# Need help for simple LED circuit

Discussion in 'LEDs and Optoelectronics' started by kenek, Apr 20, 2013.

1. ### kenek

4
0
Apr 20, 2013
We have an rc car we built for a school competition but need to have an led light in the circuit to show when the power is on. We are running with a 24v battery pack and I need help designing a circuit that will run a low power led, Any help would be greatly appreciated!

2. ### duke37

5,364
771
Jan 9, 2011
You can use a resistor in series with the led. If you could pass 20mA, this is the normal current for a low power led, then subtract the led voltage from 24, then multiply this figure by 50 to get the resistor value. With a 3V led, 21*50 = 1050, use 1k.

Note that with 24V you have sufficient voltage for ten 2V leds in series (20V) so the resistor would need to be 50*4 = 200, use 220. Always leave 10 or 20% spare voltage to be dropped by the resistor, this stabilises the current.

There is a 'sticky' somewhere describing how to drive leds. One of these next years I will learn how to find it!

The voltages that the leds use depend on the construction and colour.

3. ### kenek

4
0
Apr 20, 2013
Thanks! What watt resisitor should we use? The battery pack is running at 1300 mA

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,490
2,832
Jan 21, 2010
The battery pack is 24V (and possibly 1200mAh). Only the 24V is important.

5. ### duke37

5,364
771
Jan 9, 2011
The dissipation in a resistor is current times voltage across it. In the first case of 20mA and 21V, the dissipation is 0.42W so a half watt resistor would do but I would put in a 1W, the bigger resistor would run cooler.

In the second case 20mA and 4V, the dissipation is 0.08W so any diddy resistor that does not get lost on the floor will do.