Post by thread:[EE:] Triac driving problems

Hi Everyone, I'm having a few problems getting my triac to turn on through an opto-triac. More specifically, the opto-triac is turning on and putting mains voltage at the triac's gate, but the triac won't start conducting. A schematic: +5vDC 240vAC (live) |-R1---LED1---| |------R2-----|------------| | | | LED2 TR1 TR2 | | | | --------------| |------------- -------LOAD-| Gnd Neutral R1 200 Ohms R2 5.6K LED1 Red indicator LED LED2 MOC3041 opto-triac TR1 MOC3041 opto-triac TR2 BTA16-600 Triac (16A, 600V) My LOAD is a 60W table lamp I'm using for testing. I'm a bit nervous of experimenting too much with this because I've already welded my meter probes together and flipped the house electrics off yesterday, and if it wasn't for my (now black and sooty) safety goggles, I'd be very unhappy. TR2's data sheet says it needs a max 50mA of gate current to turn on, and 240/5600 is 42mA. From the datasheet's graphs, it looks like the minimum Igt is 16mA. I've tried it with a 9.6K resistor in there and a fresh triac, but still no light. I've read up as much as I could find on interfacing to triacs, and (to me) mine looks OK. Any ideas of what I'm doing wrong? Cheers, Tim PS Hope everyone's having a good Christmas :) -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

At 11:42 AM 12/26/2003 +0000, you wrote: {Quote hidden}>Hi Everyone, > >I'm having a few problems getting my triac to turn on through an >opto-triac. More specifically, the opto-triac is turning on and putting >mains voltage at the triac's gate, but the triac won't start conducting. > >A schematic: > >+5vDC 240vAC (live) >|-R1---LED1---| |------R2-----|------------| > | | | > LED2 TR1 TR2 > | | | | >--------------| |------------- -------LOAD-| >Gnd Neutral > >R1 200 Ohms >R2 5.6K >LED1 Red indicator LED >LED2 MOC3041 opto-triac >TR1 MOC3041 opto-triac >TR2 BTA16-600 Triac (16A, 600V) > >My LOAD is a 60W table lamp I'm using for testing. > >I'm a bit nervous of experimenting too much with this because I've already >welded my meter probes together and flipped the house electrics off >yesterday, and if it wasn't for my (now black and sooty) safety goggles, >I'd be very unhappy. > >TR2's data sheet says it needs a max 50mA of gate current to turn on, Unless you are operating in the pesky quadrant IV (in which case, 100mA), which you are not. >and 240/5600 is 42mA. Peak voltage is 1.41 * 240 = 340V, so current is 61mA at the peak of the AC line before the load turns on. Note: You really should be using more like 100R than 5K6. You want it to turn on near the beginning of the AC cycle, not some poorly determined time around 1/4-1/2 way through (getting earlier as the thyristor heats up), right? Note that the 100R will BURN UP if the triac does not turn on so you need to use a flameproof type for safety. This will also take out the MOC, most likely. > From the datasheet's graphs, it looks like the >minimum Igt is 16mA. I've tried it with a 9.6K resistor in there and a >fresh triac, but still no light. > >I've read up as much as I could find on interfacing to triacs, and (to me) >mine looks OK. > >Any ideas of what I'm doing wrong? I suspect you have MT1 and MT2 swapped. Also, how did you calculate the 200R? You need to guarantee 15mA (@25°C, MORE at lower temperatures) through the LED for this part to be guaranteed to work. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam@spam@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

