charger for your 18650's. Also, I would suggest that you only use protected 18650's (meaning that the batteries have a small circuit board in them that prevents them from being over-discharged), especially if you are using the batteries to power DIY projects. Over-discharging a lithium-ion battery can be nearly as dangerous as improperly charging one.

Finally, to answer your original questions: you can add as many binding posts as you want to any of the rails, but remember that those posts as a group will only ever provide a total amperage up to the rated amperage of that rail. If the color-standardization of your wires is normal, then the brown wire is a sense wire for the 3.3V rail and you should attach it to that rail. If you're unsure, then check if the wire is supplying any voltage (a sense wire shouldn't be) and confirm that it is in the same position as the sense wire in the ATX connector by using the wiring charts in "Step 3".

67spyder made it! (author)2016-06-22

Hi all, having built a couple of these I would like to make a suggestion that will hopefully save someone some time. Test your PSU BEFORE you go to the effort of cleaning up cleaning up the wires and mounting the power terminals. I'd like to say I'm smart enough to just know this but I did actually go to the work to convert a PSU then discovered I had started with a dead PSU lol.

LynxSys made it! (author)LynxSys2016-06-23

That is an excellent suggestion. I will add it to the Instructable for future readers. Thanks!

ADP3 made it! (author)2015-12-22

This is great. I am making a second one. I used a 12v indicator light from radio shack connected to 12v + and ground. This keeps the unit on but some powersupplies made before 2000 have a different wiring configuration. Thanks for the idea great page.

LynxSys made it! (author)LynxSys2015-12-27

I'd like to hear about the different wiring configuration that you encountered so it can be included in this Instructable for everyone's future reference. Let us know?

cwillson made it! (author)2015-12-12

Thank you for a great Informable. It helped me a great deal with my conversion.

That said, I have a problem. Even though I used a sandbar on the 5v rail, the PS won't run when switched on.

If I plug a auto tail light inot to the 12v, it powers right up, so it's apparently not getting the power draw it needs to run by itself. Any suggestions?

LynxSys made it! (author)LynxSys2015-12-22

Hey, that's a great build! I really like the fuse-holders and the professional look of the enclosure.

As to fixing your problem, it sounds like you've already figured out that your PSU is designed to require some load on the 12V rail, so the simplest solution would be to just keep using a 12V bulb to provide that load. If you want a more elegant solution, I suggest that you try to find the spec sheet for the specific PSU that you're using. The sheet should list minimum loads for each rail, from which you can calculate what size of resistor you need to put on the 12V rail. If you can't find a spec sheet, you could calculate the size of resistor that would directly replace the bulb that you're currently using by learning the bulb's wattage (you already know that it's 12V). If you would like me to explain the calculations needed to select a resistor, just ask!

Thanks for taking the time to share your hacked PSU; it looks great!

rban1 made it! (author)2015-11-13

i made it but i was lacking out of items soo i did i on my way.

Metalman000 made it! (author)2015-10-22

Great Instructable. Funny, informative and extremely readable. Thanks for the info :)

russ_hensel made it! (author)2015-01-09

Nice job on links. Also: Just a note to let you know I have added this instructable to the collection:
Encyclopedia of ATX to Bench Power Supply Conversion
Take a look at about 70 different approaches to this project.

faizal.mohdzulkafli made it! (author)2014-11-25


My PSU have a yellow with black stripe. Looking at your wire colour diagram I can see that the yellow with black stripe is +12V (2nd rail). The question is, can I combine the wire with the yellow +12V (1st rail)??

LynxSys made it! (author)LynxSys2014-11-26

Yes, that should be fine. Doing so will combine the maximum load capacities of the two rails.

arush2 made it! (author)2014-06-04

hi friend

currently I'm working with some dc motors and i need negative voltage with high current, but when I connect the GND of the "power supply 1" to the 12V of the "power supply 2", to use the GND of "power supply 2" as -12V, i have a short circuit, and therefore the "power supply 2" is switched off. Is there some way to get -12V with high current?

