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Chapter 24
• 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,
• Identify circuit applications of the different types of
• Identify the packaging used with the different types
of thyristors.
• Test thyristors using an ohmmeter.
• 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.
• 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.
– Leads are attached to only three layers to form
the anode, cathode, and the gate.
• This figure shows that the anode is positive with
respect to the cathode and the gate is open.
• 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
– An acronym for triode AC semiconductor.
– Conduct both directions of AC current flow.
– Have the same switching characteristics as
– Equivalent to two SCRs connected in parallel,
back to back.
• TRIACs are widely used to control
application of power to various types of
• 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.
• Advantages and disadvantages of SCRs and
– 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.
– Frequency:
• TRIACs from 50 to 500 hertz.
• SCRs up to 30,000 hertz.
– TRIACs have difficulty switching power to
inductive loads.
• Bi-directional (two-directional) trigger
– Used in TRIAC circuits because they have
nonsymmetrical triggering characteristics.
– Most frequently used triggering device is the
DIAC (diode AC).
• 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.
• 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.
• 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
• 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
– An ohmmeter can detect the majority of
defective thyristors.
– It cannot detect marginal or voltage-sensitive
• In Summary
– Thyristors include SCRs, TRIACs, and DIACs.
– SCRs:
• Control current in one direction by a positive signal
• Turned off by reducing the anode-to-cathode voltage
to zero.
• Used to control current in both AC and DC circuits.
• Can handle up to 1400 amperes.
• Have voltage ratings up to 2600 volts.
• Can handle frequencies up to 30,000 hertz.
• Bi-directional triode thyristors.
• Control current in either direction by either a positive or
negative gate signal.
• Can handle up to 25 amperes.
• 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.