The PN-junction diode, shown in Figure 1, is the most basic of semiconductor devices. This diode is formed by a doping process, which creates P-type and N-type semiconductor materials on the same component.
An N-type semiconductor material has electrons (represented as negative charges) as the current carriers while the P-type has holes (represented as positive charges) as the current carriers. N-type and P-type materials exchange charges at the junction of the two materials, creating a thin depletion region that acts as an insulator.
Operation of Semiconductor Diode
Diode leads are identified as the anode lead (connected to the P-type material) and the cathode lead (connected to the N-type material). The most important operating characteristic of a diode is that it allows current in one direction and blocks current in the opposite direction.
When placed in a DC circuit, the diode will either allow or prevent current flow, depending on the polarity of the applied voltage. Figure 2 illustrates two basic operating modes of a diode: forward bias and reverse bias.
A forward-bias voltage forces the positive and negative current carriers to the junction and collapses the depletion region to allow current flow.
A reverse-bias voltage widens the depletion region so the diode does not conduct. In other words, the diode conducts current when the anode is positive with respect to the cathode (a state called forward-biased) and blocks current when the anode is negative with respect to the cathode (a state called reverse-biased).
Operation of Rectifier Diode
Rectification is the process of changing AC to DC. Because diodes allow current to flow in only one direction, they are used as rectifiers. There are several ways of connecting diodes to make a rectifier to convert AC to DC.
Figure 3 shows the schematic for a single-phase, half-wave rectifier circuit. The operation of the circuit can be summarized as follows:
• The AC input is applied to the primary of the transformer; the secondary voltage supplies the rectifier and load resistor.
• During the positive half-cycle of the AC input wave, the anode side of the diode is positive.
• The diode is then forward-biased, allowing it to conduct a current to the load. Because the diode acts as a closed switch during this time, the positive half-cycle is developed across the load.
• During the negative half-cycle of the AC input wave, the anode side of the diode is negative.
• The diode is now reverse-biased; as a result, no current can flow through it. The diode acts as an open switch during this time, so no voltage is produced across the load.
• Thus, applying an AC voltage to the circuit produces a pulsating DC voltage across the load.
Diodes can be tested for short-circuit or open-circuit faults with an ohmmeter. It should show continuity when the ohmmeter leads are connected to the diode in one direction but not in the other.
If it does not show continuity in either direction, the diode is open. If it shows continuity in both directions the diode is short-circuited.
Operation of Bridge rectifier
The half-wave rectifier makes use of only half of the AC input wave. A less-pulsating and greater average direct current can be produced by rectifying both half-cycles of the AC input wave. Such a rectifier circuit is known as a full-wave rectifier.
A bridge rectifier makes use of four diodes in a bridge arrangement to achieve full-wave rectification. This is a widely used configuration, both with individual diodes and with single-component bridges where the diode bridge is wired internally. Bridge rectifiers are used on DC injection braking of AC motors to change the AC line voltage to DC, which is then applied to the stator for braking purposes.
The schematic for a single-phase, full-wave bridge rectifier circuit is shown in Figure 4. The operation of the circuit can be summarized as follows:
• During the positive half-cycle, the anodes of Dl and D2 are positive (forward-biased), whereas the anodes of D3 and D4 are negative (reverse-biased). Electron flow is from the negative side of the line, through D1, to the load, then through D2, and back to the other side of the line.
• During the next half-cycle, the polarity of the AC line voltage reverses. As a result, diodes D3 and D4 become forward-biased. Electron flow is now from the negative side of the line through D3, to the load, then through D4, and back to the other side of the line.
Note that during this half-cycle the current flows through the load in the same direction, producing a full-wave pulsating direct current.
Operation of Capacitor Filter
Some types of direct current loads such as motors, relays, and solenoids will operate without problems on pulsating DC, but other electronic loads will not.
The pulsations, or ripple, of the DC voltage can be removed by a filter circuit. Filter circuits may consist of capacitors, inductors, and resistors connected in different configurations. The schematic for a simple half-wave capacitor filter circuit is shown in Figure 5.
Filtering is accomplished by alternate charging and discharging of the capacitor. The operation of the circuit can be summarized as follows:
• The capacitor is connected in parallel with the DC output of the rectifier.
• With no capacitor, the voltage output is normal half-wave pulsating DC.
• With the capacitor installed, on every positive half-cycle of the AC supply, the voltage across the filter capacitor and load resistor rises to the peak value of the AC voltage.
• On the negative half-cycle, the charged capacitor provides the current for the load to provide a more constant DC output voltage.
• The variation in the load voltage, or ripple, is dependent upon the value of the capacitor and load. A larger capacitor will have less voltage ripple.
Three Phase Full-Wave Bridge Rectifier
For heavier load demands, such as those required for industrial applications, the DC output is supplied from a three-phase source. Using three-phase power, it is possible to obtain a low-ripple DC output with very little filtering.
Figure 6 shows a typical three-phase full-wave bridge rectifier circuit. The operation of the circuit can be summarized as follows:
• The six diodes are connected in a bridge configuration, similar to the single-phase rectifier bridge to produce DC.
• The cathodes of the upper diode bank connect to the positive DC output bus.
• The anodes of the lower diode bank connect to the negative DC bus.
• Each diode conducts in succession while the remaining two are blocking.
• Each DC output pulse is 60° in duration.
• The output voltage never drops below a certain voltage level.
Inductive loads, such as the coils of relays and solenoids, produce a high transient voltage at turnoff. This inductive voltage can be particularly damaging to sensitive circuit components such as transistors and integrated circuits.
Despiking Circuit
A diode-clamping, or despiking circuit connected in parallel across the inductive load can be used to limit the amount of transient voltage present in the circuit.
The diode-clamping circuit of Figure 7 illustrates how a diode can be used to suppress the inductive voltage of a relay coil. The operation of the circuit can be summarized as follows:
• The diode acts a one-way valve for current flow.
• When the limit switch is closed, the diode is reverse-biased.
• Electric current can’t flow through the diode so it flows through the relay coil.
• When the limit switch is opened, a voltage opposite to the original applied voltage is generated by the collapsing magnetic field of the coil.
• The diode is now forward-biased and current flows through the diode rather than through the limit switch contacts, bleeding off the high-voltage spike.
• The faster the current is switched off, the greater is the induced voltage. Without the diode the induced voltage could reach several hundreds or even thousands of volts.
• It is important to note that the diode must be connected in reverse bias relative to the DC supply voltage. Operating the circuit with the diode incorrectly connected in forward bias will create a short circuit across the relay coil that could damage both the diode and the switch.
