# Power supplies for computers



## Harold_V (Feb 2, 2013)

Would one of you guys tell me what voltages are available from power supplies for the typical desk top computer? What I'm looking for is 12 volts, DC. 

I an looking for a clean DC supply to power some LED light strips. The power supplies I purchased with the light strips are solid state, not wire wound, so they transmit a noise that is picked up by my FM receiver. Quite noisy, and annoying. I powered the strips with an old battery charger and got dead silence. 

I am assuming that the supply from a computer is most likely filtered, so there is no noise. I am using them in three different locations, so I need three power supplies. I'd need two amps for one, about four for the second, and less then ten for the third. 

Thoughts?

Thanks for anything you can offer, guys. 

Harold


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## tek4g63 (Feb 2, 2013)

Harold.

Yes a PC power supply has a 12V set of wires. Amperage will vary a bit from model to model. I would be more than happy to wire one of mine up for you and send it to you. No charge. I can send you pictures of how I do it, I just can't get my phone to upload them here. So I'll send them in your email.

I'm actually working on gathering up parts and planning on offering to modify PC power supplies for all members of the forum for free. I want to be able to give something back to the forum.

If you wouldn't mind, could you upload the pictures for me. I'm not quite ready to start modifying a bunch of these just yet, but maybe the pictures would help other members mod one them selves while I gather all the hardwear I need to start helping out.

Thad.


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## darshevo (Feb 2, 2013)

After the 12v benchtop supply I inherited from my dad a million years a go gave up the ghost I made a new on using an old pc power supply (286 era) that had its own power switch. I tore it down, got rid of the case and everything else that it would no longer need than installed it in an old Nintendo 64 case just to make it geeky. Binding posts on the front and the fan attached to the top to keep it cool. It had posts for +3.3, +5, +12 and -12 as well as a ground post. I used it up until I found a regulated supply that was adjustable at a surplus sale. 

To Harold's question the red wires are your +5 and the yellow +12.


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## resabed01 (Feb 2, 2013)

Harold,
Don't all PC power supplies have fans? This would be too noisy for your application I would think. I know you're looking at PC power supplies because the are cheap and plentiful but I'm not sure if you'll be happy with the noise they make.
Laptop power bricks might be a better choice. They are built to be small and efficient (and silent). You can get them in 12v although it probably won't supply up to 10A of current.


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## tek4g63 (Feb 2, 2013)

I also use the binding posts/ banana jack type terminals.

You could add a potentiometer or two to the PC power supply and make the voltage and current adjustable. The one I use for my sulfuric de-plating cell is set up that way and I have jacks built into it so that my multimeter plugs right into it to monitor either current or voltage. If I had two multimeters I could monitor both at the same time I guess. I usually only watch the amperage.


Edit:
Harold I just remembered that I have a power supply that is specifically designed for LEDs. It even has adjustment knobs for brightness. I'll have to do some digging, but when I find it if it will work for you its yours. I got it for free so I am more than happy to pass that kindness on. 

Thad


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## FrugalRefiner (Feb 2, 2013)

Harold,

As Thad said, computer power supplies have a +12 volt rail (yellow wires), a +5 volt(red), as well as -12v (blue) and -5 v (white). The newer ATX power supplies also supply 3.3 volts (orange). Amperage for each depends on the power supply which is usually listed on the case of the PSU. 

Instead of trying to explain the modifications needed to make the PSU operate outside a computer, here are some links I've saved that explaing the much better than I can.

http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply
http://web2.murraystate.edu/andy.batts/ps/POWERSUPPLY.HTM

There are plenty of others out there, but I found these easy to understand. Pretty simple stuff really.

Good luck,
Dave


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## oldgeek (Feb 2, 2013)

Another link that may help.
http://www.instructables.com/id/Simple-Plug-and-Play-LED-Lighting/


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## Harold_V (Feb 3, 2013)

I'm a little late getting back to this topic, but I'd like to thank you guys for all the information. 

