Using a PC power supply as driver?

[Slim Picky]

Hi all

I'm thinking of stripping out a PC case to use as a small veg box. I've got a few of the chinese 10w ic led's laying around from a custom car lighting project but no drivers. Since I'm trying to do this as cheap as possible I was wondering if I could use the atx power supply's 12v output?

The leds are rated for 9-12v dc input running at 1A.

I'm fairly handy with a soldering iron but no electronics whizz. I realise that I'd need a constant current source so is there an idiots guide on how I could convert the 12v output from the PC power supply to use with the leds?
I've heard mention of an LM317 but could someone who knows more than I do chime in to let me know if this is doable or if I should just spring for some proper drivers?

I used a 9v wall wart to test the leds and they all fire up, so there is a 3v headroom. I'm not particularly concerned with efficiency, rather simplicity of design and component count.

It just seems a waste to remove the power supply if there is some way I could reuse it easily. It would certainly help the stealth look of the case albeit at the cost of a little internal space.

Thanks for any input

 

[hempfield]

It's very easy to use a stabilized 12V power supply to power up cheap white or blue 10W LEDs. Their forward voltage vary between 10-11V , but if you directly connect them to a 12V power supply they will die soon, because the current will go way up above 900-1000mA.
To avoid this you must decrease the available voltage by adding one ore two rectifier diodes (1N4007) in series with the led. Such diodes have a typical forward voltage of 0.8-1V at maximum current, which is 1000mA.
If you want you LEDs to be bright as hell (well, not as hell, but more like the sun) you use only one diode. For a better light efficiency and less dissipated heat on LED itself, you can use 2 or even 3 diodes.

Please keep in mind that the diodes will run very hot , so I suggest you to cut one terminal very very short (3-4mm) and to solder it directly on the 10W LED fins. In this way, the heat dissipated by the diode will sink via LED fin on the heatsink.

Do not connect two ore more diodes on parallel , as this will kill your LED in case of over voltage.

P.S. For red LEDs, which have a forward voltage of 6-7V, you can connect two in series directly to the 12V power supply, without any additional diodes. They will run bellow 800mA, but this means higher efficiency, more stable wavelength , less heat and longer lifespan.

Simple, don't you think ?

Peace

L.E. I just remembered I have some explanatory pictures on my albums :

 

[Slim Picky]

Wow Hempfield thanks man!

I was thinking it was going to involve some really technical stuff to do it with the PC power supply but a single component?! I think even I can manage that!

The leds you show in your pictures are exactly the kind I have aswell. I realised that I had been looking through some of your posts when I was researching on here and there was a link to the Romanian part of the forums where I saw a build with crees that looked pretty sweet. The more I think about it the more I want to build another small panel and see how well I could flower under leds. Have you flowered anything under the cheap chinese leds?

I am currently going to put four 12v cool white 10w leds in a case for the veg box. Have another four warm whites laying around. Should I build another panel with the warms and see if I could flower or better to buy better crees or something for flower?

Also a quick question about the diodes, what does the 1N4007 actually do to the voltage and current? You mentioned it has a voltage drop, which I think I follow. So does it limit the current to a maximum of 1A, which is what the leds need? Is this then a constant current?
Also if I connect two as you state will that cut the current in half i.e one diode gives 1A so two together will halve it to 0.5A? I understand how this would help in terms of helping the led last longer but it would also mean less light, yes?

And these diodes have a wrong and right way to connect if I remember my basic electronics from years ago. Which side goes to the positive led terminal and which side do I connect the +12v to?

Sorry for firing so many questions at you, I was expecting someone to say it wasn't possible. I certainly wasn't expecting someone to come back and say a single diode was all it needed! So thanks again man!

Slim

 

[KRANKENWAGEN]

hemp mate, the math you gave was for 10W LED, correct?

 

[hempfield]

hemp mate, the math you gave was for 10W LED, correct?

Here I will try to answer to both of you :

Yes, the math is only for 10W LEDs which runs at 10-11V, 900-1000mA. Pay attention of what type of LEDs do you have, as on the market there are also 10W LEDs running at 36V 350mA.

Yes, I flowered ONLY under cheap chinese leds, and I was very pleased by the final product.

A PC switching power supply is supposed to have a pretty stable 12V output. A 10W LED only require a 10-11V voltage when it is run at nominal current of 900mA.
Because of the equation V=I*R, where V means voltage, I means current and R mean resistance, any additional voltage over this value will cause an increase of the current, because we can consider that the internal resistance of the LED remains constant (which actually is not quite true, because it varies base on temp and high current).

So the role of the 1N4007 diodes is to steal some voltage (0.8-1V) from the main 12V, just enough to prevent the running current to rise. This diodes have polarity, the negative terminal being always marked with a grey/silver line on the body. So if you use two diodes for one led (which I strongly recommend), on the positive terminal of the led you should solder a negative terminal of one diode, and on the negative terminal of the led connect the positive terminal of the second diode , like this : +++DIODE---|+++LED---|+++DIODE---

Using such diodes, also protect your LEDs by accidental reverse polarity and they will only light up when you connect the power supply properly.

