🚀 go-pugleaf

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
        |  |    |
       ------   |
          |     |
          o-----o 

   Uwe.