Triac Question

Generally, a triac will turn off when the current goes below the holding current. Reverse recovery time is a measured characteristic of a device, and that has something to do with the design of the device, such as p-n junction areas, etc. When alternating voltages are applied to two-lead devices, as the polarity of the voltage changes, the previously forward-biased junction will take time to establish a blocking state, but in the case of triacs, the previously reverse-biased junction turns on and the two junctions conduct current simultaneously until the now-reverse-biased junction redissipates its carriers and begins blocking current.

Thanks Bill, Is that mean in the case of Triac or SCR they continue conducting current after the current goes down below holding current until the now-reversed-biased junction begins blocking current (i.e. stops conducting a while after the voltage zero crossing point is reached)?

If I understand your question correctly, then the answer is, "yes."

However, in the case of a triac, the other half of the device begins conducting after zero crossing, so the reverse bias on the previously conducting p-n junction is low: 0.6 volts.

When the gate signal is removed so that the triac turns off at zero crossing, the reverse voltage rises sinusoidally (for a pure resistive load) at a rate much less than the reverse bias recovery rate of the triac's p-n junctions.

See Wikipedia's stuff on Triacs, Diodes, and p-n junctions for more information.

Thanks Bill again , so why the holding current is given in the datasheet if SCR will continue passing current below the holding current until the the recovery time is fulfilled?

The holding current is the minimum current at which the SCR will remain ON. When it drops below that level in the forward biased condition, it will turn OFF.

The recovery time and reverse current do not have anything to do with the holding current (directly, that is). Look at the reverse recovery times and draw a graph of it versus load current or line voltage, and I think it will become more clear to you what may happen during the reverse recovery period.

Actually, the specified holding current is the smallest current guaranteed to keep the device turned on. The actual device may continue to conduct at currents well below the specified holding current. The circuit designer must assume that the device is unreliable at lower currents.

that is depends upon the reverse recovery time

Triac turns off when the on-state current goes down to the holding current or when the current reaches zero or when the current crosses zero when gate current is absent.

By the way, what is the difference between the Triac's (dv/dt) and (dv/dt)c ?

Which one of these two parameters is used in circuit design?