programmable bec to power leds?

[Carlyle Harper]

Why couldn't I just use the CastleCreations BEC to power two 1 watt luxeons in series? If the forward volt drop of two of these in series is 7 volts then couldn't I just set the bec for 7 volt output?

[fdix]

Of course you can. A switching regulator would make a very efficient setup!

[W.W. Corrigan]

This is what I want to do. The real question I have is do you have to use a resistor even if you set the output voltage to exactly match the combined drops of the leds in the circuit? I put this info, 7 volt source voltage, forward voltage drop 3.5 volts, current of 20ma, and 2 leds into this Calculator http://led.linear1.org/led.wiz
The answer that it returns shows a need for a 1 ohm 1/4 watt resistor in series with the two leds. Is there somthing fundamental to the design of these devices that no matter what you need some resistance in the circuit in order to keep them from burning up?

[rkopka]

Why couldn't I just use the CastleCreations BEC to power two 1 watt luxeons in series? If the forward volt drop of two of these in series is 7 volts then couldn't I just set the bec for 7 volt output?
Might work but it is the wrong way. The LEDs are specified to draw a certain amount of current. At this current there will be a voltage drop of about some voltage over the LED. But this voltage drop will change only slightly for big changes in current. So 7V might give you 5mA or 50mA or even 500mA. You can't be sure. And this value will also change with temperature.

So the right way would be to use an electronics which will control the current. The voltage put in must then only be a little higher than whats needed by the LEDs. And depending on the voltage this electronics accepts you can easily put several LEDs in series without problems.

RK