INTRODUCTION TO OTHER ELECTRONIC DEVICES

CHAPTER 6

BYPN. RUHIYAH NAZIHAH ZAHKAI

ELECTRICAL ENGINEERING DEPARTMENTPOLYTECHNIC SULTAN IDRIS SHAH

SILICON CONTROLLED RECTIFIER (SCR)

An SCR consist of four layers of alternating P and N type semiconductor materials.

As the terminology indicates, the SCR is a rectifier constructed of silicon material with a third terminal for control purposes.

Silicon was chosen because of its high temperature and power capabilities.

PHYSICAL STRUCTURE & SCHEMATIC DIAGRAM

SCR MODES OF OPERATION

In the normal "off" state, the device restricts current to the leakage current.

When the gate-to-cathode voltage exceeds a certain threshold, the device turns "on" and conducts current.

The device will remain in the "on" state even after gate current is removed so long as current through the device remains above the holding current.

CONT’D

Once current falls below the holding current for an appropriate period of time, the device will switch "off".

If the gate is pulsed and the current through the device is below the holding current, the device will remain in the "off" state.

SCR APPLICATION

Mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded.

Suitable for use in medium to high-voltage AC power control applications, such as lamp dimming, regulators and motor control.

SCRs and similar devices are used for rectification of high power AC in high-voltage direct current power transmission.

DIAC

The DIAC, or 'diode for alternating current', is a diode that conducts current only after its break-over voltage has been reached momentarily.

The diac is basically a two-terminal parallel-inverse combination of semiconductor layers that permits triggering in either direction.

DIAC SCHEMATIC SYMBOL

A1

A2

CONT’D

DIACs are also called symmetrical trigger diodes due to the symmetry of their characteristic curve.

Because DIACs are bidirectional devices, their terminals are not labeled as anode and cathode but as A1 and A2 or MT1 ("Main Terminal") and MT2.

DIAC APPLICATION The diacs, because of their symmetrical

bidirectional switching characteristics, are widely used as triggering devices in triac phase control circuits employed for lamp dimmer, heat control, universal motor speed control etc.

TRIAC

TRIAC, from Triode for Alternating Current, is a genericized tradename for an electronic component which can conduct current in either direction when it is triggered (turned on).

The triac is fundamentally a diac with a gate terminal for controlling the turn-on conditions of the bilateral device in either direction.

For either direction the gate current can control the action of the device in a manner very similar to that demonstrated for an SCR.

TRIAC SCHEMATIC SYMBOL

CONT’D

It can be triggered by either a positive or a negative voltage being applied to its gate electrode.

Once triggered, the device continues to conduct until the current through it drops below a certain threshold value, the holding current, such as at the end of a half-cycle of alternating current(AC) mains power.

This makes the TRIAC a very convenient switch for AC circuits, allowing the control of very large power flows with milliampere-scale control currents.

TRIAC APPLICATION

Low power TRIACs are used in many applications such as light dimmers, speed controls for electric fans and other electric motors.

In the modern computerized control circuits of many household small and major appliances.

UJT (UNIJUNCTION TRANSISTOR)

A unijunction transistor (UJT) is an electronic semiconductor device that has only one junction.

A slab of lightly doped (increased resistance characteristic) n-type silicon material has two base contacts attached to both ends of one surface and an aluminum rod alloyed to the opposite surface.

The p-n junction of the device is formed at the boundary of the aluminum rod and the n-type silicon slab.

The single p-n junction accounts for the terminology unijunction.

UJT APPLICATION

The most important applications of UJTs or PUTs is to trigger thyristors (SCR, TRIAC, etc.).

DC voltage can be used to control a UJT circuit such that the "on-period" increases with an increase in the DC control voltage. This application is important for large AC current control.

UJTs can also be used to measure magnetic flux.