What resistor do i need

[Matty1973]

Hi all,

i have this torch led module which i am trying to modify for my reliant model,

http://i1103.photobucket.com/albums/g48 ... /photo.jpg

it had 24 white lED's attached running off 4.5 v or 3 1.5 volt AA batteries with a 1.20 ohm resistor.

I want to up this to a 9 volt system and will be running the 24 white led's but with leads to branch off.

The bulbs are 3.0v-3.3v

What resistor will i need? i have some 300 ohm and some 470 ohm will any of those be ok, or something stronger or weaker

[brt]

http://ledz.com/?p=zz.led.resistor.calculator

[Scott Hasty]

Hi all,

i have this torch led module which i am trying to modify for my reliant model,

http://i1103.photobucket.com/albums/g48 ... /photo.jpg

it had 24 white lED's attached running off 4.5 v or 3 1.5 volt AA batteries with a 1.20 ohm resistor.

I want to up this to a 9 volt system and will be running the 24 white led's but with leads to branch off.

The bulbs are 3.0v-3.3v

What resistor will i need? i have some 300 ohm and some 470 ohm will any of those be ok, or something stronger or weaker

Curiously, why 9V? At what current do you want this to operate? Your resistor value at 9V will be under 10 ohms.

[Matty1973]

Curiously, why 9V? At what current do you want this to operate? Your resistor value at 9V will be under 10 ohms.

9v as I am going to link to Trekmodellers federation blinker, using a ac to dc adaptor at 500ma

[Sparky]

That's a parallel circuit, so you will be wasting a lot of power as heat!

Power is calculated by Voltage * current.
assuming 3.3 volts for the LEDs we can find how much over voltage must be consumed by the resistor:
9-3.3 = 5.7 volts

Since the resistor must pass all the current that then gets divided up to each LED, it must pass the sum total of all current used.

The notes you had on the setup calculate to 1 amp draw through the resistor. I don't think that's true since AA batteries don't supply that much current. And makes the LEDs 42 mAmp LEDs. Let's assume 20 mAmp since that's a standard LED current draw.
Total current consumed by 24 LEDs= 480 mAmps

Total power that the resistor must handle is
5.7*048=2.736 Watts. That's a 2.7 watt space heater there.

Also you will need a resistor rated for 3 watts (not likely to find a 2.736 watt resistor). Using a 1/4 watt resistor (like that found in original flash light) will result in a slowly building stream of smoke. Turning it on for a short time will not reveal the issue, so if the blink rate is quick enough you may not find the under rated resistor issue, until the circuit has been running for a longer time.