Most LEDs have
their characteristics specified at a current of 20 mA. If you want really good
reliability and you are not certain you don't have worse-than-average heat
conductivity in your mounting, heat buildup in wherever you mount them,
voltage/current variations, etc. then design for 15 milliamps. Now for how to
make 15 milliamps flow through the LED:
First you need
to know the LED voltage drop. It is safe enough to assume 1.7 volts for
non-high-brightness red, 1.9 volts for high-brightness, high-efficiency and
low-current red, and 2 volts for orange and yellow, and 2.1 volts for green.
Assume 3.4 volts for bright white, bright non-yellowish green, and most blue
types. Assume 4.6 volts for 430 nM bright blue types such as Everbright and
Radio Shack. Design for 12 milliamps for the 3.4 volt types and 10 milliamps for
the 430 NM blue.
You can design
for higher current if you are adventurous or you know you will have a good lack
of heat buildup. In such a case, design for 25 ma for the types with voltage
near 2 volts, 18 ma for the 3.4 volt types, and 15 ma for the 430 NM blue.
Meet or exceed
the maximum rated current of the LED only under favorable conditions of lack of
heat buildup. Some LED current ratings assume some really favorable test
conditions - such as being surrounded by air no warmer than 25 degrees Celsius
and some decent thermal conduction from where the leads are mounted. Running the
LED at specified laboratory conditions used for maximum current rating will make
it lose half its light output after rated life expectancy (20,000 to 100,000
hours) - optimistically! You can use somewhat higher currents if you heat-sink
the leads and/or can tolerate much shorter life expectancy.
Next, know
your supply voltage. It should be well above the LED voltage for reliable,
stable LED operation. Use at least 3 volts for the lower voltage types, 4.5
volts for the 3.4 volt types, and 6 volts for the 430 NM blue.
The voltage in
most cars is 14 volts while the alternator is successfully charging the battery.
A well-charged 12 volt lead-acid battery is 12.6 volts with a light load
discharging it. Many "wall wart" DC power supplies provide much higher voltage
than specified if the load is light, so you need to measure them under a light
load that draws maybe 10-20 milliamps.
Next step is
to subtract the LED voltage from the supply voltage. This gives you the voltage
that must be dropped by the dropping resistor. Example: 3.4 volt LED with a 6
volt supply voltage. Subtracting these gives 2.6 volts to be dropped by the
dropping resistor.
The next step
is to divide the dropped voltage by the LED current to get the value of the
dropping resistor. If you divide volts by amps, you get the resistor value in
ohms. If you divide volts by milliamps, you get the resistor value in kilo-ohms
or k.
Example: 6
volt supply, 3.4 volt LED, 12 milliamps. Divide 2.6 by .012. This gives 217
ohms. The nearest standard resistor value is 220 ohms.
If you want to
operate the 3.4 volt LED from a 6 volt power supply at the LED's "typical"
current of 20 ma, then 2.6 divided by .02 yields a resistor value of 130 ohms.
The next higher popular standard value is 150 ohms.
If you want to
run a typical 3.4 volt LED from a 6 volt supply at its maximum rated current of
30 ma, then divide 2.6 by .03. This indicates 87 ohms. The next higher popular
standard resistor value is 100 ohms. Please beware that I consider the 30 ma
rating for 3.4-3.5 volt LEDs to be optimistic.
One more thing
to do is to check the resistor wattage. Multiply the dropped voltage by the LED
current to get the wattage being dissipated in the resistor. Example: 2.6 volts
times .03 amp (30 milliamps) is .078 watt. For good reliability, I recommend not
exceeding 60 percent of the wattage rating of the resistor. A 1/4 watt resistor
can easily handle .078 watt. In case you need a more powerful resistor, there
are 1/2 watt resistors widely available in the popular values.
You can put
LEDs in series with only one resistor for the whole series string. Add up the
voltages of all the LEDs in the series string. This should not exceed 80 percent
of the supply voltage if you want good stability and predictable current
consumption. The dropped voltage will then be the supply voltage minus the total
voltage of the LEDs in the series string.
Do not put
LEDs in parallel with each other. Although this usually works, it is not
reliable. LEDs become more conductive as they warm up, which may lead to
unstable current distribution through paralleled LEDs. LEDs in parallel need
their own individual dropping resistors. Series strings can be paralleled if
each string has its own dropping resistor.
Copyright: Don
Klipstein, Jr. 01/01/00
