A Light-Emitting Diode (LED) in essence is a P-N junction solid-state semiconductor
diode that emits light when a current is applied though the device. By scientific
definition, it is a solid-state device that controls current without the deficiency of having
heated filaments. How does a LED work? White LEDs ordinarily need 3.6 Volts of Direct
Current (DC) and use approximately 30 milliamps (mA) of current and has a power
dissipation of approximately 100 milliwatts (mW). The positive power is connected to one
side of the LED semiconductor through the anode and a whisker and the other side of the
semiconductor is attached to the top of the anvil or the negative power lead (cathode). It is
the chemical composition or makeup of the LED semiconductor that determines the color of
the light that the LED produces as well as the intensity level. The epoxy resin enclosure
allows most of the light to escape from the elements and protects the LED making it virtually
indestructible. Furthermore, a light-emitting diode does not have any moving parts, which
makes the device extremely resistant to damage due to vibration and shocks. These
A rectifier is a dispositive that ideally transforms the AC input voltage into a DC voltage
(voltage is always positive or zero). These diodes have the largest ratings and sometime
can be quite big in volume. As a rule of thumb, the bigger the diode (more pn surface
junction available for heat dissipation), the higher the ratings.
A half-wave rectifier is composed of a single diode that connects an AC source to a load. In
figure 3 the load is represented by a resistor. The diode conducts on AC voltage only when
its anode is positive with respect to the cathode (i.e. greater than 0.7 V for a silicon diode).
The output has therefore only a positive component with an average value:
The output peak voltage is the AC source minus the voltage drop of the diode, that in most
cases can be neglected.
In half-wave rectifiers, half of the power provided by the source is not used. To solve this
problem, we have to use full-wave rectifiers. The minimum full-wave rectifier is composed
of two diodes, but it requires a center tapped transformer. Figure shows a bridge rectifier,
composed of four diodes, that can use a “normal” transformer.
The AC current, according to its direction, flows either in the top or in the bottom part
of the bridge in each half-cycle. In the output voltage we will have a component for both
negative and positive parts of the input voltage. In both cases the current passes through
two forward-biased diodes in series, what produces a voltage drop of 1.4 V.
The average voltage of a full-wave rectifier is:
Full wave rectifier. In this case the voltage drop, not shown in the graphic, will be
1.4 V because two diodes are cros.sed.
We can distinguish the following types of diodes:
• Rectifier diodes are typically used for power supply applications. Within the power
supply, you will see diodes as elements that convert AC power to DC power;
• Switching diodes have lower power ratings than rectifier diodes, but can function better
in high frequency application and in clipping and clamping operations that deal with
short-duration pulse waveforms;
• Zener diodes, a special kind of diode that can recover from breakdown caused when
the reverse-bias voltage exceeds the diode breakdown voltage. These diodes are
commonly used as voltage-level regulators and protectors against high voltage surges;
• Optical diodes;
• Special diodes, such as varactors (diodes with variable capacity), tunnel diodes or
There are four diode ratings that apply in one way or another to all types of diodes and
1. Forward voltage drop VF : is the forward-conducting junction level ( 0.7 V for Si diodes
and 0.3 V for Ge diodes)1.
2. Average forward current IF : is the maximum amount of forward current that the diode
can carry for an indefinite period. If the average current exceeds this value, the diode
will overheat and, eventually, will be destroyed.
3. Peak reverse voltage VR, or reverse breakdown voltage. This is the largest amount of
reverse-bias voltage the diodes’s junction can withstand for an indefinite period of time.
If a reverse voltage exceeds this level, the voltage will punch through the depletion layer
and allow current to flow backwards through the diode, which is a destructive operation
(except for the case of a Zener diode).
4. Maximum power dissipation P. The actual diode power dissipation is calculated multiplying
the forward voltage drop and the forward current. Exceeding the maximum
power dissipation will result in thermal breakdown of the diode.
Excessive forward current and reverse breakdown voltage are the most common causes
of diode failure. In both cases the diode gets very hot, what destroys the pn junction. Occasional
peaks of voltage or current exceeding these rates for very short times (few milliseconds)
may not overheat the junction, but repeated peaks may fatigue the junction. By
design, diodes are selected with ratings that exceed two or three times the expected peaks
in the circuit.
When the diode is reverse-biased, a very small drift current due to thermal excitation flows
across the junction. This current (reverse saturation current, I0) is given, according to the
Boltzmann equation, by the formula:
where K0 is a constant depending on the pn junction geometry and V0 is the built-in voltage
of the diode (see chapter “Semiconductor Materials: pn junction”).
Forward BiasWhen the diode is forward-biased through a voltage V , a small drift current flows again
across the junction. In that case, however, there is an additional component, the diffusion
current Vd, given by the formula: