[SOLVED] Diac and Triac transistors - help

[7@rB]

can I have a brief explanation about these two types of transistors and about their function in a circuit...( how does they work )

for example I found a triac transistor in a board that controls the speed of a motor..what is the role of the triac in such a design!

Regards.

 

[snaiks]

A zener diode is used to limit DC voltage across its terminals. But zener won't work on AC. in this case two zeners wired as both anode short (reverse) works as zener for AC voltage. This is Diac.

A SCR have property to remain conductive if fired once and it stops conducting when potential goes below triggering level (shorted to ground) But it works on DC. A triac works as SCR for AC.

 

[BradtheRad]

Triacs can often be found in AC incandescent light dimmers and AC motor speed controls. A triac has 3 terminals.

A diac has two terminals. About the only place I've seen a diac is in schematics, when it is used to trigger a triac.

When I made my own AC fan control, I read that a neon bulb is commonly used as a substitute for a diac. So I used a neon bulb. I don't think I've ever seen a real diac, or if I did I didn't know it.

 

[kak111]

Good information about thyristors, triacs and diacs.

Thyristors and Triacs Power Semiconductor Applications
Philips Semiconductors

CHAPTER 6 : Power Control with Thyristors and Triacs

6.1 Using Thyristors and Triacs
6.2 Thyristor and Triac Applications
6.3 Hi-Com Triacs

84 pages

**broken link removed**

 

[Emergency Lights]

Hello,
The triac is similar in operation to two thyristors connected in reverse parallel but using a common gate connection. This gives the triac the ability to be triggered into conduction while having a voltage of either polarity across it. In fact it acts rather like a "full wave" thyristor. Either positive or negative gate pulses may be used.

This is a bi-directional trigger diode used mainly in firing Triacs and Thyristors in AC control circuits. Its circuit symbol (shown in figure 3a) is similar to that of a Triac, but without the gate terminal, in fact it is a simpler device and consists of a PNP structure (like a transistor without a base) and acts basically as two diodes connected cathode to cathode.

Exit Signs