Re: Basic question about volts

From: Byron A Jeff (byron_at_cc.gatech.edu)
Date: 12/30/04 

Date: 30 Dec 2004 13:34:28 -0500

In article <rLqdnQPfH4Wc30ncRVn-gg@speakeasy.net>,
mike c <mikechambersATgmail.com> wrote:
>Thanks. That helps clarify things.
>
>One more questions. If I have multiple LEDs, can they all use the same
>resistor?

Yes. Under certain circumstances.

>
>i.e. the 5 volt wire comes in, and has a resistor connected to it. Then
>multiple LEDs are connected to the resistor, and the each LED is
>connected to the ground.

That's not one of the circumstances.

>
>Is that possible? Is this running the LEDs in Parellel?

It is running the LED in parallel. It's a dangerous game though because one
LED will take almost all of the current leaving the others with none. And
since you'll choose the resistor to spread the current evenly among the LEDs,
that current hogger will get way too much current and burn up.

The circumstance that you can run all the LEDs off a single resistor is when
you put them in series. When a circuit is run in series, then each item in
the string will receive equal current. This means that the LEDs will have
even brightness.

But in order to accmplish this you must have sufficient voltage for each LED.
Blue and White LEDs can require as much as 5V each whereas red and greens will
use between 2V and 3V. So if you have 3 red LEDS with a voltage of 2.1V then
you'd need 6.3V minimum available to drive the string. You would then set the
resistor to drop the remaining voltage at the current you wish the drive the
string. So for example of you have a 9V source and you wanted to drive the
string at 10ma then you would set the resistor value at

R=V/I R=(9-6.3)/.01 = 270 ohms.

If you are running in parallel, then each LED should get its own resistor.

BAJ

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