LynxSys made it! (author)LynxSys2014-06-05

If I'm understanding you correctly, you don't actually need two PSUs for what you're trying to do. Read my reply to General Eggs below. Think of "Ground" as "0 volts", and remember that voltage is a measurement of difference in electrical potential between two contacts. In your case, if your motor is hooked up to +12V, all you need to do in order to give it -12V (and to get it to spin the other way, which is what I'm assuming you're trying to do) is to reverse the polarity by switching the contacts.

arush2 made it! (author)arush22014-06-05

thanks for your response :), the problem that i have is due to the driver that im using to control the motor, i have a analog output signal of -10v to 10v from the PC (data acquisition card), and the driver only converts this signal in "high current voltage" (the driver is similar to use the h-bridge but using two transistors), then i can control the movement speed and the rotation direction using one output, but for the driver i need -12V and 12V with high amperage, but when i use the 2 atx in parallel to get 12 and -12v volts one of them is turned off :S.... then i dont know if it is possible to connect two atx in parallel, or is necessary to do other thing

LynxSys made it! (author)LynxSys2014-06-09

It sounds like your issue is that you are connecting two PSUs to the same system, but trying to set them up with different absolute* voltages as "ground". Whatever you decide to use as "ground" must be the same across the whole system. For example, you cannot configure one side of the system to use 0V as "ground" and +12V as "+12V", while setting up the other half of the system to use 0V as "-12V" and +12V as "ground". When those disparate values meet in the middle, things will not work as expected.

*For the sake of this discussion, I have called the ground (0V) that the PSU is getting from the wall "absolute" and notated voltages that are relative to it without quotation marks. I realize that this is technically incorrect, but it adds clarity in this case.

APuckNut made it! (author)2014-05-06

Firstly, I have to say a big thank you. I just salvaged a psu from a machine set for the e-wasting station.

Secondly, I have a question. Is it safe to and could I use one for the ground connections to provide me with a safe place to plug a anti-static wrist strap into? This seems like a logical thing but i am not certain.

Any help or guidance you can offer is greatly appreicated.


LynxSys made it! (author)LynxSys2014-05-10

I'm glad that this has been helpful; let me know how your project turns out! Your idea does seem logical, and it should be fine as long as your wrist strap is a proper one with a resistor that will bleed current away slowly. Typically I just connect my wrist strap to the chassis of whatever electronics I'm working on. If you're constructing circuits from scratch with delicate components, you might also consider using an antistatic mat that has an attachment point for a wrist strap.

APuckNut made it! (author)APuckNut2014-05-10

Thanks for your reply. I will let you know how I get on. I am planning to combine your instructable(s) with this one to turn the case to turn it in to a project box

Should turn out quite nice. (I hope)

I have a mat and a strap and indeed I familiar with connecting the strap to the thing I am servicing. It just seemed to make sense to me to have it attached to something a little more permanent.

Tanmcnew made it! (author)2014-04-16

Great Instructable! very clear and informative, i;m going to build this for sure! i have like a billion PS's lying around

mahf1230 made it! (author)2014-02-04

excelent work, im working on my own


msmesm made it! (author)2013-08-20

it is very useful project thanks for all of you

comebackin made it! (author)2013-08-15

I found this schematic of a 10A adjustable regulator using the LM350 in the data sheet for the component. I hope it helps.

10A adj reg.jpg10A adj reg.jpg
Homepwner made it! (author)2013-07-15

Hello, General Eggs. I am by far an expert of any kind but what I do know is that some chips require both positive and negative voltages to get the correct output. As LynxSys said it is a voltage that is comparative to Ground or Common. I have a chip that I'm interested in exploiting, The ICL8038. It is a function generator chip, that can produce sine waves,square waves,sawtooth waves and f.m. sweep. If I understand correctly, it's like a DC representation of an AC waveform The sine wave and FM sweep requires voltages to come above 0 and dip down equally below 0v. Sawtooth and square waves start at 0 and rise and fall in the positive voltages. I hope this helps.

iTixle made it! (author)2013-06-27

If you want variable voltage that can deliver a higher current than 1-2 amperes, there is a better way than with a monstrous potentiometer. Instead, you can get a relatively low current potentiometer and use it to control the voltage flowing between the collector and emitter of a high current transistor such as the 2n3055. Since high current transistors are much cheaper than high current potentiometers, this is a great way to get high current with variable voltage output. :)

zackandreweli made it! (author)2013-05-13

Hi, could you post the link you got your binding posts from, the only ones I can seem to find are for speakers. Or are they ones that are for speakers?

LynxSys made it! (author)LynxSys2013-05-14

I just used what was available at my local electronics store for less than $1 each. They're standard "universal" or "five way" binding posts that accept a banana jack (which are often categorized as audio connectors), similar to these:

It would actually be safer to use safety banana jacks instead, but binding posts are more versatile so some people prefer them.