On Fri, 26 Dec 2003, Spehro Pefhany wrote: > At 11:42 AM 12/26/2003 +0000, you wrote: > >TR2's data sheet says it needs a max 50mA of gate current to turn on, > > Unless you are operating in the pesky quadrant IV (in which case, 100mA), > which you are not. I believe the moc3041 has a built-in zero-crossing detector, so I guess that means it can only turn on at the beginning of the 1st quadrant and the end of the 2nd? > Peak voltage is 1.41 * 240 = 340V, so current is 61mA at the peak > of the AC line before the load turns on. Errmm, what's the 1.41 all about then? > Note: You really should be using more like 100R than 5K6. You want it to > turn on near the beginning of the AC cycle, not some poorly determined > time around 1/4-1/2 way through (getting earlier as the thyristor heats > up), right? Note that the 100R will BURN UP if the triac does not turn > on so you need to use a flameproof type for safety. This will also > take out the MOC, most likely. The max forward current of the output photo-triac is rated at 1A, so surely if I use a 100R resistor in there it'll get 3.4A (using your peak voltage above)? And if the gate current is spec'd as a max of 50mA at the triac, wouldn't it break too? > >Any ideas of what I'm doing wrong? > > I suspect you have MT1 and MT2 swapped. I was under the impression it didn't matter which way round they were? I didn't see it mentioned in the data sheet? > Also, how did you calculate the 200R? You need to guarantee 15mA (@25°C, > MORE at lower temperatures) through the LED for this part to be guaranteed > to work. It was an incorrect, hasty decision :) ... I did 5v/200R (what I could find in my junk box at the time) and got the 25mA, but now I realise that the double-diode drop would make it more like 2/200, so there's only 10mA flowing. But there's still 240vAC at the triac's gate, and I didn't see any mention of coupling-transfer ratio's like in normal opto-isolators, so I assumed that if I can see voltage there, I'm getting all the current too? I'm starting to think I'll stick with relays instead. You know where you are with a switch. What sort of application would an optically driven triac be better than a relay in? I can think of dimmer circuits and motor-speed controllers, but that's all. Thanks for the input, Spehro. Cheers, Tim -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body

At 06:15 PM 12/26/2003 +0000, you wrote: >On Fri, 26 Dec 2003, Spehro Pefhany wrote: > > At 11:42 AM 12/26/2003 +0000, you wrote: > > >TR2's data sheet says it needs a max 50mA of gate current to turn on, > > > > Unless you are operating in the pesky quadrant IV (in which case, 100mA), > > which you are not. > >I believe the moc3041 has a built-in zero-crossing detector, so I guess >that means it can only turn on at the beginning of the 1st quadrant and >the end of the 2nd? There are (not surprisingly) 4 quadrants. Quadrant IV is negative voltage on MT2 and positive gate current. As the current comes from the power line and not some external supply, it can only possibly be in Quadrant I or III. (+Ve/+Ve or -Ve/-Ve) > Peak voltage is 1.41 * 240 = 340V, so current is 61mA at the peak > > of the AC line before the load turns on. > >Errmm, what's the 1.41 all about then? Peak voltage of a sine wave of 240 VRMS voltage is sqrt(2) times the RMS voltage. The waveform goes from -340V to +340V. v(t) = 340 * sin(2 * pi * 50* t) (assuming 50Hz) {Quote hidden}> > Note: You really should be using more like 100R than 5K6. You want it to > > turn on near the beginning of the AC cycle, not some poorly determined > > time around 1/4-1/2 way through (getting earlier as the thyristor heats > > up), right? Note that the 100R will BURN UP if the triac does not turn > > on so you need to use a flameproof type for safety. This will also > > take out the MOC, most likely. > >The max forward current of the output photo-triac is rated at 1A, so >surely if I use a 100R resistor in there it'll get 3.4A (using your peak >voltage above)? And if the gate current is spec'd as a max of 50mA at the >triac, wouldn't it break too? No, because the triac will turn on at 50mA and there won't be any voltage left... > > >Any ideas of what I'm doing wrong? > > > > I suspect you have MT1 and MT2 swapped. > >I was under the impression it didn't matter which way round they were? I >didn't see it mentioned in the data sheet? Ah. It very much matters. The center lead is MT2. {Quote hidden}> > > Also, how did you calculate the 200R? You need to guarantee 15mA (@25°C, > > MORE at lower temperatures) through the LED for this part to be guaranteed > > to work. > >It was an incorrect, hasty decision :) ... I did 5v/200R (what I could >find in my junk box at the time) and got the 25mA, but now I realise that >the double-diode drop would make it more like 2/200, so there's only 10mA >flowing. But there's still 240vAC at the triac's gate, and I didn't see >any mention of coupling-transfer ratio's like in normal opto-isolators, so >I assumed that if I can see voltage there, I'm getting all the current >too? The trigger current is rated at 15mA, so you need to guarantee that for it to work at 25°C. If it might be colder, you need more current. >I'm starting to think I'll stick with relays instead. You know where you >are with a switch. There are plenty of gotchas with relays too. >What sort of application would an optically driven triac be better than a >relay in? I can think of dimmer circuits and motor-speed controllers, but >that's all. A typical relay is good for 100,000 operations at full current. At one per 10 seconds, it could wear out in a week and a half. That might not be acceptable. ;-) (derating can extend the life, but not always by as much as you'd guess). So, if there are a lot of operations (eg. a temperature control where the process time constant is small), the triac is better. The acoustic and electrical noise from a relay can be bothersome. OTOH, relays are rugged electrically, seldom fail on, and generally are quite efficient. Neither is perfect. >Thanks for the input, Spehro. Sure. Hope it helps. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" EraseMEspeffspam_OUTTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

