An LED indicator
Figure 1: Various symbols may be used to represent an LED.
Because an LED will often be used in devices where the DC power supply exceeds the maximum forward voltage, a series resistor is customarily used as a simple way to restrict current through the diode. To eliminate the chore of adding a series resistor to limit current through an LED, some indicators are sold with a series resistor built in. They may be rated for use with 5 VDC or 12 VDC, but are externally indistinguishable from each other (and from LEDs that do not contain series resistors).
A through-hole LED will have two leads of unequal length. The longer lead connects internally with the anode of the diode, and should be wired externally to the “more positive” side of a power source. The shorter lead connects internally with the cathode of the diode, and should be wired externally to the “more negative” side of a power source.
Because the effective internal resistance of a diode is not a constant value at different voltages, a trial-and-error approach may be necessary to
An approximate value can be found using a very simple formula in which
R is the resistor value,
VCC is the supply voltage,
VF is the forward voltage specified for the LED, and
I is the desired current:
R = (VCC - VF) / I
Normally a series resistor rated at 1/4 watt will be acceptable, and 1/8 watt may be used in 5VDC circuits. However, care should be taken with a power supply of 9V or higher.
LEDs in parallel
If multiple LEDs are to be driven in parallel, and none of them has to be switched individually, it is naturally tempting to save time by using a single series resistor for all of them. In these circumstances, assess the maximum current carefully and multiply by the voltage drop imposed by each of the LEDs, to determine the wattage of a series resistor.
LEDs containing their own series resistors can safely be wired in parallel.
Where a series circuit receives the same current to each LED, a parallel circuit receives the same voltage to each LED and the total current to each LED is the total current output of the driver divided by the number of parallel LEDs.
The specification for an LED will include the wavelength of emitted light, luminous intensity, maximum forward voltage and current, maximum reverse voltage and current, and working values for voltage and current. All these values are important when choosing an indicator for a specific function.
- Forward current:
LEDs are usually rated for a typical forward current of 20 mA to 25 mA (absolute maximum ratings may be twice as high, but should not generally be applied).
- Forward voltage:
The forward voltage is
the voltage drop across the diode if the voltage at the anode is more positive than the voltage at the cathode(if you connect + to the anode). This is value to calculate the power dissipation of the diode and the voltage after the diode. Red is the color that requires not only the least forward current, but the lowest forward voltage.
|Color||Typical forward voltage|
|Infrared||1.6V to 2V|
|Red||1.6V to 2.1V|
|Orange||1.9V to 2.1V|
|Amber||2V to 2.1V|
|Yellow||2V to 2.4V|
|Green||2.4V to 3.4V|
|Blue||3.2V to 3.4V|
|Ultraviolet||3.3V to 3.7V|
|White||3.2V to 3.6V|