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I mean, Wouldn't it be possible to give a gate pulse on triac directly? I have seen in many phase control application circuits like fan speed control,the usage of diacs to trigger a triac.

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    \$\begingroup\$ The pulse generated by a DIAC is synced with mains automagically. You don't need a DIAC If you feel like doing the synchronization manually or don't care. \$\endgroup\$ – Oleg Mazurov Feb 1 '15 at 18:06
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    \$\begingroup\$ It is possible to give a gate pulse on triac directly by using something that produces gate pulses. A DIAC is just such a device and is doing just what you suggest. A TRIAC gate is not designed to switch at a tightly controlled voltage or current and if a rising voltage source is connected to the gate the TRIAC will turn on in a "soft" and ill defined manner and time location. The DIAC allows input voltage to rise to a wellish defined level and then dumps a more than high enough voltage, energy source (capacitor) into the gate. ~~~ Dog on long chain - left to itself you may get bitten .... \$\endgroup\$ – Russell McMahon Feb 1 '15 at 21:24
  • \$\begingroup\$ .... at extreme length of chain and retreat. If a "handler" restrains the dog until you are well inside chan length the dog can devote all its energy to you. \$\endgroup\$ – Russell McMahon Feb 1 '15 at 21:25
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See http://en.wikipedia.org/wiki/DIAC for a diagram of the breakdown behaviour of a Diac.

As you can see there, a Diac is non conductive up to a certain breakdown voltage, above it becomes conductive, until the current drops below a threshold value.

In short, the diac is forming the pulse that is needed to trigger the triac.

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I mean, Wouldn't it be possible to give a gate pulse on triac directly? I have seen in many phase control application circuits like fan speed control,the usage of diacs to trigger a triac.

Yes you can, however, if you control with DC, I recommend using an opto-isolator (like a moc3041) to keep the dc grounds isolated from the AC.

But here is the simple switch method:

schematic

simulate this circuit – Schematic created using CircuitLab

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