> I'm starting to think I'll stick with relays instead. You know > where you are with a switch Yes. Slow. If you're planning never to do anything more sophisticated than on/off then a relay is OK. Notwithstanding the size, cost and ultimate contact degradation of large relays that is But opto-driving triacs does work http://home.clear.net.nz/pages/joecolquitt/txless.html (last picture). You can also use small trigger transformers > What sort of application would an optically driven triac be better > than a relay in? I can think of dimmer circuits and motor-speed > controllers, but that's all Those would be two major uses. But there's more to it than that. Switching mains with relays can be very noisy, electrically speaking. Because of the uncertainty wrt the activation and closure time of the contacts you have no control over at which point in the cycle the actual switching will be done. This can cause a lot of annoying EMF. If though you can precisely control the switching point (for example zero-crossing) this can virtually eliminate EMF and problems caused by current in-rush -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

Tim ODriscoll wrote: {Quote hidden}>Hi Everyone, > >I'm having a few problems getting my triac to turn on through an >opto-triac. More specifically, the opto-triac is turning on and putting >mains voltage at the triac's gate, but the triac won't start conducting. > >A schematic: > >+5vDC 240vAC (live) >|-R1---LED1---| |------R2-----|------------| > | | | > LED2 TR1 TR2 > | | | | >--------------| |------------- -------LOAD-| >Gnd Neutral > >R1 200 Ohms >R2 5.6K >LED1 Red indicator LED >LED2 MOC3041 opto-triac >TR1 MOC3041 opto-triac >TR2 BTA16-600 Triac (16A, 600V) > > R2 is usually 680R IIRC. The MOC3041 datasheet should show a standard circuit with typical values. David... -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email KILLspamlistservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

> > I'm starting to think I'll stick with relays instead. You know > > where you are with a switch Y don't buy no ugly truck!!!! :-) The failure to work is a temporary problem. You are doing something wrong. It's not something major (but might as well be until you find what it is). Once you've found it you'll be very very very glad to have added triacs to your arsenal of possible solutions. Basically, triacs driven by MOCxxx zero crossing opto couplers work. That's what they are designed to do. Somewhere along the line you've failed to design the circuit to meet their needs. Once you do they'll work as intended. Lets try designing the input circuit. MOC3041 datasheet http://www.isocom.com/datasheets/db91048.pdf MINIMUM design drive current to trigger is 15 mA DESIGN LED froward current in MOC3041 is 1.4V (Both these values may be lower in practice but you MUST design to datasheet guaranteed limiting values (or else). Your series indicator LED is red so we'll allow it a 1.2v drop for now. You want 15 mA MINIMUM input. You show this from =%v. If that is a hard supply feed, well and good. If it is eg a PIC pin it will drop well below 5v at 15 mA. So available drive is (5-1.4-1.2) = 3.2v Available drive current is 3.2/200 = 16 mA. IF the drive is solid 5v and IF the series LED is 1.2V drop max at 15 mA then you have just enough drive (IF your 200 ohm resistor (+/- 5%) is not +5%). ie the input drive is JUST marginally adequate to meet design specs, probably :-). ie this probably ISN'T the probl;em, but for now add a bit more drive to be sure. For now 1. Drive from a hard 5V supply. 2. Ensure that the series LED lights when you apply 5V drive. 3. Change R1 to 150 ohms (or even 120 ohms).(Abs max LED rating is 50 mA so you have substantial safety margin) Note that IF this part had worked reliably at 10 mA they would have sold it as a MOC3042, so it will be somewhere in the 10 to 15 mA range that it actually triggers reliably. Now read what other people said about R2 and have fun. Russell McMahon -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

Stuart Meier wrote: >Built a few Solid State Relays like this the other day. Once I sorted them out >(nasty stuff 240v!) they work a treat. Saves the clacking of those relays.... > >I'd (A) reduce R2 a lot I was using MOC3020s driving less sensitive gate triacs, >with the load on the highside. I had R2 at 360R (B) I would check I had the >triac the right way round (MT1 v MT2) (C) I split R2 and put a 0.01u 240v cap >from the centre to neutral so they could (I hope) deal with some inductance in >the load. > > A word of warning here. The "240v" capacitor *needs* to be the type meant for the application. Don't use normal greencaps even if they are rated at 630Vdc. Use 250V "X2 class" types as these are the only types that are safe to use in your application. David... -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body