LED's and variable voltage

[Dougget]

I'm planning to light some buildings with white LED's. I will most likely use the DC terminals of an old powerpack for power. I could just mark the 3v position on the knob and not adjust any higher... Or I could plan to set the powerpack at 12v and use resistors for each LED.

A couple of questions...

Can LED's run below their rated voltage to allow for some dim lighting options? I know higher voltage burns them out or at least shortens their life. Will lower voltage have any ill effects?

If lower than rated voltage is okay, just not as bright, will I be able to get better (larger range) of brightness control if I use 12v with resistors vs 3v without resistors?

Thanks,
Doug

 

[tankist]

the lleds obviously shine dimmer when current through them is lower (remmember LEDs are current devices, not voltage), but they can hardly be dimmable, the range between full brightness and cut off is very narrow.

instead why not use 7805 regulator? the brightness will be constant anywhere from 5v to max your pack can crank out. or better yet, just use separate brick for those (your old cellphone charger, wireless phone adapre, etc)

 

[ncmrdispatcher]

Doug -
Always use a series resistor with each LED. Unlike a regular light bulb, they do not appreciate being driven by a fixed voltage because the current will vary wildly with tiny changes in applied voltage. It is very easy to exceed the maximum current rating, typically about 20 mA.

The red line shows an LED without a resistor. The current (which determines brightness) is too hard to control. With a 1K resistor, the current is easy to adjust.

Common practice would be a 1K ohm 1/4 watt resistor for use with a 12 volt supply. To dim just one LED, increase its series resistor to 3.3K, 10K or even more depending on the look you desire. To dim them all at once, use an adjustable power pack or make one with a 3-terminal regulator such as an LM317.

All the best.

 

[lmackattack]

dont use a the knob on a old power pack to control led brightness. Even if you can figure out how to make it work....you will surely bump it one day and blow out all the leds you installed.

 

[CSX Robert]

...Can LED's run below their rated voltage to allow for some dim lighting options?..

Yes.

...Will lower voltage have any ill effects?..

No.

If lower than rated voltage is okay, just not as bright, will I be able to get better (larger range) of brightness control if I use 12v with resistors vs 3v without resistors?..

Yes.

I would measure the maximum ouptut of the power pack(most of them actually put out more than 12 volts) and select my resistor using that value.