Molex connector current?

Raz

I'm attempting to hook up some extra LEDs to my PSU and I need to figure out what value resistor to put in series with them.
So what I need to know is what current do you get form the 5V line on a molex connector? If it helps it's an ATX 2.03 PSU giving 300W.

Also for extra info I'm wiring the LEDs in parrallell (to halve the current) and I'm putting the resistor before the parrallell split.
Like this

5V +o----------/\/\/----------|>|-----------------o -
                         |                 |
                         |----|>|----------|

My attempt at binary art

SouperComputer

that aint binary mate, thats ASCII!

Raz

And I call myself a technology geek!!!
I've been spending too much time with FTP, I'm getting my terms mixed up

flav0rflav

The relevant bit of maths is

R = V / I.

The resistance is equal to the voltage divided by the current.

Your voltage is 5V. The current is how much current the leds need. Leds are diodes and have awkward voltage-current characteristics, but basically should be about 10 - 20 mA, x2 for the 2 leds, so the resistance should be on the order of 200 Ohms.

Which implies

200 O = 5 V / 25 mA

Each led having about 12 mA. You could bump it up or down 50 and visually see the difference, or check the data for the leds you are using.

Do I have to append a warning about messing with your PC power supply?

SouperComputer

Originally posted by Raz
And I call myself a technology geek!!!
I've been spending too much time with FTP, I'm getting my terms mixed up

lol, must be all those long hours!

Raz

Originally posted by flav0rflav
The relevant bit of maths is

R = V / I.

I know that bit (I'm in an electronic engineer course and I should really know how to do this stuff but I'm getting confused )

Leds are diodes and have awkward voltage-current characteristics,

Don't I know

Which implies

200 O = 5 V / 25 mA

I did a calculation with the diodes in series and I came up with about a 150ohm value but that was based on what the value of current should be and not what it necessarily is.

I'm trying to pump up to 20 mA through each LED because that's roughly the max current for the diodes.
To the best of my knowledge the current doesn't change unless the circuit splits. So I'd need 40mA coming through the resistor. Therefore I believe I would need 40mA coming from the power supply. Hence my question of how much curent comes from it.
Now is that a bit screwy or am I right and I'd need to put a resistor in parrallell with the diodes?

Am I making sense?
Maybe I should just get a multimeter out and plug the PSU in

Do I have to append a warning about messing with your PC power supply?

That's the fun bit

flav0rflav

The supply gives you whatever current you ask for.

It's a power supply, it supplies power, but it has a fixed volatge of 5 V. Power is current multiplied by voltage. Some devices will want more power, some less, so whats different for each device if they all are using 5 V? The current. It's what varies. And it will depend on the effective resistance of the devices.

The simple way to look at the circuit and arrive at an 'engineering' solution is to consider the normal (steady state) situation:


<diagram removed, as it turned into pants>

Consider it as two resistors in parallel, the leds, and then in series with another resistor.

The leds want 20mA each, and will have about 1.5 V ? (check specs) across them when lit. You can do the circuit equations, or just looking at it, you can see there will be 40mA through the main resistor, and 5-1.5 = 3.5 V across it. So resistance should be

3.5V / 40mA = 87.5

But, yea, you need to understand a bit more about how the power supply works. I hope you can get a bit more insight from this.

Raz

I never thought of it as an actual "POWER" supply before. I just took it as 'Where the computer gets the juice from'. I thought it was more of a dressy up term than an accurate description of the piece of hardware. That clears up my thinking considerably For that alone I'm deeply in your dept.

The leds want 20mA each, and will have about 1.5 V ? (check specs) across them when lit. You can do the circuit equations, or just looking at it, you can see there will be 40mA through the main resistor, and 5-1.5 = 3.5 V across it. So resistance should be

3.5V / 40mA = 87.5

I understand things a lot better now and I reckon I can manage the rest of it on my own. I'll throw up any more questions if I have them (in the hopes that you'll answer ) and if things go horribly wrong, well, I'll let you know that too

Thank you for your help, I'd have been stuck at this for another 2 weeks if you weren't here.
Cheers,
Raz.

Raz

Well here it is.
I've soldered and pondered and bought resistors and now I have two pics to show you of what I've done

Thanks for you're help in this flavOrflav.
I havn't plugged it in yet. I'll do that this evening and hopefully I'll be able to show you all the pretty lights

So pic1 is the solder connections I made (not my best work!!)

Raz

You may have noticed my avatar change. I felt like going superhero!! (ish)

So this is what it looks like all closed up with the molex connector added.