Post by thread:[EE]: Driving a triac

Hi guys, I'm having a bit of a problem driving a triac from a pic. The electrical main is 220Vac 50Hz. I have the Vdd of the pic connected to the live and Vss is 5v less than live. I have a PNP transistor: emitter to live, collector to triac trigger,base via resistor (1k) to the pic i/o pin. The base is also connected to the live via 10k resistor (to prevent false trigger at powerup). My theory is that with the pic i/o pin high the transistor is off and therfore high z to the trigger so no leakage current can trigger it.Thus the triac doesn't conduct. With the pic i/o pin low the transistor is switched on and the trigger gets all the current it needs and the Triac conducts. The triac I'm using is a BTA12 from Philips (logic level driven device). My target use is for a small motor but I'm using a lightbulb for testing. The Problem: The bulb is always on! I thought I had all the bases covered but obviously I'm missing something. Has anyone got an idea what this 'something' is. Thanks in advance John -- http://www.piclist.com hint: To leave the PICList spam_OUTpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

John Walshe @Inpact wrote: {Quote hidden}> > Hi guys, > I'm having a bit of a problem driving a triac from a pic. > The electrical main is 220Vac 50Hz. > I have the Vdd of the pic connected to the live and Vss is 5v less than > live. > I have a PNP transistor: emitter to live, collector to triac trigger,base > via resistor (1k) to the pic i/o pin. The base is also connected to the live > via 10k resistor (to prevent false trigger at powerup). > > My theory is that with the pic i/o pin high the transistor is off and > therfore high z to the trigger so no leakage current can trigger it.Thus the > triac doesn't conduct. > With the pic i/o pin low the transistor is switched on and the trigger gets > all the current it needs and the Triac conducts. > > The triac I'm using is a BTA12 from Philips (logic level driven device). > > My target use is for a small motor but I'm using a lightbulb for testing. > > The Problem: The bulb is always on! > > I thought I had all the bases covered but obviously I'm missing something. > > Has anyone got an idea what this 'something' is. > > Thanks in advance > John The BTA12 is a pretty high gain device, seems you are relying in "high z" to stop the triac being gated. I would think you need some resistor to hold the triac gate "off" rather than relying on high-z which basically allows leakage or even RF to activate the traic. Also check you haven't fried the transistor, what current limiting (resistor?) are you using in the collector circuit? -Roman -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu

Sounds like you're using a transistor in an AC circuit, so half the time the transistor has reverse polarity on it. I just did a triac drive circuit using the MOC3023 opto coupler. For noninductive loads, it's real simple. See Motorola Ap Note AN780A, if you can find it... Harold On Tue, 7 Nov 2000 14:35:27 -0000 "John Walshe @Inpact" <John.WalsheKILLspamINPACTMICRO.COM> writes: {Quote hidden}> Hi guys, > I'm having a bit of a problem driving a triac from a pic. > The electrical main is 220Vac 50Hz. > I have the Vdd of the pic connected to the live and Vss is 5v less > than > live. > I have a PNP transistor: emitter to live, collector to triac > trigger,base > via resistor (1k) to the pic i/o pin. The base is also connected to > the live > via 10k resistor (to prevent false trigger at powerup). > > My theory is that with the pic i/o pin high the transistor is off > and > therfore high z to the trigger so no leakage current can trigger > it.Thus the > triac doesn't conduct. > With the pic i/o pin low the transistor is switched on and the > trigger gets > all the current it needs and the Triac conducts. > > The triac I'm using is a BTA12 from Philips (logic level driven > device). > > My target use is for a small motor but I'm using a lightbulb for > testing. > > The Problem: The bulb is always on! > > I thought I had all the bases covered but obviously I'm missing > something. > > Has anyone got an idea what this 'something' is. > > Thanks in advance > John > > -- > http://www.piclist.com hint: To leave the PICList > .....piclist-unsubscribe-requestKILLspam.....mitvma.mit.edu > > > > FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ YOU'RE PAYING TOO MUCH FOR THE INTERNET! Juno now offers FREE Internet Access! Try it today - there's no risk! For your FREE software, visit: dl.http://www.juno.com/get/tagj. -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu

Roman is pointed in the right direction. Triacs have a parameter called dv/dt. The gate of the triac is capacitive, and as such a high dv/dt can push enough charge over to the gate to false trigger the device. This dv/dt parameter also applies to opto triacs when they are in use. You may have to use some form of RC snubber to protect the Triac and properly hold the gate off. I saw some great app notes in SGS territory that covered the microprocessor driven tirac with details, and even more in the area where they have a new multi channel triac trigger device that mates up with your processor. Do you have to pass UL and CE? You might carefully study your non-isolated strategy to see if you will pass. Chris~ Roman Black wrote: {Quote hidden}> > The BTA12 is a pretty high gain device, seems you are relying > in "high z" to stop the triac being gated. I would think > you need some resistor to hold the triac gate "off" rather than > relying on high-z which basically allows leakage or even RF > to activate the traic. > > Also check you haven't fried the transistor, what current > limiting (resistor?) are you using in the collector circuit? > -Roman -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestspam_OUTmitvma.mit.edu

part 1 1432 bytes content-type:text/plain; charset="US-ASCII" (decoded 7bit) Hi John, Attached is a Triac schematic I have used for controlling an AC source with a PIC I/O. The SCR is really a TRIAC device. Also, you will have to find a different TRIAC as I think the T2800 is rated up to 200V. Cheers, Paul In a message dated 11/7/00 8:39:31 AM Central Standard Time, @spam@John.WalsheKILLspamINPACTMICRO.COM writes: << Hi guys, I'm having a bit of a problem driving a triac from a pic. The electrical main is 220Vac 50Hz. I have the Vdd of the pic connected to the live and Vss is 5v less than live. I have a PNP transistor: emitter to live, collector to triac trigger,base via resistor (1k) to the pic i/o pin. The base is also connected to the live via 10k resistor (to prevent false trigger at powerup). My theory is that with the pic i/o pin high the transistor is off and therfore high z to the trigger so no leakage current can trigger it.Thus the triac doesn't conduct. With the pic i/o pin low the transistor is switched on and the trigger gets all the current it needs and the Triac conducts. The triac I'm using is a BTA12 from Philips (logic level driven device). My target use is for a small motor but I'm using a lightbulb for testing. The Problem: The bulb is always on! I thought I had all the bases covered but obviously I'm missing something. Has anyone got an idea what this 'something' is. Thanks in advance John >> part 2 6523 bytes content-type:application/pdf; name="triac.pdf" (decode) part 3 107 bytes -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu