ZENER DIODE

A zener diode (fig. 4-3) is designed to allow a current to flow through it in a direction that is reverse to the normal flow of current that would occur if it were used as a rectifier. Current can flow through a zener diode in both directions. In the forward direction, current will flow at a low voltage, usually about 1 volt. In the reverse direction, no current will flow until the voltage impressed across it is equal to the zener voltage. At this point, a current will flow and an extremely small increase in voltage will cause a large increase in current. Most importantly, it should be noted that the current flow through the zener diode is in the reverse direction to that of a normal rectifier.

When the zener diode is connected to a source of direct current through a resistor of the correct value, the voltage drop across it can be made to be relatively constant or remain at the same value with wide variations or changes in the voltage of the direct current source. With care in selection of the zener diode and the series resistor, the voltage drop across the diode will remain the same regardless of changes in the source voltage and temperature.

The purpose of the zener diode is to provide a voltage reference.

SILICON CONTROLLED RECTIFIER

Figure 4-4 shows the physical outline and the schematic symbol of the silicon controlled rectifier (SCR). The SCR looks much like an ordinary rectifier and has many of its characteristics. Current will flow through the SCR in only one direction, from the cathode to the anode. Unlike the rectifier, the SCR must be electrically turned on before any current will flow through it.

A current from an external source must flow from the cathode to the gate to turn it on. When the SCR is turned on by this method, it is said to be "fired." Circuits that supply this firing current are called firing circuits.

Another characteristic of the SCR is its ability to stay turned on or fired without the need of a continuous current or signal supplied to the gate. It will remain turned on even if the gate signal is cut off until the current flow of the alternating current main supply reverses its direction of current flow. Because the SCR cannot conduct in the opposite direction, no current will flow and

Figure 4-3.\Zener diode.

Figure 4-4.\Silicon controlled rectifier.

it is said to be cut off. It will not conduct again regardless of the current direction from the source until the gate signal is again applied and the SCR is fired. It will conduct only when fired by the gate signal and when the alternating current flow is in the correct direction (cathode to anode).

The ability of the SCR to stay turned on after it is fired allows it to be fired with short bursts or pulses of current applied to its gate circuit. These pulses must be timed so that they will occur when the alternating current flow is in the correct direction; otherwise, no current will flow. This is known as pulse firing and is the method used to fire the SCRs in voltage regulators.