P-N Junction Diodes
Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. See Figure 1
Forward Voltage Drop There is a small voltage across a diode when it is conducting, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon.
- Si 0.4 – 0.7V
- Ge 0.2 – 0.3V
Reverse Voltage When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.
Testing diodes You can use a multimeter or a simple tester (battery, resistor and LED) to check that a diode conducts in one direction but not the other. See Figure 2
Forward Bias A diode is said to be forward bias when the positive terminal of a battery is connected to the anode (p-type material) and the negative terminal to the cathode (n-type material). When this happens current flows. See Figure 3
Reverse Bias A diode is said to be reverse bias when the negative terminal of a battery is connected to the anode (p-type) material and the positive terminal to the cathode (n-type) material. When this happens NO current flows. See Figure 4
Signal diodes (small current) Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA.
General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V.
Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio/sound signal from the weak radio signal.
Silicon diodes are better because:
1- they are less easily damaged by heat when soldering
2- they have a lower resistance when conducting
3- they have very low leakage currents when a reverse voltage is applied.
Diode Applications
1. Radio demodulation
2. Rectification (AC to DC)
3. Over-voltage protection
4. Logic gates
“Diagram of Diode OR Gate” (Figure 5).
Rectifier diodes (large current) - Rectifier diodes are used in power supplies and in circuits where a large current must pass through the diode.
- Rectification. The process of converting alternating current (AC) to direct current (DC).
- All rectifier diodes are made from silicon. See Figure 6
Bridge rectifiers They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled .
Zener Diodes See Figure 7
Zener diodes are used to maintain a fixed voltage. They are designed to 'breakdown' in a reliable and non-destructive way so that they can be used in reverse to maintain a fixed voltage across their terminals. The diagram shows how they are connected, with a resistor in series to limit the current.
Zener diode codes can begin BZX... or BZY... Their breakdown voltage is printed with V in place of a decimal point, so 4V7 means 4.7V for example.
Zener diodes are rated by their breakdown voltage and maximum power:
· The minimum voltage available is 2.4V.
· Power ratings of 400mW and 1.3W are common.
Forward Voltage Drop There is a small voltage across a diode when it is conducting, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon.
- Si 0.4 – 0.7V
- Ge 0.2 – 0.3V
Reverse Voltage When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.
Testing diodes You can use a multimeter or a simple tester (battery, resistor and LED) to check that a diode conducts in one direction but not the other. See Figure 2
Forward Bias A diode is said to be forward bias when the positive terminal of a battery is connected to the anode (p-type material) and the negative terminal to the cathode (n-type material). When this happens current flows. See Figure 3
Reverse Bias A diode is said to be reverse bias when the negative terminal of a battery is connected to the anode (p-type) material and the positive terminal to the cathode (n-type) material. When this happens NO current flows. See Figure 4
Signal diodes (small current) Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA.
General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V.
Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio/sound signal from the weak radio signal.
Silicon diodes are better because:
1- they are less easily damaged by heat when soldering
2- they have a lower resistance when conducting
3- they have very low leakage currents when a reverse voltage is applied.
Diode Applications
1. Radio demodulation
2. Rectification (AC to DC)
3. Over-voltage protection
4. Logic gates
“Diagram of Diode OR Gate” (Figure 5).
Rectifier diodes (large current) - Rectifier diodes are used in power supplies and in circuits where a large current must pass through the diode.
- Rectification. The process of converting alternating current (AC) to direct current (DC).
- All rectifier diodes are made from silicon. See Figure 6
Bridge rectifiers They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled .
Zener Diodes See Figure 7
Zener diodes are used to maintain a fixed voltage. They are designed to 'breakdown' in a reliable and non-destructive way so that they can be used in reverse to maintain a fixed voltage across their terminals. The diagram shows how they are connected, with a resistor in series to limit the current.
Zener diode codes can begin BZX... or BZY... Their breakdown voltage is printed with V in place of a decimal point, so 4V7 means 4.7V for example.
Zener diodes are rated by their breakdown voltage and maximum power:
· The minimum voltage available is 2.4V.
· Power ratings of 400mW and 1.3W are common.