Can a TRIAC vary AC voltage? - Page 1
Forum section: electronics / beginners

author: scott216
author_rank: Regular Contributor
timestamp: March 17, 2014, 11:47:00 am
content: I'm trying to learn about triacs. It's clear they work well to switch AC voltage on and off, but can they vary the AC voltage? I have a toaster oven I want to use as a reflow oven and with all the heaters on it draws 30 amps - which is too much. I'd like to use a circuit to lower the 120 volts going into the heating elements so it draws less current. That circuit would be controlled by an Arduino.

author: uwezi
author_rank: Unknown
timestamp: March 17, 2014, 12:02:06 pm
content: Triacs can not vary AC voltage, but they can vary the power delivered to the load:http://en.wikipedia.org/wiki/Phase-fired_controllershttp://www.littelfuse.com/~/media/electronics/application_notes/switching_thyristors/littelfuse_thyristor_phase_control_using_thyristors_application_note.pdf.pdf

author: rexxar
author_rank: Frequent Contributor
timestamp: March 17, 2014, 01:03:57 pm
content: Like uwezi said, they're for controlling current. You probably wouldn't want to be using a triac on 30A, though. Toaster oven reflow mods usually use relays to cycle the heaters on and off to control the temperature-- like PWM dimming an LED. You'd need a relay to turn it off automatically anyway, so why not go that route instead?

author: scott216
author_rank: Regular Contributor
timestamp: March 17, 2014, 05:46:57 pm
content: The toaster came with two triacs to control the heating elements and I was consider just tapping into the existing circuit board and having the Arduino just control them. I've also own a 40 amp SSR I could use, which is probably what I'll end up doing. The triacs peaked my curiosity so I wanted to learn more.

author: uwezi
author_rank: Unknown
timestamp: March 17, 2014, 07:03:35 pm
content: There is no upper limit for the current which you can control with a triac.Actually you don't want to quickly switch on and off 30A with an electromechanical relay. And a semiconductor relay is nothing else than triacs with some control circuitry.When running the heater from AC switching-off is not the problem, it will happen automatically twice every 20 ms/16 ms - depending on whether you have 50 Hz or 60 Hz at home.

author: scott216
author_rank: Regular Contributor
timestamp: March 17, 2014, 07:54:41 pm
content: I should point out that using a 40 amp SSR still leaves me with heaters that draw 30 amps - which will blow my breaker. There are 6 heating elements all in parallel, I plan to rewire them so their are 3 pairs of two in-series. This should will lower the amps to a reasonable amount. I'll have to run a test to see how quickly the oven heats up, I think it will still heat up pretty quick. If not, I'll probably look into the triac scenario some more.This is how I'll wire them:+--- \/\/\/\-----\/\/\/\/ ---+| |---+--- \/\/\/\-----\/\/\/\/ ---+----| |+--- \/\/\/\-----\/\/\/\/ ---+

author: tonyarkles
author_rank: Regular Contributor
timestamp: March 18, 2014, 12:25:45 am
content: That seems like a pretty powerful toaster oven! Depending on where you're located, that's either 3 or 6kW!

author: scott216
author_rank: Regular Contributor
timestamp: March 18, 2014, 01:19:45 am
content: Quote from: tonyarkles on March 18, 2014, 12:25:45 amThe toaster's original controller never let all the elements turn fully on at the same time. It would alternate top and bottom and I'm pretty sure the triacs limited the load going to the elements.

author: scott216
author_rank: Regular Contributor
timestamp: March 18, 2014, 06:43:07 pm
content: Quote from: scott216 on March 17, 2014, 07:54:41 pmI tired this setup and the toaster took 10 minutes to get to 225C, so I'm going to revert to original wiring and see if I can get the triacs to control the amps to a reasonable amount.

author: David_AVD
author_rank: Super Contributor
timestamp: March 18, 2014, 09:14:33 pm
content: Quote from: uwezi on March 17, 2014, 07:03:35 pmIt's actually every 10ms / 8.3ms for 50Hz / 60Hz as it turns off after each half cycle.

author: scott216
author_rank: Regular Contributor
timestamp: March 26, 2014, 08:55:45 pm
content: I spent a little more time on my toaster oven. For voltage regulation it uses aL7905CVwhich outputs negative 5 volts. I'm not sure why the circuit uses -5 volts, but it is what it is. There are two triacs (BTA26-600B) to control top and bottom heaters. When ground is applied to the triac's gate, it's off. When -5v is applied, it's on. The each heating element draws about 14 amps. I want to have an Arduino control everything. I'd like to use the toaster's PCB to power the Arduino. It's easy to tap off the board because it has a connector that goes to the old controller board which I have removed, so I can have the connector go to a proto board with and Arduino on in. Since the regulated voltage is -5v, can I hook up Arduino ground -5v from the L7905CV and Arduino Vcc to ground on the L7905CV? I assume this will work, but I'm not sure how I should control the Triacs. The negative voltage regulator his me a bit confused.If I have both heating elements on at the same time, it would draw to much current. My plan is to have the Arduino alternate between top an bottom heaters. Probably every second or so.I have two scenarios I want to consider for having the arduino control the triacs. First would be each triac is controlled by an output on the Arduino (probably via a transistor or maybe a photocoupler). The second scenario is the same, but with an added feature where the circuit would prevent both triacs from coming on at the same time.I'd appreciate any advice on how to wire this up.I attached a couple pictures of the PCB that's in the toaster.