LynxSys made it! (author)2013-04-14

It sounds like the initial surge of the motor starting up is tripping the PSU's internal protection circuits, which disabling the unit until it's turned off and back on. I'm a little surprised that a motor that's supposed to only draw 14A appears to be peaking above 21A, so hopefully there's nothing else going on. Assuming my guess is correct, the best way around this issue would be to use a soft starter. Of course, that's complicated. For a quick and dirty fix you might try powering the motor from both of your 12V power sources at the same time to increase your max amperage and spread the load. As always, be careful!

nick.brett made it! (author)nick.brett2013-04-15

That's what I was afraid of.  I have a request into the company that builds the compressors to see if they have any ideas.  I suppose I could test this by putting a 20 amp fuse inline and see if that blows.  My multimeter only goes to 10 amps, so I can't check the draw that way.

To use both of the power supplies at the same time I would just wire the compressor's positive to the positive of both supplies and do the same with the negative correct?  Is there a chance that may cause a problem for the power supplies?

nick.brett made it! (author)nick.brett2013-04-15

When I got home from work today I put a 20 amp fuse inline with the compressor and to my surprise it did not blow, so it doesn't appear to be the startup surge. I also tried a 3 amp, and that did blow. I guess this is good, but now I don't know where to go from here.

I'm also not sure why Instructables won't let me reply. The captcha is missing.

LynxSys made it! (author)LynxSys2013-04-16

The issue could still be a surge. Fuses don't blow instantly--it takes some time for a fuse to blow, and that time becomes shorter as the overcurrent becomes greater. Fuses are also available in different time characteristics, which are marked on the fuse (from fastest to slowest) as either "FF", "F", "M", "T", or "TT".

About using both PSUs simultaneously: yes, you would wire them in parallel as you described. Such a setupmight be your best bet for now, although it's a little bit jury-rigged for permanent use.

General Eggs made it! (author)2013-04-10

What would you use the negative voltage terminals for? Do they have any actual applications?

LynxSys made it! (author)LynxSys2013-04-10

I typically use them to get voltages that the PSU can't normally supply. For instance, hooking up a device to the -12V and the +12V is equivalent to hooking the device up to ground and +24V. The key is to think of "ground" as just being "0 Volts", and remembering that voltage is just a measurement of difference in electrical potential between the positive and negative leads.

There are other rarer and more technical instances in which negative voltage might be required (some sensors need negative voltage, for instance), but those examples get fairly specific and I'm much less familiar with those applications.

A note specifically about ATX PSUs: The negative voltage rails seem to be less electrically protected than the ground. I once connected the +12V to the -12V through water during an electrolytic rust removal experiment and it smoked the PSU (seriously, it smelled like burning) in a matter of seconds. The very same circuit had been fine for hours when connected between the ground and +12V terminals.

General Eggs made it! (author)General Eggs2013-04-11

Ok thanks, that was helpful. Another question; the +12v rail on my psu I want to modify runs excruciatingly low (its never topped out at more than 10v). Would it be safe to hook the negative terminal to the -12v and positive to ground to get a positive 12v?

LynxSys made it! (author)LynxSys2013-04-11

That would work in all of the examples that I can think of. The only downside is that the -12V output is usually rated for a relatively low amperage. On the PSU used in this Instructable, the +12V rail is rated to 8A, whereas the -12V rail is rated to a tenth of that at 0.8A.

About your PSU, I'd be wary of using a unit that seems to have some type of current-regulation failure. Typically, PSU voltages should be quite well regulated. Check if there are any "sense" wires that are disconnected, and also see if the grey "self-test OK" wire is actually returning +5V. If you can't figure out the issue, I would suggest that you find a different PSU.

Good sources for a serviceable (and free) unit might be the old computer and TV disposal area at your local waste disposal facility (they're usually classed as "hazardous waste" because of the lead solder, among other things), or even a local computer shop that might have some old machines that they're getting rid of. I can usually find a computer somewhere that's too old for anyone to want, but has a perfectly good PSU.

ZenerD made it! (author)2013-04-01

My PSU works without the Sandbar resistor, probably because of it's age. I'm thinking I may just use the resistor though to ensure a stable supply.

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Bio: I'll tinker with anything, but I'm often to be found repairing, improving, and restoring things. I'm always looking to develop new skills ... More »
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