Electrical engineer type help needed.

[gixer]

Hello.
I have some 3.5 volt led’s that I want to run off of 12v and 9v.
What would I need to drop the voltage down to get the led’s to run on 3.5v?
Thanks

 

[Lou]

Use a 390 Ohm, 1/4 Watt resistor (in series with LED)
You can use a lower value like 330 but be carefull about going over 14V.

12V(supply) - 3.5V(across the LED)=8.5V (across the resistor)

if you want 20mA through the LED, then 8.5V/0.02A = 425 Ohms (resistor required)

If you apply 9V through the 425 Ohm setup, there will be (9V-3.5V=5.5V, 5.5V / 425 Ohms = 0.129A

 

[gixer]

Thanks
Should it go in a certain way? What I mean is does it flow in a certain direction?
Thanks

Use a 430 Ohm, 1/4 Watt resistor (in series with LED)
----------------------------------------------------------

12V(supply) - 3.5V(across the LED)=8.5V (across the resistor)

if you want 20mA through the LED, then 8.5V/0.02A = 425 Ohms (resistor required)

If you apply 9V through the 425 Ohm setup, there will be (9V-3.5V=5.5V, 5.5V / 425 Ohms = 0.129A

 

[Lou]

Flat side of LED is negative.

See here... and here

 

[gixer]

Thanks
You are the man.
I am trying to gathere all this info quick so I have it in the am,
Does the resistor need to go in a certan way. do they flow in one direction or can they go in any way?
I will be up later reading the info in the links you provided.
Thanks again

Flat side of LED is negative.

See here... and here

 

[PaulJ]

The resister can go in any way, in series of course.

 

[gixer]

thanks just wanted to make sure. some one was saying it went in a certain way. I know you have to read the color bands a certain way on the resistor but I thought it would flow in either direction.

The resister can go in any way, in series of course.