What is Diac?

[lethenl]

can u tell me what is Diac? it's something juz like a diode...
thanx for ur help

[Kevin Weddle]

I have heard two different stories when it comes to these 5 layer devices. Some would say it's about the current, but I found one that deals with voltage. I like to deal with voltage, as the current is just the requirement which will maintain the voltage. A diac is basically a switch that is activated when the voltage across it exceeds a threshold value. Once it starts to conduct, the voltage across it drops to a minimum.

[surajbarkale]

DIAC is a 3 layer device N-P-N (take a transistor cut base lead & you get a cheap DIAC ;D). You can also view it as two zeners connected back to back. There was a topic about DIAC some days ago you can search this forum for details.

[ante]

Hi Suraj,

I do not completely agree to your description of the diac as

[Guest Yevgenip]

Yeah, a transistor without the base is a chip (and not very good) resistor.

[Kevin Weddle]

http://www.play-hookey.com/semiconductors/diac_triac.html

[Guest Yevgenip]

I think this is more for the Theory Articles board. Can the moderator please redirect this post?

Thanks.

[surajbarkale]

Okey so next time I should say it's a TRIAC without gate ;D
Anyway, I have used BC547 with base removed in a light dimmer project instead of DIAC. The project is still working (Don't know how though ;D).

[audioguru]

Hi Suraj,
If you use an ordinary transistor instead of a a Diac in an AC circuit it would not drive the Triac properly and would likely overheat.

A Diac has equal threshold voltages for both polarities of the AC mains voltage. When the threshold voltage is reached across a Diac, it conducts heavily and therefore supplies a fairly high current (limited by a series resistor) to the Triac's gate.

An ordinary transistor with its base not used would breakdown like a Zener diode at about 50V to 80V with its normal polarity, and turn-on like a very high-voltage Diac. With the dimmer control set very dim or very bright, the Triac might not even turn-on during these half-cycles and operate like a half-wave rectifier.
When the voltage is reversed across the transistor, its C-B junction becomes forward-biased in series with its E-B junction that becomes reverse-biased. A silicon transistor's reverse-biased E-B junction breaksdown like a zener diode at about 6V, so will provide gate current to the Triac for this half-wave of the mains. With 6V across the E-B junction and the full Triac's gate current through it, the junction must dissipate a power that it was never designed to do.

I bet that your light dimmer does not dim smoothly without a Diac.

[ante]

Hi Suraj,

Your nose is not very sensitive to smoke I guess! ;D