24 LEDs via PC PSU

[alan_fx8150]

Hello everyone,

I'm new to these forums and opened the account to ask for help (please, please, please) with my planned circuit...

Power Source:
Basically, I want to power an LED circuit from my PC's PSU Molar (12v red wire). The internet shows the red wire to be 12+ and the yellow 5+ - does this mean that its safe to assume its 12v, or could I end up overpowering and destroying everything?

Circuit Specs:
Voltage input: 12v, LED through power: 3.1v, MA 30v x24 LEDs

Aim of the Plan:
I will have 4 fan shrouds at the rear of the case and 2 in the middle (above the motherboard). Each has 4x5mm LED holes so I'm trying to plan a circuit of 16 (rear) & 8 (middle) LEDs for 24 in total.

Research:
I've used LED online calculators but they only seems to show 3 LED's in series per resistor and this would be very difficult to plan due to distances. The calc's say to use a parallel connection of 8 wires - leading to 3 LED's in Series followed by a 100ohm 1/4w Resistor on the ground wire?? Shouldn't the resistor be on the positive wire or is the ground wire okay?

Solution?
Can I plan out a parallel connection of 6, leading to 4 LEDs in series, then which resistor should I use? Would another option be to have maybe 3 wires in series, leading to 3 resistors, then splitting to 6 LEDs (to keep things simpler?). I cant see how 12v can be enough to power 24x 3.1v LEDs regardless of being series or parallel. Can it actually even work at all? I know the calc's say it can but I'd rather a 'real' person say yay, nay or maybe please

I know there's are a few queries here so if anything doesn't make sense, I'd be happy to clarify. Please could you take a look as my research has taken me this far, but my experience is zero and I'm struggling to find a similiar situation in forums!

 

[Richard Burns]

You may find it easier to purchase 12V LEDs for car dash indicators as they will have integral resistors and flying leads, so they could be connected up directly to the supply.

 

[tazz]

The simplest was to think of this, it the voltage divide by the amount of leds.....this will give you the voltage across each led wire in series. Most leds required 1.8v -2v at 20ma, so if we wire 6 leds in series, we would get 2v across each led, and will work fine, but the leds will draw as much current as they can. placing a 100ohm resister in series with a diode, acts as a current limiter. As a led is a diode, this will limit the current to 20ma, governed by the leds. Now you can connect up a set of series leds in parallel, your only limit to amount is the power supplies output current.........hope this helps

 

[shanky887614]

You will need to solder in a button to start them if using a pc psu

Only the 5v rail is on till the pins on psu are shorted together to start it

 

[alan_fx8150]

You may find it easier to purchase 12V LEDs for car dash indicators as they will have integral resistors and flying leads, so they could be connected up directly to the supply.

Thanks very much Richard. It would save a lot of hassle but it'd cost £162.24 for 24 of them and they're 1mm too big to fit in the holes unfortunately but I appreciate you taking the time to have a look for me, cheers.

The simplest was to think of this, it the voltage divide by the amount of leds.....this will give you the voltage across each led wire in series. Most leds required 1.8v -2v at 20ma, so if we wire 6 leds in series, we would get 2v across each led, and will work fine, but the leds will draw as much current as they can. placing a 100ohm resister in series with a diode, acts as a current limiter. As a led is a diode, this will limit the current to 20ma, governed by the leds. Now you can connect up a set of series leds in parallel, your only limit to amount is the power supplies output current.........hope this helps

Thanks Tazz, thats great. I learnt a lot ysterday and I do apologise - I didnt realise that parallel connections would all draw the source voltage of 12v - I guessed it would be divided down but thats actually the way it works in series. I ordered a multimeter yesterday and I'm going to check the 'actual' voltage of the PC molex, then try to find some LED's with a forward voltage of 3.0v (to allow a tiny bit more headroom). When thats all set in stone, I'll re-calculate the required ohms and (hopefully) job done!

You will need to solder in a button to start them if using a pc psu

Only the 5v rail is on till the pins on psu are shorted together to start it

Ohhh, I wasn't aware of that. Do you mean that the 5v rail is constantly on but the red wouldnt work unless I turn on the P.C or will a switch be needed even if the P.C's on? I wouldnt want the LED's on when the PC's off anyway mate.

I'd be happy to solder a switch onto the circuit but wouldnt that just give me the option to 'stop' the current? Presumably the current would flow as long as the actual computers on?

Cheers,
Alan

 

[shanky887614]

If this is for a pc, have a look at overclockers uk they will be more helpful there.

They will likely also sell leds that can plug into power supply without soldering


Thats overclockers.co.uk


(No need to solder switch, I thought you wanted to run it without a pc.)

Ive seen people do stranger things so thought nothing of it

 

[alan_fx8150]

If this is for a pc, have a look at overclockers uk they will be more helpful there.

They will likely also sell leds that can plug into power supply without soldering

Thats overclockers.co.uk

Cool, cheers Shanky. Yes I'm registered there already but didn't think of checking for LED's there. I had a look at some P.C sites before but most of them were a molex to 2 LED connection so I was hoping to put a lot of effort in, to save using all the molex ports. I'll have a fresh look and I could even modify some I guess

(No need to solder switch, I thought you wanted to run it without a pc.)

Ive seen people do stranger things so thought nothing of it

Haha, yes I'd seem crazy projects too. Like having a P.C power external LED's for a house! I feel like my task is minor in comparison

 

[telectrix]

i'd rethink the supply arrangement and use a separate 12V PSU . anything from 3-5A will give you plenty of juice for the LEDs without frigging round in ther PC.

 

[shanky887614]

Cool, cheers Shanky. Yes I'm registered there already but didn't think of checking for LED's there. I had a look at some P.C sites before but most of them were a molex to 2 LED connection so I was hoping to put a lot of effort in, to save using all the molex ports. I'll have a fresh look and I could even modify some I guess



Haha, yes I'd seem crazy projects too. Like having a P.C power external LED's for a house! I feel like my task is minor in comparison

Usually they have passthrough molex ports so you can string loads together.

You could always cut, solder and braid the cables so the connectors are hidden in the case.

I know a few people that have done this, I can't as my case is rammed.

A full atx and 7970 really fill up an antec 300

 

[tazz]

There will be a few molex plugs not used, you can nick power off.......another trick if your interested, place one flashing led, (1.8-2v type) in the series chain....and all the leds will flash...(used this to make small led christmas tree lights for the kids years ago)