> From: Andre Miller[SMTP:andre.miller@BLUERIVER.CO.ZA] > Sent: Thursday, March 11, 2004 2:01 AM > To: PICLIST@MITVMA.MIT.EDU > Subject: Re: [EE:] Switching 24V AC inductive load with PIC, coupling & protection? > Hi, > Would I still need to do pulse triggering when I just want to switch > something on / off? I was thinking I could just keep the gate 'high' to turn > the load on, and then make it 'low' to turn off the load. Timing isn't > critical, the load will be on for 10 mins - 2 hours, and then turned off > again. > ____ > Andre Miller >> I've seen a sample circuit that uses a half-wave rectifier and a zener >> to drive the logic circuit which then directly drives the TRIAC, but >> not sure how suitable this would be? I don't know enough about TRIAC's >> to know how to design such a circuit. >> Triacs turn off when the current through them goes to zero. This >> means that there is no inductive "kick" on turn off. There are other factors, >> though, that will affect the choice of driver. >> Because main terminal current can continue to flow after the gate >> drive is turned off, you have to be careful about how the triac is >> triggered. Suppose that you are using pulse triggering. You trigger the >> triac 1/4 of a half-cycle after the zero crossing. With a highly inductive >> load, the current from the . . . . Yes, of course. I just got carried away. The one consideration with driving the triac directly is protecting the PIC if the triac fails (shorts), dumping 24 volts into the port pin. A series resistor is required between the port and the gate of the triac, but the value needed for reliable triggering may be too small to protect the PIC. An optocoupler would be useful here; in fact, that would be the only reason for using one. John Power -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads