Regarding MT1 and MT2 Being a bidirectional device, it does not matter whether 'Phase' (Live) or 'Return' (Neutral) is conected to either, or on which side the 'Load' is placed. It does matter to which terminal the gate signal to 'fire' the triac is referenced, being MT1 as Bob has pointed out. A diac is only appliedf it's characteristic breakover voltage at which it starts conducting is being used to derive the triac firing point at some interval after the phase crosses zero. This is usually established with an RC network which causes a phase delay dependant upon a potentiometer resistance, as in a budget designed 'Dimmer Switch' When controlling the triac via an optocoupler such as the MOC20xx, which is a 'random phase' device, having no internal ZCD, the zero crossing point of the mains phase will need to be established by an external zero crossing detector, which will then serve as a timing reference from which a phase delay may be generated. Optocouplers such as the MOC30xx are essentially ON/OFF devices, the triac beng fired at ZC if the LED in the opto is on (100% phase switched) or never fired if the opto is off. 'Control' is effected by some or other sensor such as a thermocouple being used to alter the ON time to OFF time ratio, as in the case of a heater element. Regards, On Thu, Oct 4, 2012 at 3:25 AM, Bob Blick wrote: > > > On Wed, Oct 3, 2012, at 05:44 PM, IVP wrote: > > Are MT1 and MT2 of a triac interchangeable ? > > No, because gate is referenced to MT1. Required gate current is > different depending on polarity, both absolute and relative, but is > lowest when gate polarity is the same as MT2. > > Best regards, > > Bob > > > > > > I see circuits with either MT1 or (more often) MT2 to phase. I > > don't use triacs a lot, but do recall making a heater control (an > > immersion element) that didn't work with MT1 to phase but did > > with MT2 to phase. Gate control was DC from a CMOS gate, > > a 4017 I think. Are the positions of MT1 and MT2 less important > > if gate control is AC, through a diac for example ? > > > > When I've used low-power optotriacs like MOC302x to drive > > triacs, I've never had a diac. Gate current is AC via a resistor > > from phase through the opto's triac > > > > In this document > > > > Thyristors & Triacs - Ten Golden Rules for Success In Your Application > > > > www.nxp.com/documents/application.../AN_GOLDEN_RULES.pdf > > > > MT2 is to phase, as it is here > > > > http://en.wikipedia.org/wiki/TRIAC > > > > with note that filters should be referrred to MT1, likening it to the > > 'cathode' of an SCR, implying that MT1 is to neutral > > > > In the previous version of his dimmer > > > > http://www.circuitstoday.com/simple-lamp-dimmer-fan-regulator > > > > the author has MT2 to phase, but in the revision of my original > > post MT1 and MT2 are not identified > > > > I'm asking for general knowledge and so that I can make a > > small controller PCB > > > > TIA > > > > Joe > > -- > http://www.fastmail.fm - Access all of your messages and folders > wherever you are > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 Perry Curling-Hope Research and Development --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .