🚀 go-pugleaf

On Sat, 05 Oct 2002 01:52:45 GMT, john_wr@NOSPAM.hotmail.com (Johnny)
wrote:

Before someone jumps in here, there's a potential problem with
Negative Temperature coefficient of LEDS for Parrallel connection..  I
have a feeling that the series resistance of LEDs may be enough so
that "Thermal run-away" is not a problem.  Others may know better - I
never actually tried it.

regards,
Johnny.


>Here's another idea:
>
>1) Wire LEDS in 9 groups of 4 series connected leds.  Then all 9
>groups wil conduct with a total voltage of about 9V, and can be
>connected in parrallel. 
>
>2) Use a LM317T voltage regulator as a "Current regulator"  This will
>limit the current to what ever amount you decide. The current can be
>adjusted by changing the resistor value.  The circuit looks like this:
>
>(+12~13.8V)--------[ LM317T ]------- (output pin)
>                            [               ]      >
>                                   |               <  5.6 ohm 1W
>                                   |                >    Resistor 
>                                   |               < 
>                                   ------------------  (adjust pin)
>                                                   |
>
>                                              [ LED]
>                                              [ LED]  Parrallel
>                                              [ LED]     x 9
>                                              [ LED]
>                                                   |
>GND----------------------------------------------
>
>
>So if you use a drive current of 25mA per LED it will be 225mA in
>total.  Othwerwise you can calculated.  The LM317 will dissipate just
>over 1W when the input volage is maximum.  So it will need a small
>heatsink, or if you reduce the current to about 100-150mA you probably
>dont need any heatsink, although it will still get quite warm. 
>
>You can calculate the resistor value by:
>R = 1.25 / (Current in Amps)
>
>Hope this helps,
>
>regards,
>Johnny.
>