LEDs from variable DC?

[cncproadwarrior]

Just wondering if I can power my LEDs from variable DC to be able to control brightness.

 

[UP2CSX]

Yes...for about three microseconds, before they blow out. LED's need a constant source of current to light. You could, I suppose have one LED and several power routes with different resistors and then be able to pick from which one you want to make it brighter or dimmer, but you can't use variable DC like a light dimmer on LED's. Incandescent lamps work much better for this purpose.

 

[MGWSY]

use a 1000 ohm resistor with the LED and you will be able to use it up to 12-14 volts. It will go out around 3 to 4 volts with the resistor depending what color the LED is.

 

[RexHea]

Bernie: an LED has a set amount of current that it will operate with and with a relatively small +/- range. Too low and it's out; too high and you have blown it. While some adjustment is possible I don't think it will give you what you want. Follow Jim's advice and use incandescent for dimmer bulbs.

 

[tankist]

or build pulse width modulation circuit. the duty cycle will determine the apparent brightness.

something like this.
http://www.reuk.co.uk/LED-Dimmer-Circuit.htm
i've seen better but can't seem to find them at the moment...

EDIT:
hek, or just buy one - http://www.hollysolar.com/control/dimmer/dimmer.html

 

[RexHea]

Anton: Good ole 555 timer. Wonder how many thousand circuits that it is used in.

 

[tankist]

Anton: Good ole 555 timer. Wonder how many thousand circuits that it is used in.

this is an area that I would like to be so much more verse in. i still can't forgive myself sleeping through electronics classes in HS. i really could use the knowledge now, calculating transistor cascades in particular

oh well, there will always be a smart guy out there and i am just going to copy his stuff...

 

[RexHea]

No sweat! The 555 timer IC in the circuit you posted is used in the most simple of circuits to the most complex of electronic circuitry. Great for home grown (hobby) electronics.

 

[ncmrdispatcher]

Bernie -

Mark G. offered the simplest solution: just put a 1K ohm resistor (or thereabouts) in series with the LED. An LED tries to limit the voltage applied to it by exponentially increasing the current through it until it melts, sort of like a water tank that starts to overflow (water depth = voltage, current = gallons or liters/second.) We tame this bad behavior by adding a series resistor, also known as a current limiting resistor.

Here is a chart of a typical white LED operated without and with a 1K ohm dropping resistor. The horizontal axis represents the voltage out of the power supply, and the vertical axis represents the current going through the LED.

The red line shows what happens without a resistor. A tiny change in the voltage of the power supply causes very steep changes in the LED current. The LED brightness would be extremely difficult to control and we would risk quickly burning it out. The small LEDs we typically use in our models usually have a maximum current rating of about 20 milliamps.

With a 1K resistor wired in series with the LED, the LED current can be smoothly and easily adjusted over a wide range. A 1K resistor is just a handy value. If you wanted a greater maximum current, you can use a lower value of resistor (say, 560 ohms.)

I'm just trying to help clarify why we use dropping resistors, and hope that you find this to be helpful.

All the best.