It's true that the power supplies can be noisy (because of the fan), but I may be able to place the units where that may not be an issue. I also won't be putting a big demand on them, as it's not likely I'd run all of the strings of lights all the time. The one with the greatest demand would most likely be used sparingly (lighting in a record/CD storage cabinet with many shelves). Pretty good chance the lights would be on only briefly, and not all of them at one time, so the supply would not be putting out much amperage. I would want a supply that was capable, however. 

The other option I have at my disposal is to use wire wound transformers. I tried an old battery charger---which allows for perfectly silent operation. Maybe I need to pursue a source for some small (wire wound) transformers. It appears that switching transistors (to create the wanted voltage) are the problem. 

Again, thanks!

Harold


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## butcher (Feb 3, 2013)

Harold,
I do not know what Kind of LED light strips you are trying to power, knowing that would help in making a decision on the power supply, most LEDs or even light strips are not that critical of how they are powered.

Switching power supplies like computers or digital electronics use can create harmonics on your systems power lines, but the switching power supply do produce a very smooth DC, and you can get them without fans, and in a very wide variety of sizes physically and electrically rated, many types of electronic equipment use this type of power supplies as most digital circuits or computer circuits need very smooth DC, some of these are so common you can buy them very cheap or use on from a scrapped out piece of electronic scrap.

LEDs normally will run just fine even without the DC being smooth you could run them even without capacitor filters on a DC power supply, a capacitor filtered DC power supply's are also very common or can easily be built with a few components.

This is of course unless you did not have some kind of digital circuit as drivers to drive the LED harder or brighter or some type of computerized circuit involved, where would be filtering would be needed, or if computerized then you would need a switching supply.

LEDs do not use much current at all, so you may even be able to get by with a simple wall wart power supply, depending on your current the string of LEDs draw.

Many of the LED light strings are not much more than just LEDs with resistors to limit the current the LED would try to pull from the power supply.

Without knowing more about the LED string it is hard to say what supply you would need, but I would think you could most likely use a simple supply to light the string.

Your supply would need to be rated for at least twice the current the lights pulled, so the supply would not overheat.


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## Harold_V (Feb 3, 2013)

Butcher,
Here's a link to the type of strip I'm using. 

http://www.ebay.com/itm/Blue-5M-300LED-3528-SMD-Flexible-LED-Light-Lamp-Strip-/180727542037?pt=US_Car_Lighting&hash=item2a14337115 

I also have some that are larger, which would require a larger power supply. While larger (and 600 LED's per 5 meters), the lengths in use are short---no greater that about 10 feet, so the overall amperage is not high. 

I am of the opinion that the power supplies that they provide are switching type, but of low quality, with no means to filter noise. The small units plug in (wall wart?) directly to a receptacle, while the larger capacity sizes are hooked up via a cord. The strips work very well, and I'm not the least but unhappy with them, but the noise that comes from the power supply is not acceptable. It manifests itself as a raspy snapping sound. Simply turning on the power supply creates the noise---I need not turn on any of the strips, although when I do, the noise changes. It gets better or worse, but it's always there. 

I'm all ears if you have a solution. Right now I'm thinking it's a matter of making a computer power supply work (assuming it's filtered, and I expect it is), or going to a wire wound transformer and a rectifier. I'm open to anything you care to offer. I know a little about electronics, but it's generally just enough to get me in trouble. 

Harold


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## g_axelsson (Feb 3, 2013)

Try a Quick 'n Dirty (Tm) solution.
First, disconnect the leads to the LED:s on the power supply. Even when the LED:s are turned off the wires could act as antennas. If the noise is still there then the electro magnetic interferance (EMI) is going over the power leads.
Next step, if the noise died when disconnecting the leads to the LED:s then try wrapping them around an empty iron core from a transformer (where the copper core goes) before connecting it back to the power supply.
If the noise didn't stop in the first step then try to wrap the power cord through the transformer core, about four turns usually helps.

