Download LEDs : Cool Light

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.
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
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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.
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.
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
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with a single resistor. If four LEDs are placed in series, two thirds of the power makes it to the
LEDs, quite an improvement.
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.
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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.
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Omrekening naar 24V
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