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Thread: LED Challenged
face BY : Robert A. LaBudde email (remove spam text)

<x-flowed>At 08:06 PM 5/28/00 -0400, you wrote:
>I guess I am really "LED Challenged" as I can't find a circuit that will dim
>an LED used as a backlight.  The LED has a Vf of 3.5 to 4.0 volts @ If of 20
>mA.  I want to run it off 5 volts and want to vary the intensity from "Off"
>to "On" (less the drop across the Vbe) using a 2N3904?  I figured the LED
>limit resistor to be about 45 ohms (5 - .6 - 3.5)/.020) ?  Is this right?
>How do I vary the base drive to vary the LED intensity?  A resistive divider
>with a pot?  How do I calculate the values for the divider?  The 2N3904 has a
>hfe of 100 to 300.  Does anyone care to help me out?

The emitter voltage is the base voltage V(B) minus the diode drop of 0.7 V.
The voltage across the emitter resistor determines the emitter current. The
emitter current is the collector current I(C) (99+%).

So use a voltage divider to generate a voltage V(B) such that

        I(C) = (V(B)-0.7))/R(E)

The maximum current you will be able to get with 5.0 V is

        I(C)max = (5-0.7-3.5)/R(E) = 0.8/R(E)

For this to equal 20 mA, you'll need R(E) = 40 ohm.

However, I believe you've got a mistaken impression on the LED voltage
drop. They usually start about 1.7-2.0 V. In this case

        I(C)max = (5.0-2-0.7)/R(E) = 2.3/R(E)

and you'd need R(E) = 110 ohm.

Also, modern diodes have the same brightness at 5 mA as the original ones
did at 20 mA. You might do better to get by with 0-10 mA instead of 20 mA.

Assuming a 150 ohm resistor for R(E) (to be conservative), then

        I(LED) ~ I(C) ~ (V(B)-0.7)/R(E)

and you set V(B) with a voltage divider. The r(B) will be about beta*150
ohm, or about 20k. You need voltage divider resistors of about 2k to avoid
feedback from the base current.

In summary, use a 2k pot as a voltage divider, backed by 200 ohm resistors
at its ends (unless you want to go fully to saturation and cutoff), use a
150 ohm emitter resistor, and put the LED across the +5 V and collector.

Once you've determined the brightness you want, replace the pot with fixed

Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: STOPspamralspamBeGonespamlcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causas scire"


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