Thyristors are semiconductor devices that can operate only in the switching mode.
Thyristors are typically used in highpower switching applications to control very large amounts of current.
A diac is a two-terminal device that consists of three semiconductor layers and two p-n junctions. Diacs are often used in conjunction with triacs to provide symmetrical triggering.
The silicon controlled rectifier (SCR) is a four-layer pnpn device with three leads, the anode, gate, and cathode.
Unlike an ordinary rectifier diode, an SCR will not conduct until the forward breakover voltage is reached, even though its anode-cathode is forward-biased.
The gate current in an SCR controls the forward breakover voltage. The higher the gate current, the lower the forward breakover voltage.
Once an SCR turns on, the gate loses all control.
The only way to turn off an SCR is to reduce the anode current below the holding current, IH, which is the minimum anode current required to keep the SCR conducting.
A triac is a bidirectional thyristor used to control the power in ac circuits.
A triac has two leads designated MT1 and MT2 or A1 and A2.
Like an SCR, a triac has a gate lead which is used to control its conduction.
A triac is equivalent to two SCRs in parallel.
The unijunction transistor (UJT) has two base leads, B1 and B2, and an emitter (E) lead.
The interbase resistance, RBB, of a UJT is the resistance of its n-type silicon bar. RBB appears as two resistances designated RB1 and RB2.
The ratio <a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg::::/sites/dl/free/0072988215/363997/32_14sum.jpg','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (1.0K)</a> is called the intrinsic standoff ratio, designated η.
UJTs are used in conjunction with SCRs and triacs to control their conduction angle.