Also this diodes have a maximum running current of 1000mA, so if the current rise above this limit , they will die, protecting the LED. They are so freaking cheap and I always buy 100pcs pack when I replenish my spare parts.

When you will test this , you probably notice that the current will be slightly bigger when the LED is cold and grow with a few miliamps. when it reach the running temperature (which I always try to keep as low as possible).

 

[KRANKENWAGEN]

you know, the saddest thing is that i actually understand the theoretical part, i just have exacty zero real world experience, and THAT makes it so frustrating

 

[hempfield]

And old saying here in my country states : Hunger is the best chef !

If you understand the theoretical concepts, the experience will start to accumulate when you will begin to play with LEDs. They are so cheap now that you can't be upset if something goes wrong.

You only need :
- leds ;
- diodes ;
- wires ;
- a 12V power supply ;
- a solder low power solder iron (30-40W) , solder alloy and some flux ;
- spare time - the more, the better

 

[Jhhnn]

It's been awhile since i fussed with PC power supplies, but there are a couple of things that might be helpful.

Some need to have a load on the +5V rail to work at all, even if that's just a high value resistor to ground.

Some have internal potentiometers to adjust output voltage.

Some have separate discreet output rails of the same voltage. If the maker says 300W on the 12V rail, for example, that can be 2 outputs of 150W each, on separate wires intended to power different parts of the computer.

At the motherboard connector, jumper the green wire to any black to turn on an ATX supply.

Really cheap PSU's won't last for long at anything near their "rated" output capacity.

 

[KRANKENWAGEN]

ok
so, a bit of theorycrafting:
i have an old 450W PSU
let's assume i have 3 12v rails
i can use 2 for 10W red LEDs, and use the remaining 12v, in combination with the resistor, for blue/white LEDs
i can than use remaining low voltage rails for supplemental red/blue 1w LEDs...
am i making any sense here?
thanks

 

[Slim Picky]

Hey all

Jhhnn - I have already jumpered the green and black wires and it works without needing a load.

Krankenwagen - I hadn't considered what you're suggesting but it sounds entirely plausible since as you say there are 3.3 and 5v rails on the psu which lend themselves to various combinations of lighting for 1 and 3 watt chips. Hmm interesting. ..

Hempfield - Since you mentioned you have had success flowering under these chips I'm tempted to build a dedicated flower panel. As you said these things are cheap enough top test with. What spectrum did you use to flower with? Could I use 3 10w warm white with 1 10w cool white for the blue spectrum? I was thinking to use 10w 630nm and 660nm chips but they would require 18-22v to run. I'm trying to keep things as simple as possible so do you think the reds would make enough of a difference our just go with the cool and warm whites? Maybe a neutral white?

Sparked off an interesting discussion for us micro growers!

 

[hempfield]

It's been awhile since i fussed with PC power supplies, but there are a couple of things that might be helpful.

Some need to have a load on the +5V rail to work at all, even if that's just a high value resistor to ground.

Some have internal potentiometers to adjust output voltage.

Some have separate discreet output rails of the same voltage. If the maker says 300W on the 12V rail, for example, that can be 2 outputs of 150W each, on separate wires intended to power different parts of the computer.

At the motherboard connector, jumper the green wire to any black to turn on an ATX supply.

Really cheap PSU's won't last for long at anything near their "rated" output capacity.

I have a PSU bought in 2001 which runs 24/7 for the last 3-4 years and it does not show any signs of malfunction. When I start using it to power things on my grow box/room I modified it a bit, connecting the green to ground directly on the PCB , and connect longer and thicker wires for the 12V output. The 5V output was left unused for a while, but I remember I use it to power some fans.

However, I also have a few server PSU recovered from some recycled 1U IBM servers, which have only 12V output, but higher amperage - up to 47Amps.

On Ebay you can find also cheap PSU : http://www.ebay.com/itm/Universal-1...er-Supply-Driver-for-LED-Strips-/221238405438

ok
so, a bit of theorycrafting:
i have an old 450W PSU
let's assume i have 3 12v rails
i can use 2 for 10W red LEDs, and use the remaining 12v, in combination with the resistor, for blue/white LEDs
i can than use remaining low voltage rails for supplemental red/blue 1w LEDs...
am i making any sense here?
thanks

You must read the maximum available current on the 12V rails (which are all connected on the same point on the PCB, but they are more than one for loadbalancing).
The 5V output could be eventually used to power 2x3W red leds , which require ~2.4V at 700mA. Connecting 2 leds in series without a current limiter could be to much for them, so a resistor of 0.5~0.6Ohm at 2W should be used.

Hey all

Jhhnn - I have already jumpered the green and black wires and it works without needing a load.

Krankenwagen - I hadn't considered what you're suggesting but it sounds entirely plausible since as you say there are 3.3 and 5v rails on the psu which lend themselves to various combinations of lighting for 1 and 3 watt chips. Hmm interesting. ..

Hempfield - Since you mentioned you have had success flowering under these chips I'm tempted to build a dedicated flower panel. As you said these things are cheap enough top test with. What spectrum did you use to flower with? Could I use 3 10w warm white with 1 10w cool white for the blue spectrum? I was thinking to use 10w 630nm and 660nm chips but they would require 18-22v to run. I'm trying to keep things as simple as possible so do you think the reds would make enough of a difference our just go with the cool and warm whites? Maybe a neutral white?

