Chapter 24 Thyristors • Objectives – After completing this chapter, the student should be able to: • Identify common types of thyristors. • Describe how an SCR, TRIAC, or DIAC operates in a circuit. • Draw and label schematic symbols for an SCR, TRIAC, and DIAC. 2 • Identify circuit applications of the different types of thyristors. • Identify the packaging used with the different types of thyristors. • Test thyristors using an ohmmeter. 3 • Silicon-controlled rectifiers. – Best known of the thyristors. – Referred to as SCRs. – Three terminals: • anode • cathode • gate – Used primarily as switches. – Controls current in only one direction. 4 • A power transistor would require ten times the trigger signal of an SCR to control the same amount of current. • Constructed of four alternately doped semiconductor layers. – Made from silicon by the diffusion or diffusionalloy method. – Three junctions are formed. 5 – Leads are attached to only three layers to form the anode, cathode, and the gate. 6 • This figure shows that the anode is positive with respect to the cathode and the gate is open. 7 • SCRs are used: – primarily to control the application of DC and AC power to various types of loads. – As switches to open or close circuits. • A small gate current can control a large load current. 8 • TRIACs – An acronym for triode AC semiconductor. – Conduct both directions of AC current flow. – Have the same switching characteristics as SCRs. – Equivalent to two SCRs connected in parallel, back to back. 9 • TRIACs are widely used to control application of power to various types of loads. 10 • TRIAC construction – A four-layer NPNP device in parallel with a PNPN device. – Designed to respond to a gating current through a single gate. – Not equally sensitive to the gate current flowing in opposite directions. 11 • Advantages and disadvantages of SCRs and TRIACs: – Current ratings: • TRIACs up to 25 amperes. • SCRs up to 1400 amperes. – Voltage ratings: • TRIACs maximum rating is 500 volts. • SCRs maximum rating is 2600 volts. 12 – Frequency: • TRIACs from 50 to 500 hertz. • SCRs up to 30,000 hertz. – TRIACs have difficulty switching power to inductive loads. 13 • Bi-directional (two-directional) trigger diodes – Used in TRIAC circuits because they have nonsymmetrical triggering characteristics. – Most frequently used triggering device is the DIAC (diode AC). 14 • DIAC construction: – Three alternately doped layers. – The doping concentration around both junctions is equal. – Leads are only attached to the outer layers. – Packaged like a PN junction diode. 15 • A DIAC has the same effect on current regardless of the direction of flow. – One junction is forward biased. – The other is reverse biased. – Performs as if it contained two PN junction diodes connected in series back-to-back. 16 • DIACs are most commonly used in conjunction with TRIACs to provide fullwave control of AC signals. • Useful for controlling – lamps – heaters – speeds of small motors 17 • Thyristors can be tested with commercial test equipment or an ohmmeter. – Refer to the operator manual for proper settings and readings when using commercial test equipment. – An ohmmeter can detect the majority of defective thyristors. – It cannot detect marginal or voltage-sensitive devices. 18 • In Summary – Thyristors include SCRs, TRIACs, and DIACs. – SCRs: • Control current in one direction by a positive signal gate. • Turned off by reducing the anode-to-cathode voltage to zero. • Used to control current in both AC and DC circuits. 19 • Can handle up to 1400 amperes. • Have voltage ratings up to 2600 volts. • Can handle frequencies up to 30,000 hertz. – TRIACs: • Bi-directional triode thyristors. • Control current in either direction by either a positive or negative gate signal. • Can handle up to 25 amperes. 20 • Have voltage ratings up to 500 volts. • Can handle frequencies up to 400 hertz. • Require the use of a DIAC because they have nonsymmetrical triggering characteristics. – DIACs: • Bi-directional trigger diodes. • Mostly used as triggering devices for TRIACs. – Testing is done using commercial transistor testers or ohmmeters. 21