Download Lighting with LED

Lighting with LED
LED (Light Emitting Diode) has become very popular as indicators, displays and lighting applications.
The main reason is that they produce minimal heat, consume very little power compared to incandescent
lamps, come in all kind of shapes and sizes and colours and, as they are solid-state, last a long time. The
down side is that they need external current limiting, are not as bright as incandescent lamps and need
special treatment when it comes to dimming and use on AC supplies.
The problem is that most LEDs need to be powered from voltage sources simply because that is what we
find in most electronic equipment.
But why all the fuss?
While incandescent lamps are voltage driven, LEDs are current driven. 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 changes colour to orange and starts emitting more foul smelling smoke than light.
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.
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 with a single resistor. If
four LEDs are placed in series, two thirds of the power makes it to the LEDs and supply current is 75%
lower, quite an improvement.
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.
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.
To calculate resistor values for other configurations, use the following formula:
R = (supply voltage – forward voltage drop of LEDs at rated current)/rated current.
Use the next larger available resistor value. Use 1% resistors to get the brightness of all the LEDs as even
as possible.
True current control
Current control opens new windows of opportunity at the cost of (some) increased complexity. We need
to replace the resistors in the above examples with active devices. Now we will have the ability to adjust
the current through the LED without worrying about the voltage drop over the LED – as long as the
supply is higher than the combined voltage drop over the LEDs plus some left over for the control circuit
to function.
Although only 2 LEDs are shown, it could be anywhere up to 8 red
LEDs in series. We adjust the current with the pot. For voltages at the
pot wiper up to 0.6V nothing happens, but thereafter the current through
the LEDs rise by 10mA per V increase. That means the pot supplies a
voltage to the control transistor to set the current, could one pot supply
many transistors and so adjust to current through many sets of LEDs?
YES! We just need to buffer the pot output and tweak the circuit around
the pot to make sure the control voltage (and with it LED current) does
not go too high and the full sweep of the pot is usable.
The extra diode eliminates the dead range at the bottom of the pot travel and the preset enables us to set
the maximum LED current between15 and 45mA. The 10k resistor limits the current through the Zener
that ensures a stable control voltage. The op-amp and transistor buffers to output voltage. Make sure to
use an op-amp that can handle output voltages close to (1.2V) the negative supply voltage.
The fuse and Zener diode in the dashed box are optional extras. If the control voltage goes to high,
causing the current through the LEDs to rocket, the Zener will conduct, clamping the control voltage and
cause the fuse to blow. One blown fuse instead of 300 blown LEDs, what a bargain!
Special thanks to Mike Powell of www.mikesflighdesk.com for the use of information and diagrams
from his web site. The rest of the info comes from blowing up lots of LEDs and then reading the spec
sheets. No other persons or animals were hurt during research for this article.
Disclaimer:
This article is meant for information purposes only and as a starting guide for other experimenters. Don’t
blame me if you build a circuit and it destroys itself or anything else.