Sparked off an interesting discussion for us micro growers!

There are various version of red 10W leds. Here you can find leds which require just 6-7V : http://www.ebay.com/itm/1pc-High-Po...ht-Bulb-Beads-10W-Lamp-Bulb-Red-/130810638751

I don't think you will find cheap 10W leds available on neutral white (I never saw something like this). Cool white leds provide enough 'blue' spectrum (and have higher light intensity also) so a 3:1 (WW:CW) combination should be enough. But wait a minute, there is always fun to experiment, so you can try to enhance the spectrum with red or blue - there are endless possibilities.

 

[Slim Picky]

Thanks for all your input hempfield. You've really started me thinking about building a small dedicated flower box. I have to keep things small and stealth.
Thanks for the link to that red led, I hadn't seen that version before.
To start off I think I'll just use 3 warm white and 1 cool white in a PC case which should be enough for the size of the case. I'm just waiting on the diodes.
I may just order some of those red chips to supplement. If I wire two of the reds on 12v would I still use diodes for current limiting?

 

[hempfield]

Two LEDs of that type wired together will run under nominal current, which means lower light output but higher efficiency. Normally you don't need to add extra diodes for current limiting, but if you think the leds are to bright you can add one diode.

 

[Jhhnn]

ok
so, a bit of theorycrafting:
i have an old 450W PSU
let's assume i have 3 12v rails
i can use 2 for 10W red LEDs, and use the remaining 12v, in combination with the resistor, for blue/white LEDs
i can than use remaining low voltage rails for supplemental red/blue 1w LEDs...
am i making any sense here?
thanks

That would work, I think, if your psu has multiple 12V rails. If it's just one rail, you're golden, anyway. If you pull too much power from any rail, the voltage will sag, tipping you off to the possibility of multiple rails. Some psu's are labeled as having multiple rails, some aren't, and some have a single rail, really the best possibility. The other voltage outputs can be used within their power handling capabilities. You can even exploit the voltage difference between outputs, like 7V between 12V & 5V, provided there is an adequate 5V load.

Electronics is not my area of expertise, but I know a little from being a computer enthusiast some years ago.

 

[Neekz]

I am running 5 of these Diodes in parallel, on a single 12VDC 1300Mah wallwart. You can basically use any 12vDC source, SO LONG as you HAVE A WAY to REGULATE the amperage. I did this by using a P.W.M. between the power source and the Diodes, It's basically like an adjustable driver. You can dial down the juice for less heat (at the expense of less light.), and longevity. The P.W.M. I am using is rated for up to 8Amps so theres room for a bigger power supply (12v wallwart) and more diodes. I dont think the 12v source on the P.C. PSU is giving more then 2Amps so it should work out if you decide to go that route...

 

[KRANKENWAGEN]

thanks all for the info

i'm also shit in electronics

 

[hempfield]

thanks all for the info

i'm also shit in electronics

You must spoil before you spin.

 

[Slim Picky]

Hey guys, apologies for resurrecting this thread again but had a question and didn't really want to start a new one since it follows on from this one. Hoping Hempfield has another golden nugget of electronic information to help me out!

So I'm wiring up the 10w ic leds I've got with the 12v rail on the pc power supply using the diodes as per hempfields instructions, things are working well!

A friend has, however, given me some dragon plus warm white leds for free! 5 for now and there may be another 5 on the way. It would be a shame not to be able to incorporate them into this box so was wondering how to do this using the pc power supply? They each have a forward voltage of 3.2v and from what I can gather can be powered up to a constant current of 500ma to produce 2w per led. Now the power supply has a 3.3 and a 5v power rail supplying 28 and 35amps respectively!

So hempfield, is there an equivalent to the 1n4007 diode that I could use to do the same job to produce a constant 500ma current for these leds? With a voltage drop across whatever current limiting component I used would it be better to use the 5v rail?

I was inspired by tremolo's PC case and since I now have the same leds he used I really want to use them. I will buy a dedicated power supply if I have to but I'm hoping the techy geniuses on here will be able to provide a far simpler, cheaper option to this.

Thanks for any input

Slim

 

[Neekz]

Get a little 12v input driver for them, like these (Cheap, used for car lighting...), and you can wire the golden drags into the main 12v line . Or you could use this wired into the main line going into the PSU... Good luck!

 

[hempfield]

I'm glad that my suggestion helped you !

3.2V is the typical or maximum voltage drop of the dragons ? If typical, you can use the 5V output to power them, connected with 2 1N4007. The maximum voltage drop across 1N4007 can reach 0.9V at 1000mA. If the current though the dragon led start to rise, the voltage drop off 1N4007 also rise, which decrease the voltage drop on the led, causing a self balancing loop. You can test this configuration and measure the current to see if it is below 500mA.

If the voltage drop for reds is 2.4V , you can power them using just one `1N4007 on the 3.3V rail.

I'm to stoned now to continue

P.S. It would be nice to see some pics !