newbie to LED's

[t-bird]

I want to ceate something like this.

I am going to us 8"AA" batteries. This particular light uses 33x55,000 MCD White LED's.

My question pretty much is would i need to use resistors in the build.

Again I am new and don't know much. Please be kind.

T-bird

 

[knabsol]

Hi!
For creating a led string or array like on the picture you would first need to calculate the voltage you have for driving them.
In your case you´re going to use 8AA batteries. Probably connected in series? Then you will get 8x1.5V=12Volts

Second you have to decide which LEDs you´re going to use because you need to know what forward voltage they have. A red maybe got 1.7V and a white one maybe 3.2V. This is the voltage each diode need between + and - for emitting light.

Third, you need to know the current you need to drive them, or to get the light you want from them. Typically you would need 20 or 25mA running through them. This also reflects on how long battery time you will have. If you use a 1000mAh battery (milliampere-hour) and the LED wants 25mA you can drive it 1000/25=40hours.

So lets say you want to use 33 white leds with a forward voltage of 3.2volts and 25mA. For restraining the led from burning due to high current you need a resistor to restrain the current flowing. It´s like putting in a smaller pipe in a water line.

The easiest way to see how to connect them is to use this calculator:
http://led.linear1.org/led.wiz

We have:
Source voltage: 12v
Diode forward voltage: 3.2v
Diode forward current: 25ma
Number os leds: 33

Put in those number and hit the design my array buttom and you´re done
The it´s just to connect them in that way.

Good luck.

 

[t-bird]

So I understand it that I would need 11 100 ohm resistors for every 3 LED's?

Just want to make sure so I don't burn the whole thing up.

Thanks,

T-Bird

 

[knabsol]

Yes that´s right!
One 100ohm resistor for each string of 3 in the example.

 

[Ken_McE]

knabsol

, that was nicely done, clear and concise. Would you consider adding it to the CPF Wiki?

 

[knabsol]

Ofcourse! I´ll do it asap.

 

[russell_pta]

ok, what if you wanted to make somthing like this, maybe even somthing with around 300 led lights. but you want to use a wall socket. what exactly would you need to use. i am ok with the lights and resistors, i just don't know anything about ac/dc converters and full-wave bridge rectifiers and stuff like that.

thanks,
russell

 

[Christexan]

I won't go into rectifier designs (a bit dangerous to build from scratch for someone not familiar with them), but once you have rectified DC voltage, you really should consider using a current regulator (not a simple resistor) to output to the LEDs, you could easily run 10 strings of 30 LEDs from a 120VDC circuit, with a 1-ohm resistor in each string to help balance current sharing, off a 250mA output (slightly overdriven 25mA per 5mm LED typical, at a Vf of around 3.5 per LED) current regulator (a linear regulator capable of this would cost all of about $2 to build if mounted to the chassis as a pseudo-heatsink, a switching regulator would cost more obviously). Anyhow, you'd want to use a regulator because voltage sags and spikes on the line could cause havoc with a simple rectified/resistorized setup and result in blown LEDs, leading to blown strings and then a blown fixture (potentially in about 0.2 seconds total from start to end, more likely a slower but still eventual death one LED at a time at first.
In other words, for an extra $1.75 over basic resistors, you can run a much more reliable linear current regulator.
Still have to get the DC conversion done, best bet would be an off-the-shelf wall-wart or in-line converter of some sort.

 

[Lyndon]

For the battery application you're best off with a switching current regulator because it's much more efficient that resistors and will waste much less power (better battery life). I don't know where you can get a cheap off the shelf one, so I just started designing one for my own use. If you're interested let me know and maybe I'll build two when I'm finally finished

For running it from a wall socket, I definitely second the idea of using a wall-wart. They are cheap and safer than assembling a transformer/rectifier etc. You should use an active current regulator, perhaps a design with a 7805 voltage regulator because then you don't need to worry about changing the number of LEDs in the string.

I should probably put a schematic up on my website and link to it from here: 7805s are cheap and make good current regulators.

-l