LEDs 101
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.
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 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.
For example let's use the following vlaues : 6 volt supply, 3.4 volt LED, 12 milliamps. Subtract the 3.4
volt LED voltage from the 6 volt supply voltage to get a difference (which
is across the "dropping resistor") of 2.6 volts. Divide 2.6 by .012. This
gives 217 ohms. The nearest standard resistor value is 220 ohms.
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.
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. |
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