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From: firstname.lastname@example.org (Byron A Jeff)
Subject: Re: Best way to power array of LEDs?
Date: 4 Oct 2002 07:39:02 -0400
Organization: College of Computing, Georgia Tech
In article <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net>,
>Have assembled an array of LEDs for use as a 3rd brake light in my car,
Cool. What's the specs on the LEDs? Forward voltage? max current?
>configured as 4 rows of 9 jumbo LEDs, as of yet unwired. Of course the power
>supply will be 12 volts nominal (13.8 volts, maximum).
>My options, as I see them, are:
>1) Wire individual resistors to each LED based on its operating current
This is waste a lot of power in the LEDs.
>2) Wire each row in parallel and connect one resistor based on operating
>current for 9 LEDs
Probably won't work as you won't have enough headroom. At a forward voltage
of 1.7V (typical for Red LEDs) you'll need 15.3V to power 9 of them.
>3) Install voltage regulator to output voltage (ie, 1.3 volts?) to operate
>all LEDs in parallel.
You'd still need current limiting.
And you've missed a very important issue: Will the LED's be bright enough at
their nominal operating current?
Here's what I'd do:
* Build a 555 astable pulse circuit with a 40 percent duty cycle or so.
* Use this to drive a crude current limiter consisting of a standard bipolar
transistor and emitter limiter. Crude ASCII drawing:
V1 and RL sets the current through the string using the formula I = (V1-Vbe)/RL
where VBe is the base emitter voltage (nominally 0.6V) and V1 is the divided
output voltage of the 555. So for example if we wanted to set the current to
80ma and the output of the 555 is 5V because of the 5V regulator we use to
power it then 0.08 = (5-0.6)/RL -> RL = 4.4/0.08 -> RL = 55 ohms. So a 56 ohm
resistor would be perfect. Note that you can get a higher resistor value simply
by driving the 555 with a higher voltage. Also note that V1 still must be
current limited using a series resistor between the output of the 555 and the
base of the transistor. The 555 can easily drive 2 or 3 dozen transistors if
* Finally build an LED string that covers most of available voltage leaving
some headroom. 5 looks like the right number so you'd need 7 strings. The
the anode of the string directly to Vcc and the cathode to the collector of
the current limiter.
And there you have it. Even pulsed very high brightness.
>What is the best method to power these from an intermittent (whenever I press
>the brake pedal) power source?
>Other options for powering this array?
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