As others have mentioned, the triac is shown upside-down. It is=20 imperative that MT1 be connected to the 24 Vac line and the load=20 connected between 24 Vac common and triac MT2. This is a standard configuration in many of my products. In=20 particular, we use a variation of this topology for driving 4 or 8=20 triacs in our Christmas Light sequencers. We do things a bit differently, though. Because our stuff has to=20 work outside in the cold (as low as -50C), we smack the triac gates=20 *really* hard. -20 Vdc rail discharging through a 100n ceramic=20 capacitor with a 47R series resistor. Call it about 400 mA; pulse=20 duration is 6 us. We have found that hitting the triac gates this hard ensures that the=20 entire triac die is fully conducting and seems to eliminate the hot=20 spots on the die that lead to early failure. You may not run into triac failures at room temperature but if you=20 do, short, very hard pulses to the triac gate will likely reduce or=20 eliminate those failures. dwayne At 04:56 PM 1/1/2016, Jason White wrote: >Hello, > >I am a student, I saw this low voltage TRIAC driving circuit on the >internet and am puzzled as to why it uses a negative voltage rail? > >As far as I know, TRIACS are not sensitive to the polarity of the signal >applied to the gate (with respect to MT1). Could this configuration work >with a positive supply (with respect to neutral) in all four quadrants? > >-- >Jason White --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .