# Red Green Led 2 Leg

Discussion in 'LEDs and Optoelectronics' started by EdW, Feb 7, 2017.

1. ### EdW

6
0
Feb 7, 2017
I have a circuit that changes polarity and it is in range of 6 to 12 dc. What would be a simple circuit so I could limit the voltage to use this led 3.3v light up red an green .
Thanks

Last edited: Feb 7, 2017
2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,839
Jan 21, 2010
Place a 470Ω resistor in series with the LED.

3. ### Audioguru

3,389
732
Sep 24, 2016
3.3V is probably the green forward voltage and the red will be about 1.8V to 2.2V.
Use a single resistor to ground that is (12V - 1.8V)/20mA= 510 ohms or more. The resistor will dissipate (heat) with 0.2W so use a half-watt resistor.

4. ### EdW

6
0
Feb 7, 2017
The led is green when + - and red when - + and the voltage is variable at times.

25,497
2,839
Jan 21, 2010
Yes, we know

6. ### EdW

6
0
Feb 7, 2017
Even with the voltage can be anywhere from 6 to 20v a one size res will work.

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

25,497
2,839
Jan 21, 2010
Yes it will. The brightness will vary, but the LED will be safe.

Does the variation in brightness concern you?

8. ### EdW

6
0
Feb 7, 2017
You say res on ground side. Polarity is changing all the time. And how could I use a 5v regulator with the polarity changing every few minutes?

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

25,497
2,839
Jan 21, 2010
The resistor can be on either side of the LED, it doesn't matter.

The following circuit will limit the LED current to 10mA for an input voltage between +/-6V and probably +/-30V.

To operate correctly at 6V the diodes should be Schottky diodes.

Edit: transistors can be BC548 or similar.

10. ### EdW

6
0
Feb 7, 2017
Are you saying to just use a 6v Schottky. I can't find 6v Schotty do you mead zener?

Last edited: Feb 8, 2017
11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,839
Jan 21, 2010
No, I mean the 4 diodes shown on that circuit need at be Schottky diodes.

Having them rated for maybe twice the max input voltage would be safe. In this case 20V or more would be fine.

6
0
Feb 7, 2017
Thanks