Powering up lights. HELP!!

Today i was bored so i put together a little light thing in a 3.5" floppy drive bay. Its pretty basic, just 5 LEDs (just dodgey ones we use at school in electronics that ive collected over the years) a switch, and 9V power conector thing for a 9V battery. The only problem is its not gettin enough power from the 9V batery. The circuit all works fine but its not gettin enough power.

The LEDs are 1.5v so ill need atleast 7.5v to run them all. Or alternatively is it worth just rewiring the whole thing so the LEDs are in paralel, not series like they are now.

my question is, how much power goes through the computers power cabling (eg. cable to a fan) and also if its safe to add in a circuit to that power suply.:? :? :?

All help is apreciated because i dont want my mastepiece;) to go to waste.

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Originally Posted by Mr_Happy

All help is apreciated because i dont want my mastepiece to go to waste.

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I hope that hasen't already been the case! Let's start with the basics.

An LED really needs a current-limiting resistor in series with it. Otherwise you run the risk of frying the LED by allowing too much current through it!

Common currents for an LED that takes 3.6V is around 20mA. I'm not sure about the current your 1.5V ones require but I'm sure it can't be more than that. Your average, fresh 9V battery is capable of producing upwards towards 5~6A when it first starts to flow!!! You try sticking an LED to a nine volt battery with out anything to limit that current (that resistor I spoke of earlier) you're LED cooks. So if the 9V that you were trying to use was either dead or you fired your LEDS.

So now to answer you question: Yes, you could use the computer power supply (assuming the LED's are still good). There are two main voltages coming out of it. The connector that hooks into your HDDs and your optical drives (commonly called molex connectors) has both a 5V and a 12V available. Someone correct me here if I'm wrong (I always get them mixed up) but the red wire is the 5V and the yellow wire is the 12. The black wires in the center are ground, or the place you reference these voltages to. A computer power supply is also capable of a bunch of amps so there's more than enough juice in that thing to power your circuit. It's alright to have more voltage that you need but not current.


To answer your series/parallel question: it would be easier to leave it in series. like this:

current
limiting
+ - resistor
powersupply---vvvvvv----|<-----|<------|<---|<---|<---
|________________________________________________|

In this way, you only need one CLR because you're only needing to limit the current in one branch. If you were to try to do this in parallel, you'd need a CLR for each branch:


+ - CLRs
powersupply---vvvvvv----|<-----|
| |---vvvvvv----|<-----|
| |---vvvvvv----|<-----|
| |---vvvvvv----|<-----|
| |---vvvvvv----|<-----|
| |
|--------------------------------|

So now to figure out what resistor you need. If you're going to design it is series, you have 5 LEDs you're needing to power. So that comes out to 7.5V (1.5Vx5) and 100mA (20mAx5). You calculate resistance as Voltage/Current.

Because you would have to use the 12V rail, that's the voltage you'd use.

So: 12V/100mA = 120ohms

If you were to go with your parallel option, your would only need 20mA per branch and therefore your formula would change to:

12V/20mA = 600ohms.

So hopefully that answers your questions. Let me know if any of that is confusing or you need more info!

Grr, the parallel circuit didn't come out right, but I don't have the time to fix it right now. Let me know if you understand it or need it fixed.

also if you know what your LED power are (Voltage & Ma) as well as your source Voltage.

use the LED wizards on this site .. Plus Linear has some really good information about LED's as well.

http://linear1.org/

tableleg, i already know what im doin with the LED circuit i made, ive done electronics for 2 yrs now at school. The LED setup is all just on one 3.5" floppy drive bay cover thing. What i wanted to know if its going to be safe to connect my circuit to the computer power supply and how best to do it. What i didnt know was the voltage provided by the molex connectors.

For a more detailed description...the circuit consists of 5 yellow LEDs (1.5v) a diode, a switch and +ve & -ve wires for the power. I tested out the rig on a variable power supply unit i have that gives out 3v, 4.5v, 6v, 7.5v, 9v or 12v, depending on which setting you have it on as. So it all works fine. only problem now is that i put the diode in backwards (:rolleyes: careless mistake) and i burnt out the last LED when i was testing the circuit out with the above mentioned power supply. I accidently had it on 12V when i was testing induvidual LEDs. so pretty much the LED went from dull getting brighter & brighter then sizzled and went pop. And so now i need another yellow LED.

Thanks for all the help aswell, very usefull info.

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Originally Posted by Mr_Happy

so pretty much the LED went from dull getting brighter & brighter then sizzled and went pop. And so now i need another yellow LED.

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LOL... If I could count the number of times I've seen that reeeally bright flash before that pop... ;)

Well good times then amigo. I'd say it should work fine with the computer power supply. Just attach your +ve wire to the yellow wire and your -ve to the black one right next to it.

How you want to couple this is up to you. Whenever I've done projects like this, I've found those molex->fan converter connectors, snipped the fan part off, and soldered my the +/- V wires to the corresponding wires in the connector which makes it 1)look better when it's in the case and 2)be much easier to connect/disconnect. Since then I've found a place where I can just buy those connectors and pins so I just make my own. Makes life a lot easier that having to rummage around for those converters. :p

The only question I have about your circuit is what's limiting the current? Besides the LED's and the control components (the switch), all you have is the diode; Is this what's limiting the current? I've never thought of doing it that way. Let us know how it works out!

Well now that ive pretty much gotta re-do a few things im gunna have a look to see what spair resistors i have and probly put in a 220 ohm one. Now what ive got to do is:
1) replace the yellow LED
2) turn the diode around
3) add the resistor
4) connect it to my pc's power supply

I would take a couple pics of what it looks like but I dont have access to a digital camera, and i dont feel like waiting for normal film to get doveloped (with the rate my parents take photos you could be waiting a long while just for 2 or 3 pics of the final product)

220ohm? What current would that produce?

12V/220ohm = 54.54mA

With that resistor you'd end up with 55mA (or around 275% of what you need!!) and although the LED's would shine brighter you would make them 1) run hot and 2) shorten their lifetimes by a bunch. I'm not sure by how much, I've never bothered to find out, but anything's lifetime shortens if you push it too hard. ;)

The ideal current is 20mA. So:

12V/20mA = 600ohm

So you can either use a combination of resistors or pilfer a 600ohm from school.