lethenl Posted December 29, 2004 Report Share Posted December 29, 2004 can u tell me what is Diac? it's something juz like a diode... thanx for ur help Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 29, 2004 Report Share Posted December 29, 2004 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. Quote Link to comment Share on other sites More sharing options...
surajbarkale Posted December 30, 2004 Report Share Posted December 30, 2004 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. Quote Link to comment Share on other sites More sharing options...
ante Posted December 30, 2004 Report Share Posted December 30, 2004 Hi Suraj,I do not completely agree to your description of the diac as Quote Link to comment Share on other sites More sharing options...
Guest Yevgenip Posted December 30, 2004 Report Share Posted December 30, 2004 Yeah, a transistor without the base is a chip (and not very good) resistor. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 30, 2004 Report Share Posted December 30, 2004 http://www.play-hookey.com/semiconductors/diac_triac.html Quote Link to comment Share on other sites More sharing options...
Guest Yevgenip Posted December 31, 2004 Report Share Posted December 31, 2004 I think this is more for the Theory Articles board. Can the moderator please redirect this post?Thanks. Quote Link to comment Share on other sites More sharing options...
surajbarkale Posted January 4, 2005 Report Share Posted January 4, 2005 Okey so next time I should say it's a TRIAC without gate ;DAnyway, 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). Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 4, 2005 Report Share Posted January 4, 2005 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. Quote Link to comment Share on other sites More sharing options...
ante Posted January 4, 2005 Report Share Posted January 4, 2005 Hi Suraj,Your nose is not very sensitive to smoke I guess! ;D Quote Link to comment Share on other sites More sharing options...
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