LED and resistor question.

gunpowder · Post by **gunpowder** » Sun Oct 14, 2007 9:10 am

I have just bought a set of 100 LED Christmas lights. Now i intend to cut some off and run them from a 9V battery but i have no idea of the resistor i need. Heres the information i have

Bulb type 3.5v/0.07W
It then says that the transformer supplied is 24v/10.8V output (dont know if that makes any difference )

Im normally OK with this kind of thing but for some reason i am hving a mental block!

Cheers

gunpowder · Post by **gunpowder** » Sun Oct 14, 2007 9:18 am

I did a little calculating on the web and apparently i need a 330 ohm resistor. Anyone know if thats correct?

USS Atlantis · Post by **USS Atlantis** » Sun Oct 14, 2007 10:19 am

>Bulb type 3.5v/0.07W
>It then says that the transformer supplied is 24v/10.8V output (dont know if that makes any difference )

Well, lets break it down

W=V*A
A=W/V

So .07/3.5 = .02 or 20ma - which is normal for a LED

So to calc the resistance needed for the power supply -> LED

R=(V1-V2)/A

R=(24-3.5)/.02 = 1025 or a 1.2k resistor
R=(10.8-3.5)/.02 = 365 or a 370 ohm resistor

So depending on what your PS is switched to - I hope there's a switch to control the output - you'll need either a 1.2k or a 370 ohm resistor - always go UP in ohm from the calculated to the next available standard

If that PS can't be switched between it's two output voltages, dump it and use either a battery pack or get a new wall-wart.

Atlantis

gunpowder · Post by **gunpowder** » Sun Oct 14, 2007 10:24 am

Cool thanks!

I will be using a 9v battery to power them

Thanks

USS Atlantis · Post by **USS Atlantis** » Sun Oct 14, 2007 10:29 am

>I will be using a 9v battery to power them

Ok

R=(9-3.5)/.02 = 275 or 290 ohm - I think that's the next standard - don't have my list in front of me

Atlantis

USS Atlantis · Post by **USS Atlantis** » Sun Oct 14, 2007 10:41 am

Oh, BTW, I'd use a AA pack rather than a 9v battery to power that thing

9v Batteries have a much shorter lifespan compared to a AA or even a AAA pack

Typical 9v rechargable has less than 200 MA - that's 10 hour lifespan for 1 LED
Typical AA rechargable has 2000MA or more - that's 100 hour lifespan for 1 LED

I always use AA rechargables for lighting when I don't use a Wall-wart - you can get packs that hold 2,3,4, or 6 batteries

I'd use the 3 pack for that application and that would work out to be

R=(4.5-3.5)/.02 = 50 or 62 ohm for your standard resistor

Atlantis

macfrank · Post by **macfrank** » Sun Oct 14, 2007 1:31 pm

Why not just use The LED calculator for single LEDs or the Series/Parallel Wizard for multiple LEDs (or 1 as well)

Frank

jwrjr · Post by **jwrjr** » Sun Oct 14, 2007 10:52 pm

To get back to the original question, if you are going to run 3.5 volt leds (which would be blue or white) from a 9 volt power supply, then 330 ohms 1/4 watt will be the smallest easily available value to use. Using a larger value is fine. Using a smaller value will sooner or later be bad for the led.Don't worry about a little lower. If you go much lower then it will be well to have a supply of replacement leds.

Sparky · Post by **Sparky** » Mon Oct 15, 2007 12:32 am

You could hook 3 in series and need no resistor. They will already be under powered by the 9 volt. There's no hope of the 9 volt over powering the LEDs in that configuration. A resistor wastes power from the battery so if you need more than 1 LED try to build a circuit that uses the lest number of resistors.