led power help

[skullkeeper]

can someone tell me what 12v i should use to power these leds i think they come with 510ohm resisters.

would it be 12v 2amp?

im going for max brightness all hooked to same 12vdc.

should i use a diforent resister?

thanks
"skullkeeper"

[xplod1236]

If you will be running them at 25 mA each, then the power supply needs to provide 12v 0.025 A per LED. But since the resistors are 510 ohm, the LED will only pull 17 mA at 12v. Therefore, the supply only needs to provide what the amperage that the LED will pull (in this case 17 mA).

You could use a resistor with a min value of 350 ohm (or closest standard value), which will provide 25 mA for the LED. Using a smaller resistor will increase the brightness, but will also decrease the life of the LED.

For multiple LEDs, add up the amperages, and that will tell you how much current your supply needs to provide. As for the resistors, use a separate one for each LED.

[skullkeeper]

so if i use a 470ohm how much power would my 12v need for all 100 leds 12v 2amp?

[xplod1236]

With a 470 ohm resistor, each LED will pull 18.3 mA. That times 100 is 1.8 A, so a 2 amp supply will be fine.

[skullkeeper]

col thank you verry much

[AndrewT]

Hi,
have a search for a constant current supply to the LEDS.

Next, the LEDs need about 5V, so you can run a pair in series.
50 pairs only need 900mA to 1A.

So a constant current supply to each pair (50off) controlled by one active device. There's an app note somewhere (just to prove my filing system is c..p).

You can use them at higher current if you flash them at about 30 to 35mA and at about 100 to 200Hz. Duty cycle unknown, but around 50% should be safe.

[skullkeeper]

sorry im new to all this and i dont understand what you mean. only way i know is with the 12v.

[AndrewT]

Hi,
the spec sheet you linked to shows that each LED needs about 5V to work. You also need to limit the current to about 20mA. The 12Volt supply and a dropper resistor do that for you. But 100 of the LEDs needs 100 times the current and all that wasted power through 100 dropper resistors.

Put two LEDs in series. The pair now needs 10 Volts to operate. Limit the current to 20mA as before. However there is only 2volts to spare and a limiting resistor is going to be quite inaccurate trying to limit current with just 2Volts drop. Instead use an active current limiter. Set the current limiter to 20mA and supply two LEDs from it. Wasted power is reduced by about 75%. This allows a smaller power supply to be used, 1A instead of 2A.

So take your 100 LEDS and set up a series pair with a current limiter and do this fifty times.

Look up active constant current source. There you will find a two transistor + resistor circuit. Use that to supply the two LEDs.

There is a better way that combines the sources and saves on components and automatically controlls current to the operating LEDs if one goes dead on you.
I cannot recall who's application note it was featured in (maybe Nat Semi). Try that search I suggested.

[stelleg151]

You really only want about 3.5v across each one(see forward voltage), I would recommend (perhaps in addition to the constant current circuit) using this wizard to get the basic idea of how to arrange the LEDs.

linear1 wizard

[skullkeeper]

here is an idea of what im doing i think 2a will be easyer and cheaper sence i have the led's and the 470ohm resistors and the 12vdc 2a already.

thats what i thought 3.5v not reverse volt like hes saying of 5v. iv been using that site i found it searching and a few others.

only thing i dont like about the site is it shows 1 resister to 2-3 leds im using 1 r to 1 led