This could help with high frequency noises travelling over the power lines.

For a better effect the wires in the power cord should be wrapped in opposit directions around the core for killing common mode interfearance and noise supressing ferrites should be used.

I don't know what the FCC regulations is stating but in Europe, if some electrical equipment is too noisy then someone isn't following the regulations and I could bring it back to the seller for a replacement.

Göran


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## Palladium (Feb 3, 2013)

I know nothing about electricity really, but can't you put a capacitor in the line to stop the noise?


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## butcher (Feb 3, 2013)

Harold
You can run these off your trucks electrical system, with a terribly noisy alternator, and the voltages swinging wildly from zero to 14.5 volt, nothing fancy on those strips just 3 LEDs and a resistor, so your power supply does not have to be anything fancy, just properly rated, for the length you cut the strip.

I will post a little more info later when I have time.


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## lazersteve (Feb 3, 2013)

Stray EMI can also be reduced via twisted pairs of wires. Try weaving two of the LED lines together and see if the noise is negated. From your description of the sounds (raspy snapping) it's most likely stray RF (radio frequencies) or a faulty component in the 'wall wart'. 

Goran has made an excellent suggestion with the RFC (radio frequency choke). These are sold at your local Radio Shack and also can be found on just about every old computer monitor cable. It's the large cylinder shape in line with the video input cable. Find an old computer monitor with the RFC on it and carefully strip back the plastic plenum of the input cable to get to the ferrite choke. You'll see how it's installed when you remove the ferrite core from the cable. More or less you just loop the LED wire a few turns around the RFC.

Steve


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## butcher (Feb 3, 2013)

Harold,
The LED strip is a string of 3 blue LEDs and a resistor, these three LEDs and the resistor is wired in series, to run on 12 volt, then the strip wires these (three LEDs in parallel with the others) to light any length of these you wish use.

There is nothing fancy here, so most any DC power supply will work as long as it is rated to carry the load, and the voltage is fairly close to the operating range, a little lower voltage the light would not be as bright, but if the voltage was not too low you probably could not tell much difference in how much light they produced as long as the voltage was not below a few volts,
(So if you wanted to dim your lights, you could actually wire in a variable resistor in series to the strip of LEDs if you wished to dim the light string this would lower current available to them dimming them). 

LEDs are current operated devices, and can pull more current from the power source than they need, they will pull current until they burn themselves up, this is the reason for the resistor on the strip for each of the three LEDs, the blue LEDs run @ about three volts (a little higher than other color LEDs), so three of these with the resistor to limit current use up the 12 volts supplied.
You know your LEDs (3 each) need 12 volts DC, and these other LEDs on this strip are wired in parallel so 12 volts is supplied to each of these groups of 3 LEDs, which can be cut to any length, so the longer the strip the more the wattage thus the more current needed to run the lights.
The info given to the LED strip was about 24 watts for 5meters (16.4 feet) of LED strip, so with this information we can use OHMs law to figure amperage:
24 watts/12 volts = 2 amps 
So about 16.5 feet of the strip would pull about 2 amps, you would want a power supply rated for double this so a 4 Amp 12 volt DC power supply would work fine.

For about 8.2 feet of the LEDs light strip 12 watts/12 volt would run @ about 1 Amp, so doubling this you could use a 2 Amp, 12 to 13 DC volt plug in wall wart power supply to run eight foot of the lights.

http://www.bing.com/images/search?q=wall+wart+power+supply&qs=AS&sk=AS1&FORM=QBIR&pq=wall%20wart%20power%20&sc=8-16&sp=2&qs=AS&sk=AS1

You can find these wall warts at second hand stores, you can also get switching power supplies just a little bigger than wall warts.

You can also calculate the light string length you could run for a certain size wall wart, if you wished.
You can also use other power supplies if you had long lengths of these to power.

Small switching power supplies can be used but for just running the strip you really do not need anything fancy.

I guess I did not read this close enough to understand the question was about radio frequency noise and not about a computer fan noise.

