An SCR is basically used as a switch to turn on and off the power to a load. A refined technique can also control variable power fed to the load. Another important application is to convert a power source from one waveform to another. The gate terminal is the control where a small gate-current pulse controls a much larger power flow. In comparison to other three-terminal power switches such as the bipolar transistor and the MOSFET, a continuous voltage or current source to the gate to maintain the on-state is not necessary. It can also handle much larger current, and has a lower . The disadvantage of an SCR is the low-speed operation, typically below 10 kHz – 50 kHz, depending on the power level.
It is probably safe to say that the SCR is the most widely used power device. It is capable of carrying thousands of amperes and supporting thousands of volts. The majority of the applications can be classified into the following functions:
1. Static switch: An SCR replaces the electromechanical relay. Typical examples are automobile ignition switch, gas-appliance ignition system, battery charger, and alarm system.
2. Power control: In AC operations, the SCR controls the amount of power delivered to the load. Examples are light dimmer, variable-speed motor, heater control, etc. The settings of the variable resistor determines the phase-lag of the gate pulse with respect to the source. Once the SCR is turned on, it remains to be on until it is turned off by the negative cycle. The average power received by the load is related to the area under this curve.
3. Power conversion: The SCR is used in power conversion and regulation circuits. These include conversion of power source from AC to AC, DC to AC, AC to DC, and DC to DC.
4. Surge protection: The characteristics of an SCR are suitable for a protection device for ICs and discrete components. When a voltage rises beyond a critical value, the SCR is turned on to dissipate the charge or voltage quickly. An example is protection of telephone equipment when the telephone line is exposed to lightning.
A rectifier is a specific type of diode that has the properties of low impedance (high current) under forward bias and high impedance (low current) under reverse bias. A diode is a more generic name for a two-terminal device that has non-linear DC I-V characteristics. A diode is not necessarily a rectifier. The tunnel diode is a good example (see discussions in Introduction). General applications of a rectifier are listed below:
1. Half-wave rectifier: In general, when a waveform containing positive and negative voltage passes through a half-wave rectifier, only one polarity is allowed to pass through. A symmetric waveform such as the sinusoidal becomes a “half-wave.” When the output is passed through a low-pass filter, DC voltage can be obtained from the original AC voltage.
2. Full-wave rectifier: Here the circuit does not block negative voltage, but converts its polarity. It is a more efficient AC-to-DC converter than the half-wave rectifier. Two popular versions are the bridge rectifier and the configuration using a center-tapped transformer.
3. Clipper: The circuit clips one polarity of the wavef to within a selected value, without distorting the rest of the waveform. W V0 = 0, it becomes a half-wave rectifier.
4. Clamper: The circuit shifts the DC level of the wavef without distorting its shape. Notice that the shifted waveform is of polarity, with the minimum voltage at zero. This circuit can also be use an AC-to-DC converter as a “doubler” such that the DC value obtained is twice that from the original AC voltage.
5. Peak detector: A circuit detects the local peak value of a fast signal. It can be viewed as a half-wave rectifier with a low-pass filter. One practical example is the demodulation of AM broadcasting. The time constant of the output signal can be varied by R and C.
6. Logic: Simple OR and AND logics can be implemented.
7. The forward impedance changes with bias and the diode can be used as a varistor.
8. Microwave modulation, mixing, and detection: Although any nonlinear device, not necessarily rectifying, can theoretically perform these functions, the Schottky-barrier diode is most commonly used for this purpose because of the high-frequency capability. In systems such as broadcasting, the signals that contain the information are modulated with a much higher frequency, called the carrier frequency. The advantages of modulation to these higher frequencies are: (1) more efficient transmission through the atmosphere, (2) reduced antenna size, and (3) higher number of channels available. Popular modulation schcmes are amplitude modulation (AM) and frequency modulation (FM).
. The disadvantage of an SCR is the low-speed operation, typically below 10 kHz – 50 kHz, depending on the power level.
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