LEDs and Dimming

LEDs: Cool Light

Light emitting diodes are an inexpensive source of light for instrument, panel and task lighting. They offer a choice of colors and even “white” LEDs are now affordable. They have no filaments to burn out, and efficiencies have grown to the point that some are amazingly bright.

Sometimes so bright, you’d like to be able to dim them.

Less current : less light

LEDs are current driven devices. Fewer amps in means fewer photons out. But we don’t generally power LEDs from a true current source. The vast majority of power supplies are voltage sources, and that’s what we usually find powering LEDs.

Of course, it’s not quite that simple

The relationship between current and voltage in an LED is non-linear. As the voltage increases from zero there is only a trickle of current and no noticeable light. At about a volt and a half (we’re talking red LEDs here) the current begins to increase appreciably and the first glimmers appear. At two volts the LED is bright and with a fraction more it’s very bright. Once over about 2.2 volts, the current rapidly soars beyond safe operation. The LED soon overheats and dies.

LED current graph

Nonetheless, you can power an LED directly from a voltage source if you control the voltage carefully, but there are problems. As the temperature of the LED changes the operating voltage for constant light output changes. Generally we would prefer that the light levels stay put. Further, if you parallel a number of LEDs and power them directly from a voltage source, they won’t all have the same light output. Each will have a slightly different operating voltage for the same light output. All in all, powering LEDs directly from a voltage source is not such a good idea.

So make it act linear

The usual approach is to put a resistor in series with the LED. The combination is still non-linear, but in a much more well behaved manner. In fact, over the range of safe operating current, it acts incrementally linear.

LED with resistor

The catch is that it wastes power. If a 12 volt supply is powering a single LED-resistor combination, 2 volts goes to the LED and 10 to the resistor. Only one sixth of the power makes it to the LED.

LED wasted power

If you only have a few LEDs this is not a big deal. If you’re lighting several panels you may have a couple hundred LEDs. Depending upon your budget and power supplies, this may become a big deal.

There are several approaches with varying degrees of complexity that address this. An effective compromise between complexity and efficiency is to place several LEDs in series with a single resistor. If four LEDs are placed in series, two thirds of the power makes it to the LEDs, quite an improvement.

Serial LEDs

Back to dimming LEDs

If you simply reduce the voltage to a resistor-LED series combination, you will dim the LED. The slight hiccup is that the LEDs will not begin to light up until there’s about 1.5 volts across each LED. If you have four LEDs in series, the power supply has to be cranked up to 6 volts to get those red photons started.

So if you’ve got a 0 to 12 volt variable power supply dimming your 4-LED chain, you have to turn the knob half way around before you get some action. A subtle point, to be sure, but an important one to the true dimming aficionado.

Here’s an inexpensive dimmer that starts at about 5.6 volts and goes up to 2 volts below the supply voltage. It uses readily available parts and can supply up to an amp of lighting current if the 7805 has an adequate heat sink. You could have up to about 50  groups of four LEDs, although the 7805 may start to get a bit toasty.

LED dimmer

Not all LEDs have the same forward voltage drop. The 2.0~2.2 volts common for red LEDs is not shared by all LEDs. Blue and white LEDs may have up to 5 volts across them when operating normally, although 3.5 volts is more common. If you want to use this dimmer circuit with 3.5 volt blue or white LEDs,  put just two LEDs in series and use 150 ohm resistors in place of the 82 ohm units.