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Triac and diac

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Triac and diac

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Triac and diac

  1. 1. 1 • 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.
  2. 2. 2 • 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 diffusion- alloy method. – Three junctions are formed.
  3. 3. 3 – Leads are attached to only three layers to form the anode, cathode, and the gate.
  4. 4. 4 • This figure shows that the anode is positive with respect to the cathode and the gate is open.
  5. 5. 5 • 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.
  6. 6. 6 • 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.
  7. 7. 7 • TRIACs are widely used to control application of power to various types of loads.
  8. 8. 8 • 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.
  9. 9. 9 • 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.
  10. 10. 10 – Frequency: • TRIACs from 50 to 500 hertz. • SCRs up to 30,000 hertz. – TRIACs have difficulty switching power to inductive loads.
  11. 11. 11 • 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).
  12. 12. 12 • 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.
  13. 13. 13 • 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.
  14. 14. 14 • DIACs are most commonly used in conjunction with TRIACs to provide full- wave control of AC signals. • Useful for controlling – lamps – heaters – speeds of small motors
  15. 15. 15 • 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.
  16. 16. 16 • 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.
  17. 17. 17 • 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.
  18. 18. 18 • 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.

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