Basic Electronics Question


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HairyArse
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As a complete electronics noob this problem could probably be rate as being for 10+ yrs, but fuck it.

If I'm powering 3v LEDs off a 12volt power supply (using a resistor), obviously I can only use 4 of the bulbs. But is there anyway of wiring the circuit so that the power doesn't weaken after every bulb is added to the circuit? I.e. if I use 3 bulbs only they're all brighter than if I use 4 bulbs.
#1 at 11:34:30 - 04/06/2007
peej
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Yeah - whack a resistor in there instead of the bulbs. Simple as.

And if you're running these in parallel it'll help a lot too.

Peej
#2 at 11:37:53 - 04/06/2007
HairyArse
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If I put a resistor in instead of the bulbs how can I still have 4 bulbs which is what I need? I actually need 16 bulbs but am having to create 3 batches of 4 from a PC power supply.

(It's for the mame cabinet in case anyone is wondering, illuminated, translucent, blue buttons for the chav win!)
#3 at 11:48:10 - 04/06/2007
Tiger_Walts
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You may need a resistor for each LED if you're putting them in parallel.
#4 at 11:49:48 - 04/06/2007
HairyArse
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So one resistor for each bulb and make the circuit a parallel one? Interesting...
#5 at 13:10:58 - 04/06/2007
peej
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That's kinda what I meant if you're going for consistency of brightness...

This would be so much easier to draw!

Peej
#6 at 13:56:42 - 04/06/2007
boabg
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Just knock up something like this.
#7 at 14:39:50 - 04/06/2007
HairyArse
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Piece of piss!

O_o
#8 at 14:46:08 - 04/06/2007
Alastair
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HairyArse said:As a complete electronics noob this problem could probably be rate as being for 10+ yrs, but fuck it.

If I'm powering 3v LEDs off a 12volt power supply (using a resistor), obviously I can only use 4 of the bulbs. But is there anyway of wiring the circuit so that the power doesn't weaken after every bulb is added to the circuit? I.e. if I use 3 bulbs only they're all brighter than if I use 4 bulbs.


Err, I'm not so sure you can only use 4 LEDs. I think the potential difference (voltage) will be the same no matter how many LEDs you use. Difference will be in how much current is used. Or something.
And if you're running them in parallel, I reckon you'll only need one resistor.

-----------------
| | | | |
= | | o o o
| | | | |
| | | | |
-----------------

My god that's a crap bit of ASCII, basically your power supply is the =, the o are the LEDs and the 2 || are the resistor. You'll need a bit of Ohm's law action to calculate what resistor you need based on the resistance of the LEDs. I suspect it will be a lower resistance than them so that most of the current passes through it rather than the LEDs.

/awaits someone who knows what they are talking about to debunk this entire post....
I haven't thought about electronics since GCSE (which was a long time ago!)


Edit: Bah, the ASCII doesn't line up :o(
#9 at 15:44:05 - 04/06/2007
duncan
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Surely you only need one resistor, in series with the power supply, then all the LED's you want in parralel with the PSU and resistor combined.

additional bonus of parralel is that if they're like this if one goes the circuit will keep running.

/teaches IGCSE Physics.
#10 at 17:46:37 - 04/06/2007
TriodeBliss
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You can use one resistor, but it's considered bad practice, since you can't be sure that all the LEDs are closely matched - they're current controlled devices, so a tiny variation in characterisitcs can result in quite a wide variation in voltage drop.

It's OK though if you're not concerned about maintaining even brightness.

Edit: too many sinces
#11 at 18:05:26 - 04/06/2007
HairyArse
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Well this is the thing - I want even and maximum brightness so what would be the best way of achieving climax, errr I mean achieving this?
#12 at 20:05:42 - 04/06/2007
TriodeBliss
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The easiest way would be to get some LEDs with built-in current limiting resistors, which are usually a few pence dearer than the garden variety.

Assuming you've already got the LEDs, then parallel with a resistor each is best, certainly if you're wanting consistency.

Do you know the forward current, Hairy?
#13 at 23:07:16 - 04/06/2007

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