Re: Series resistor for LED?

On 2 Oct 2005 21:49:38 -0700, "Bill Bowden" <wrongaddress@xxxxxxx>
wrote:

>Anybody know a formula to work out the resistor needed for several
>series LEDs operating from a rectified AC source with no filter
>capacitor?
>
>Say the AC rectified source is 24 volts RMS, 34 volts peak,and we have
>5 white LEDs at 3 volts each in series drawing 20mA RMS. What series
>resistor and power rating is needed?
>
>Since the LEDs will not light until the voltage rises above 15 volts,
>the conduction time will be some fraction of the half cycle. The peak
>current will be 34 minus 15 divided by whatever resistor is used, which
>will yield some RMS value. But since the duty cycle will be less than
>100%, the RMS value needs to be higher by some amount as a function of
>duty cycle.
>
>Is there some formula to work out the needed resistor?

---
Since the diodes are going to be conducting even if they haven't lit
up, I think if you use the RMS value of the AC source it'll come out
right. That is:

Vrms - Vled 24V - 15V
Rs = ------------- = ----------- = 450 ohms
Iled 0.02A


Closest safe 5% is 470, so the RMS current will go down to:


Vrms - Vled 24V - 15V
Iled = ------------- = ----------- ~ 0.019A
Rs 470R


And the power dissipation in the resistor will be:


P = Iled * Vrms - Vled = 0.019A * 9V ~ 0.172 watts

--
John Fields
Professional Circuit Designer
.

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