In this
article, we will discuss the concept behind some of the most frequently asked
questions about diode in academics as well as interviews for Electrical
Engineer, Electronics Engineer and other related in engineering fields like, How
to use a Diode in a circuit and how it behaves for DC supply as well as for AC
supply?
This
article is specifically designed to provide a practical approach to using a
diode in electronic circuits.
A Diode is a very basic and important component in electronics. It has a unidirectional nature of current conduction i.e. current only flows in one direction only. A diode conducts current only when it is forward biased and it does not allow conduction of current into reverse bias.(discussed in detail in working of diode)
In the
above statement the conduction of current indicates that a forward biased diode
acts as a short circuit so that conduction of current takes place while a
reverse biased diode acts as an open circuit.
A forward
biased diode only conducts if the voltage applied to the terminals of the diode
is greater than the threshold voltage or Knee voltage. This knee voltage offers
a small voltage drop in the path of conduction of current hence a forward
biased diode acts as a short circuit with a small voltage drop.
Working of Diode in a Circuit
Let us understand the above discussion with the help of a circuit. Consider a circuit as shown in given figure, in which a load is connected to DC supply through a diode.
In the
above figure we see that the voltage supplied to the load is V, which positive
terminal is connected to the anode of the diode and its negative terminal is
connected to the cathode of the diode through the load. Means the diode is
forward biased and it is ready to start conduction.
If the
applied voltage to the diode is greater than the threshold voltage of diode VT then this diode acts as a short circuit with a voltage drop VT and starts
conduction as shown in the given figure.
The voltage
drop VT across diode indicates that there is some energy dissipated in the
diode, hence a diode is characterized as a passive element.
But if we
reverse the polarity of the voltage applied to the circuit then the diode does
not allow conduction of current and it behaves like an open circuit as shown in
the given figure.
If we
change the polarity of the voltage applied to the circuit continuously in every
20 milli-second like AC supply then, how does the diode behave in a circuit?
Working of Diode in AC and DC
In case of DC
supply the behavior of a diode in a circuit is the same as discussed above. As
DC supply has constant polarity and direction throughout the time until it does
not change intentionally.
In case of
AC supply the polarity and the direction of supply changes continuously in
every 20 milli-second. In that case a diode conducts for that 20 milli-seconds
for which the supply is positive and it does not conduct for that 20 milli-seconds
for which the supply is negative.
If we
analyze this behavior of the diode then we find that it acts like a switch
having switching frequency is the same as that of the supply frequency.
- For positive half of AC supply (20ms) Diode D is on.
- For negative half of AC supply (next 20ms) Diode D is off.
Let us
understand the working of a diode in AC by a circuit.
Consider a
circuit as shown in the given figure, in which a resistive load having
resistance R is connected to AC supply VS through a diode.
In the
above circuit, if positive half of the AC supply is given, then the terminal a
of the diode is positive with respect to the terminal b. That means the diode
is forward biased and it conducts. In this case the load voltage VL is equal to
the positive half of the supply voltage Vs.
When
negative half of the AC supply is given to the circuit, then the terminal a of
the diode is negative with respect to the terminal b. That means the diode is
reverse biased and it does not conduct. In this case the load voltage VL is
equal to zero.
If a
continuous pulse of AC signal is given to the above circuit, then the output of
the circuit is shown as in the given figure.
If we
analyze the output waveform of the above circuit then we will understand
that in this circuit a diode works as a rectifier (A rectifier is an electronic
circuit that converts AC signal into pulsating DC). The application of a diode
in a rectifier circuit is very basic and the important application of a diode.
Above
circuit which we will discuss is the half wave rectifier circuit. As we will
see in this circuit only the positive half of the AC supply is rectified or the
circuit only works for the positive half of the AC supply.
Apart from
the above discussion, there are several most popular applications of diodes,
some of which are listed below.
- Clipper circuit
- Clamper circuit
- Voltage multiplier
- Filter design
- Logic Gates
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