Probably Simple Electronics Question ... n00b :o

jellybeard999 · 26 Jun 2007 at 03:41

Im going to be building myself a media bench thingy at some point in the very near future, and fancy integrating some LEDs into it for a bit of ambient lighting.

2 shelves, ~90cm each, how many 5mm LEDs spaced evenly, pointing downward would each shelf need for decent coverage ? Im also considering if it'd be worthwhile having some pointing upward on the top shelf, centrally, behind the TV ?

Stop me now if this is a bad idea

Otherwise continue

I will obviously need to run the lot to a mains socket, and the LEDs are ~3v ... so i have the following options (IMO - these may or may not cause explosions, so i thought id better ask first :eek:

)

* Phone charger or similar with a 3v output - cut end and wire to LEDs
* 12v resistor(s ?) and 12v transformer

either way, would the wire go from one side of the transformer, into the leds, pass along all LEDs, out the other side of the chain, and back to transformer ? If i go resistor route, how many resistors ? Where do they go in the link ?

If possible, a little help

MNuTz · 26 Jun 2007 at 04:15

would it not be simpler to buy some very simple LED spot lights?

You can get various sizes with various depths that could easily be screwed to the underside of the shelf, and some also come with mains adapters just to make it really easy for you.

jellybeard999 said:
either way, would the wire go from one side of the transformer, into the leds, pass along all LEDs, out the other side of the chain, and back to transformer ? If i go resistor route, how many resistors ? Where do they go in the link ?

that sounds like you are thinking of putting them in series which means the poewr diminishes as it does along the LEDs meaning some would be brighter than the others. You should put them in parallel.

At least, thats what i can remember from my Alevel physics

Adam_151 · 26 Jun 2007 at 07:18

MNuTz said:
that sounds like you are thinking of putting them in series which means the poewr diminishes as it does along the LEDs meaning some would be brighter than the others. You should put them in parallel.

At least, thats what i can remember from my Alevel physics

Thats not what kirchoff's laws say...

You may get that problem if you had some in parallel and the volt drop long the supply cables to the ones at the end was significant, however with LEDs I'm not sure it would really happen significantly in the same way as its notifable witrh filament lamps (eg selv xmas lights with many segments parralleled up)

Series is the sensible way to wire them, if you had three LEDs; assume a 12v DC supply for example, a 2v diode forward voltage and a current of 15ma

in parallel you'd need to drop 12v-2v across the resister for each one, and each would disapate 10vx0.015A = 0.15W each, a wasteage of 3x0.15W = 0.45W (resistors needed btw would be 10/0.015 = ~680 ohm)

In series we would need to drop 12v-(3x2v) = 6v, so the resistor would be dispating 6vx0.015 = 0.09w (resistor needed would be 6/0.015 = ~390 or 430 ohms

oddjob62 · 26 Jun 2007 at 07:34

MNuTz said:
that sounds like you are thinking of putting them in series which means the poewr diminishes as it does along the LEDs meaning some would be brighter than the others.

Nah they would all be the same brightness.

Dave · 26 Jun 2007 at 10:25

MNuTz said:
that sounds like you are thinking of putting them in series which means the poewr diminishes as it does along the LEDs meaning some would be brighter than the others. You should put them in parallel.

At least, thats what i can remember from my Alevel physics

In a series circuit the current remains the same throughout, and therefore the brightness is the same as well.

Mr. Brightside · 26 Jun 2007 at 11:18

I think you'll be better off buying some of those LED spot lights as mentioned.

james.miller · 26 Jun 2007 at 11:22

cold cathodes? much better coverage and dispersion - led's are very directional

Clinkz · 26 Jun 2007 at 12:04

Well this is what i would do. Adjust current limiting resistors according to led specification, supply, whatever.

The only important thing is that your power supply must be able to supply the required current determined by the number of LEDs in the system. If you have 10 20mA LEDs, whether in series or parallel the power supply still needs to be able to provide 200mA of current.

MNuTz · 26 Jun 2007 at 16:25

Well i guess i was wrong

Electronics was never my strong point, i think it stems from when my GCSE physics teacher told us that no one really knows what electricity is, that confused me then and tbh, i never really took grasp of it.

Clinkz · 26 Jun 2007 at 16:40

MNuTz said:
Well i guess i was wrong

Electronics was never my strong point, i think it stems from when my GCSE physics teacher told us that no one really knows what electricity is, that confused me then and tbh, i never really took grasp of it.

Your not really 'wrong'. There was definitely something about diminishing brightness but i cant really remember what it was.

Anyway, for those saying it would work better in series, please remember that in order to have a potential difference of 5v across each led would require a 5 x (no of LEDs in system)V regulated power supply. As fun as lining shelves and work surfaces with high voltage electrical systems is, i don't think thats very wise