Download Principles and Applications of Electronic Devices and Circuits Unit

Principles and Applications of Electronic Devices and Circuits
Unit 35
Session Objectives (P1)
Explain the purpose of two different types of diode, each in a different
electronic circuit application
The General Purpose Diode 1N4001 in Forward Bias Mode
Construct the following circuit in Multisim. To connect a diode in forward bias mode
the + terminal of the power supply (V1) is connected to the positive terminal of the
diode and the negative terminal of the diode is connected to the negative terminal of
the power supply.
Note that no voltage is applied to the circuit and that no current flows.
Raise the voltage (V1) to 0.3 volts, note that now a very small current has begun to
flow.
Now double the voltage (V1) and note the change in the magnitude of the current
flowing.
Finally, raise the voltage (V1) to 1 volt
Try plotting the results on a graph. If you wish, you could collect more data for the
graph by getting more samples at different voltages.
From the graph you should be able to give an approximate value for when the diode
switches on and conducts a reasonable amount of current.
For the purposes of this exercise we will call a reasonable amount of current 1 mA.
Task P1 (1a)
Briefly explain the function of a diode in forward bias mode
The General Purpose diode 1N4001 in Reverse Bias Mode
To connect a diode in reverse bias mode we simply reverse the connections. Do this
and set V1 to 49 volts.
Now the circuit is conducting 718 nano amps. That is .718 millionths on an amp. To
all intents and purposes the circuit is not conducting at all.
Now raise V1 to 50.1 volts and note the change in current. It has increased by an
enormous margin.
146/0.0007618 = ?
The point we have reached is known as the reverse breakdown voltage of the diode.
In reverse bias mode a diode will not conduct current until this point is reached. For
general purpose diodes the reverse breakdown voltage is usually much higher than
the forward bias voltage. You can find the reverse breakdown voltage for any diode
by looking at its data sheet.
Task P1 (1b)
Briefly describe the function of a diode in reverse bias mode.
The 1N4001 Diode and The Half Wave Bridge Rectifer
Now connect up the following circuit and simulate it as shown.
Note how the diode only conducts on the positive half cycles of the input sine wave.
This process is called half wave rectification because only half of the waveform
conducts.
Note now what happens if we connect a capacitor in parallel across the output.
The capacitor smooths the output of the halfwave bridge rectifier. For many
applications such as charging car batteries this is all that is required to turn AC into
DC. However for more critical applications such as computers and logic circuits it is
more efficient to use something called Full Wave Bridge Rectification.
A full wave bridge rectifier converts both halves of a sine wave into DC. Now
consider the next circuit.
Now both halves of the waveform conduct. For one half of the cycle D2 & D3 operate
in forward bias mode (with D1 & D4 in reverse bias mode). For the other half of the
cycle this situation is reversed.
Tasks P1 (2 & 3)
What is the purpose of the 4 diodes outlined in red in the circuit, and what is this
configuration known as?
Do the diodes operate in forward and reverse bias mode?
The Zener Diode
A zener diode is a special type of diode that is manufactured to operate in reverse
bias mode. Its reverse breakdown voltage is usually much smaller than that of a
general purpose diode and it can be more precisely achieved. zeners can therefore
be used as stable and accurate voltage references with a large number of potential
applications not the least of which is their use in regulated power supplies.
The circuit on the next page shows the simplest type of zener regulator. The half
wave regulator made up of the 1N4001 diode and smoothing capacitor supplies a DC
voltage across the zener diode which is greater than the zener reverse breakdown
voltage.
For clarity I have removed the dc component from the supply voltage so that you can
see the ripple. We can clearly see that the voltage across the zener diode remains
stable despite fluctuations in the supply voltage.
P1 (Zener)
Briefly explain the difference between and general purpose diode and a zener diode
What is the normal mode of operation for a zener diode?
Briefly explain the purpose of the zener diode in this circuit
Voltage across the
Zener
Zener diode
Voltage from half wave
rectifier