From: Uwe Zimmermann
Subject: Re: Best way to power array of LEDs?
Date: Fri, 04 Oct 2002 11:02:49 +0200
Organization: Royal Institute of Technology, Stockholm, Sweden
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Tilmann Reh wrote:
> DaveC schrieb:
> > Have assembled an array of LEDs for use as a 3rd brake light in my car,
> > 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).
> > What is the best method to power these from an intermittent (whenever I press
> > the brake pedal) power source?
> Better use many LEDs in series to avoid unnecessary power loss.
> With 12V supply, you can use about 5 LEDs in series and a smaller
> resistor for each chain. These chains are then connected in parallel.
The series connection of as amny LEDs as the supply voltage allows is
indeed the way to go. As others stated correctly the bus voltage in a
car is far from constant and therefore the use of a resistor might
result in significant current variations through the LEDs, but here we
actually can use the simple constant current source I suggested in a
different thread recently.
For a min voltage of 12V in a car and a typical LED voltage of 1.6V you
can wire up 6 LEDs in series, giving exactly 6 identical chains in your
case. Even if the LEDs should drop 1.8V, this would be sufficient...
Now take one BF245B/BF245C JFET transistor for each chain and connect
source and gate together. This will give a constant current source with
the characteristic current Idss. Since this current spreads from
transistor to transistor you should get yourself some more of those and
select 6 which have a current which suits the parameters of your LEDs.
You then connect the drain of the transistor to the positive supply and
the gate/source to the anode of the first LED in each chain. The cathode
of the last LED is connected to the negative supply.
12V o---o o----o---|>|--|>|--|>|--|>|--|>|--|>|----o 0V
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