Radio frequency noise can be reduced sometimes by a simple Choke (a coil of wire wound on an iron, or ferrite core, radio shack have some snap on chokes) or sometimes a simple as looping the wire around a ferrite bead, radio shack have some snap on chokes used for this purpose, other times it may be a problem in the circuit (you could have a faulty wall wart), some time more complicated filters are needed in the circuit. 

Not all wall warts are created equal some may use a single diode while others use a bridge rectifiers, half wave DC will can create more noise than full wave DC.

The wall wart may or may not have filter capacitors (one could be added external if needed), this may help.

The noise RF generated can come from many things, or a combination of things even the light string itself or it can act as an antenna broadcasting a little noise in the power supply into a big noise.

Try plugging the wall wart into a long extension cord to see if the noise goes away, the long wire extension cord resistance can act something like a choke.

See if the power supply itself is noisy, maybe load it with an incandescent car tail lamp from a truck and see if it still produces the same noise.

I am sure they are not generating the noise themselves, but to verify you can operate the LEDs on a battery.


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## Harold_V (Feb 4, 2013)

butcher said:


> The noise RF generated can come from many things, or a combination of things even the light string itself or it can act as an antenna broadcasting a little noise in the power supply into a big noise.


That has proven to be the case. I powered a string near my stereo system, and got more than enough noise. It was absent with a battery charger (old wire wound transformer type). 



> Try plugging the wall wart into a long extension cord to see if the noise goes away, the long wire extension cord resistance can act something like a choke.


I haven't tried that, but I expect it wouldn't be a long term solution, as I have four different places where I'll run these lights, and each of them is prewired for 120 volts AC. A choke might work, but a longer extension isn't possible. 



> See if the power supply itself is noisy, maybe load it with an incandescent car tail lamp from a truck and see if it still produces the same noise.


I've already done that. The power supplies themselves are transmitting noise. I have the option of turning them on, with and without a load. They make noise in all states, so long as they're on. Adding or subtracting more lights may or may not make the noise louder. It changes with each change, sometimes louder, sometimes quieter. 



> I am sure they are not generating the noise themselves, but to verify you can operate the LEDs on a battery.


Unfortunately, they are, indeed, generating the noise. If they are powered up, I get noise, with or without light strings attached. I have five different supplies, two amps, five amps and one ten amp. They all make noise, but they vary widely. I must have total silence, otherwise the lights can't be used. 

Right now, my next move is to try a power supply from a computer. They have the potential to yield more amperage than I require, so they won't run hot. The fan noise will most likely make no difference, as the power supplies will dwell in cabinets, and will rarely be powered on for prolonged periods of time. The one that may see long service is open to air flow, so heating shouldn't be an issue. 

Thanks, Richard.

Harold


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## Geo (Feb 4, 2013)

i remember in older vehicals that had points in the distributor, if you didnt have the little canister type condenser with a single wire, the radio waves from the spark would come through the radio speakers.there may be a simple filter or condenser you can put inline to filter out the noise.


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## Harold_V (Feb 6, 2013)

Well, guys, thanks for all your efforts.. 
I picked up an old power supply (which wasn't dead), and made the necessary conversions so it could be fired up. Hooked up a string of lights and still get noise. I'm going to have to use wire wound transformers if the problem persists when the stereo system is properly set up in the house. I'll be using a roof mounted antenna then, so it may not pick up the noise, which is detected only through the FM receiver. All other modes are dead silent. 

I consider this matter closed at this point in time. 

Harold


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## kronix (Feb 6, 2013)

Harold, have you tried hooking up the antenna in with a coaxial cable from your t.v.? I used to do it with an old VHF or uhf converter ( 2 wires to a coax cylinder ) and got no static. Alternatively you can ground the antenna by plugging the - into the ground hole in any outlet to get the same result. Give it a try, it might work at filtering out the interference.

