The hysteresis (snap back) effect is somewhat similar to
the action of a kerosene lantern. That is, when the control
knob is first rotated from the off condition, the lamp can be
lit only at some intermediate level of brightness, similar to
turning up the wick to light the lantern. Brightness can then
be turned down until it finally reaches the extinguishing
point. If this occurs, the lamp can only be relit by turning
up the control knob again to the intermediate level. Figure
AN1003.10 illustrates the hysteresis effect in capacitorDIAC
triggering. As R1
is brought down from its maximum
resistance, the voltage across the capacitor increases
until the DIAC first fires at point A, at the end of a halfcycle
(conduction angle θi). After the gate pulse, however,
the capacitor voltage drops suddenly to about half the
triggering voltage, giving the capacitor a different initial
condition. The capacitor charges to the DIAC, triggering
voltage at point B in the next half-cycle and giving a steadystate
conduction angle shown as θ for the Triac.