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I am controlling fan speed using Triac phase control with a circuit similar to this. The problem that i am facing is that my triac burned out after using it for 2 3 days and after burning it will remain switched on continuously without gate trigger. I concluded that i am triggering it in 1- and 3+ quadrant resulting in slow degradation, and eventual shorting of MT1 and MT2.enter image description here

Referred following document for 3+ quadrant switching details. (http://www.nxp.com/documents/application_note/AN_GOLDEN_RULES.pdf

enter image description here

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Triggering a triac in QI/QIII is not going to cause "slow degradation".

More likely you're allowing it to greatly overheat or there is a voltage transient that has occurred.

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  • \$\begingroup\$ what about QIV (3+)? It can not be voltage transient as it has happened twice with same characteristics. Please note that i have inverted MT1 and MT2 terminals in circuit \$\endgroup\$ – Nirmala Apr 20 '15 at 16:11
  • \$\begingroup\$ The quadrant doesn't matter (to the triac)- it either triggers or it doesn't. Neither should damage the triac. Billions of triac light dimmers all over the world uses those quadrants and minor variations on that circuit. \$\endgroup\$ – Spehro Pefhany Apr 20 '15 at 18:30
  • \$\begingroup\$ i read this fact about 3+ quadrant in document by NXP(link above). Can the heating be observed from Triac surface if overheat is the case. \$\endgroup\$ – Nirmala Apr 21 '15 at 3:22
  • \$\begingroup\$ First, the phenomenon mentioned causes harder triggering (failing off or half-waving), not failing on, and secondly, you're not using it in that quadrant (what most people call quadrant IV)- the gate voltage has the same polarity as the MT2 (what you show as MT1) always, so what is called 1+ and 3- in that document, and quadrants I and III by most everyone else. Fans are inductive and do not have rapid di/dt on switch-on. \$\endgroup\$ – Spehro Pefhany Apr 21 '15 at 3:34
  • \$\begingroup\$ I am using this dim a dimmable LED bulb \$\endgroup\$ – Nirmala Apr 21 '15 at 5:42

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