Mikes Tips - LEDs, More LEDs, and Dimming

LEDs, More LEDs, and Dimming

We typically place a resistor in series with a LED to limit the current. By choosing the appropriate resistance we can power the LED from whatever voltage happens to be handy in our circuitry. This is very convenient, but not very efficient. The resistor usually burns more power than the LED. If we’re just using a few LEDs for power-on or gear-up lamps, the wasted power isn’t a big deal.

But LEDs can be used for more than just simple indicators. A few small LEDs can illuminate engine and navigation instruments. A dozen will backlight a small panel. A small cluster of high output LEDs makes a great map light; a handful can even provide cabin lighting. When we start using a lot of LEDs the amount of lost power from current limiting resistors adds up, and wasted power becomes big deal.

It’s not just the cost of the power. There’s the cost of larger or additional power suppliers. There’s the waste heat. And as home simulators get larger and more power hungry, there’s the question of where the power comes from. Normal house wiring places a limit on how much power is available before you start popping breakers. It can pay to keep an eye on overall efficiency, and how LEDs are wired is one factor to keep in mind.

The way to boost efficiency is to run several LEDs in series with a single, current limiting resistor.

Just how many is “several” depends on factors we probably don’t have information about like power supply stability, manufacturing spread of the LEDs, and so on. Not having firm numbers, we can turn to a rule of thumb: simply choose a number of LEDs that leaves at least 15% of the power supply voltage across the resistor. This provides a workable if perhaps not optimum answer.

So, an example:

We have a 12 volt lighting power bus and LEDs that drop 2.1 volts at the desired operating current of 20 milliamps. Since we want at least 15% of the 12 volts to be across the resistor, we’ll have no more than 85% across the series of LEDs. We multiply 12 volts by 0.85 to get 10.2 volts then divide that by 2.1 volts, the LED voltage. This gives us 4.86 which means we can use 4 LEDs in series.

Four LEDs will drop 8.4 volts leaving 3.6 volts across the resistor. We now use Ohm’s Law to calculate the resistance value to limit the current to 20 milliamps. R = V/I = 3.6/.02 = 180 Ohms.

By stacking four LEDs, we cut power consumption by a factor of four compared to using individual LED-resistor pairs, plus we need fewer resistors.




Dimming

We can dim LEDs by varying the voltage, though the dimming action is nonlinear. Most of the dimming happens at the high end. As the voltage decreases from max, the light drops off rapidly so that for most of the dimmer’s knob rotation there is little or no light. This effect is more pronounced when we stack LEDs to gain efficiency. Fortunately there is a better way.

Pulse width modulation (PWM) is a much better method of dimming LEDs. The light output changes in a linear fashion. We also get a bonus in that PWM circuitry is more efficient than the old style variable voltage dimmers.

PWM alternates between supplying full power to the LEDs, and turning them off completely. The dimming action comes from varying the percentage of time the LEDs are on. Because the PWM dimmer switches so fast, we don’t see the light flickering; we just see the average light output.

Here’s a basic PWM dimmer that can be used over a range of voltages. It works equally well with LEDs and incandescent lamps. This can handle up to 10 amps of lighting supply current.



The dimmer uses standard parts available from all the usual electronic parts suppliers like Jameco (www.jameco.com), Digikey (www.digikey.com), Mouser (www.mouser.com), etc.

Mike Powell, author of

Building Recreational Flight Simulators and

Building Simulated Aircraft Instrumentation.

www.mikesflightdeckbooks.com