-Ian


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## kkmonte (Feb 6, 2013)

How about 1 or 2 batteries hooked in parallel with a solar panel and charge controller? LOL maybe too much money then you wanted to spend but that shouldn't have any noise.


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## butcher (Feb 7, 2013)

LED power supplies
http://www.digikey.com/product-search/en/power-supplies-external-internal-off-board/led-supplies/591038

Dummies guide to wall warts and stopping RF noise with simple regulator circuit:
http://www.dxing.info/equipment/wall_warts_bryant.dx


One of the switching power supplies you can buy from Digikey, these are good power supplies and very good prices, this one is similar to a computer power supply only smaller, cost around $20, for this 12 volt 2 amp, they have other types as well with good prices.

http://www.digikey.com/product-search/en/power-supplies-external-internal-off-board/ac-dc-converters/590377?k=102-1326-nd


Harold the only reason to try a long extension cord was to test and see if a filter like an inductor would help (the long extension cord can act similar to an inductor filter coil) before you went out and bought one …


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## Harold_V (Feb 7, 2013)

kronix said:


> Harold, have you tried hooking up the antenna in with a coaxial cable from your t.v.?


At this point, no, I have not, but the permanent installation will be hooked up that way. I have conduit and a weatherhead installed on the roof to accept the cable. Right now I'm using a piece of lamp cord (that's what I had on hand at the time). It works fine as far as reception is concerned. That's part of the reason I decided to put this aside until we move in to the house. I may be pleasantly surprised to discover all of the problem simply evaporate. 



> I used to do it with an old VHF or uhf converter ( 2 wires to a coax cylinder ) and got no static. Alternatively you can ground the antenna by plugging the - into the ground hole in any outlet to get the same result. Give it a try, it might work at filtering out the interference.


Thanks for the idea. I'll employ each of them when I make the move, resorting to a different power supply only after exhausting all avenues. I already have several of the required power supplies. I'd like to use them, especially after spending the money to buy them. 

Harold


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## Harold_V (Feb 7, 2013)

kkmonte said:


> How about 1 or 2 batteries hooked in parallel with a solar panel and charge controller? LOL maybe too much money then you wanted to spend but that shouldn't have any noise.


Yep, noise free, of that I'm sure, but there are several locations (four) that will require power. Couple that with the fact that we live where the sun rarely shines in the winter, and the vast majority of use would occur at night, it's not a viable option for me. I really need to have something that works when I throw a switch and not have to jump through a series of hoops. Beyond that, one of the circuits will have a respectable demand if all lights are turned on simultaneously. That isn't likely to occur, but I must plan for the worst case scenario, so to that end I purchased one ten amp power supply. The others are all smaller, but larger than needed, so they should run cool. 

Thanks for your very welcome comments. 

Harold


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## Harold_V (Feb 7, 2013)

butcher said:


> Harold the only reason to try a long extension cord was to test and see if a filter like an inductor would help (the long extension cord can act similar to an inductor filter coil) before you went out and bought one …


Well, that may explain why I get a reduced noise level when I tested the computer power supply as well as one of the 5 amp supplies that were purchased. My previous test was accomplished by plugging in to the same strip that powers my preamp and tuner. As I had to disconnect another piece of gear to do that, I decided to drag an extension cord in to make it more convenient. I used a 50' extension cord and thought, initially, that the noise was gone, but when I tuned to a distant station, it was clear it was still there, just greatly reduced. 

There's a few things that will be different when the stereo system is installed where it's intended to live. I'll wait until that time before going any farther with this pursuit---I may find the problems take care of themselves. Frankly, I was somewhat surprised to discover the computer supply wasn't dead silent, which makes me think I have other issue that may be part of the problem. 

Thanks for the added information. I especially like the article whereby filters can be added. My power supplies are all larger than the typical wall wart, but I expect that duplicating the filters for larger units would still work. I'll certainly give that idea a go before resorting to wire wound transformers, which I determined to be dead silent (an old analog battery charger). 

Harold


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