author: David_AVD
author_rank: Super Contributor
timestamp: March 26, 2014, 09:28:21 pm
content: The -5V supply may be to do with the type of TRIAC used and the way they are firing it. A schematic (or portion of) would help clear that up.

author: miceuz
author_rank: Frequent Contributor
timestamp: March 26, 2014, 11:35:27 pm
content: One way to tame your heaters is to make a simple triac based dimmer. If you want to go arduino route, you have to detect the zero corssing event (when AC sinusoid crosses 0V line) and turn on the triac at some point between zero crossings. The later you turn on the triac, the less power is dissipated thru the heater. I have built an I2C controllable dimmer module based on attiny if you are interested:http://wemakethings.net/2013/10/27/triac-bloc/Another option to alternate the heater is to use an optotriac with ZC detection, this way arduino does not have to know about ZC, it's just fire and forget, optotriac will switch on after ZC automatically - the benefit in this case is less electrical noise generated.Youcouldpatch in your arduino without optocoupler, but note, that you can fry your computer and shock yourself if you mess anything up. Try tracing the schematics of existing triac solution to get more insight. Your idea about powering the arduino is correct, this way, you whould fire the triac by applying LOW to the gate.

author: David_AVD
author_rank: Super Contributor
timestamp: March 27, 2014, 12:27:28 am
content: Quote from: miceuz on March 26, 2014, 11:35:27 pmThe OP would be very silly to even consider using a TRIAC without an opto imo.

author: scott216
author_rank: Regular Contributor
timestamp: March 27, 2014, 12:54:09 am
content: Attached is the circuit for how the toaster controls the triac. Basically there is some sort of microcontroller chip that turns on an NPN transistor which turns on the triac.

author: David_AVD
author_rank: Super Contributor
timestamp: March 27, 2014, 01:21:30 am
content: Hard to say without knowing how the mains is referenced to the micro circuit, but maybe that TRIAC can only be fired that way due to the MT1 & MT2 orientation. For more info, search on quadrants with respect to TRIACs gate trigger current.

author: ludzinc
author_rank: Unknown
timestamp: March 27, 2014, 01:35:24 am
content: This power supply arrangement is typical of a lot of white goods.This gives you a 5V DC offset from mains active, and lets you switch active from the control card - the rest of the wiring in the device is neutral returns, which is 'safer' for a repair guy to work on. IMHO you're nuts working on dishwasher etc if still plugged in but some do...(To clarify - logic 0 is now at mains - 5V, logic 1 is at mains)BE WARNEDEverything on the control card is floating on AC mains and poses a definite risk to you!

author: scott216
author_rank: Regular Contributor
timestamp: March 27, 2014, 01:49:21 am
content: Quote from: ludzinc on March 27, 2014, 01:35:24 amFor "Control Card" do you mean the PCB with the triacs on it, or the PCB that's got the microcontroller, LCD, buttons etc on it?I think I want to keep this simple, so I'm leaning towards using a couple of opto-isolators to turn on the triacs.

author: IanB
author_rank: Super Contributor
timestamp: March 27, 2014, 01:59:52 am
content: Quote from: scott216 on March 18, 2014, 06:43:07 pmWhen you posted that schematic I assumed you were going to run it off 240 V. Are you saying you still tried to run it off 120 V? If so, no wonder you got slow heating times.If you want to halve the current you have to double the voltage, since power = voltage x current. Otherwise you will just halve the power...

author: scott216
author_rank: Regular Contributor
timestamp: March 27, 2014, 02:07:21 am
content: Quote from: IanB on March 27, 2014, 01:59:52 amI'm using 120 volts. I knew the power would go down with the change in wiring. I was hoping it would pull about 15 amps at 120 volts and still heat up fast enough, but it pulled less then 15 amps. I think the solution for me is to just alternate heating the top and bottom elements.

author: scott216
author_rank: Regular Contributor
timestamp: March 27, 2014, 02:09:02 am
content: Quote from: miceuz on March 26, 2014, 11:35:27 pmYour triac-bloc is very cool. I'd like to keep the triac-power supply PCB that came with the toaster too keep things simple. I'll remove the PCB that held the microcontroller, LCD, buttons etc and replace that with my Arduino and related circuitry. I'll probably use theArduino pro-mini(5v) that Spartfun sells. I've got the pro-mini working with aNokia displayfrom Adafruit.

author: ludzinc
author_rank: Unknown
timestamp: March 27, 2014, 02:21:44 am
content: Quote from: scott216 on March 27, 2014, 01:49:21 amALL the electronics will be floating at mains.You will need to optoisolate the Arduino if you want to change programme 'on the fly'.Please, don't take this the wrong way, but as you;re reasonably unsure about this type of electronics, I'd make the following suggestion:Ditch the original control board, forget interfacing to the Triacs. Use a bunch of relays to control your heater elements and drive those from your Arduino.

author: scott216
author_rank: Regular Contributor
timestamp: March 27, 2014, 02:54:39 am
content: Quote from: ludzinc on March 27, 2014, 02:21:44 amThe reason I use the forums is so I can get info form people who know a lot more them me. I may take your advice, it's clearly less hazardous. I am unfamiliar with this "floating at mains" concept. I like electronics and programming because I enjoy learning new stuff and figuring things out. I'd like to understand this floating at mains better. It sounds like it's common in appliances; do you have any suggestions for resources where I can learn more about it?

author: miceuz
author_rank: Frequent Contributor
timestamp: March 27, 2014, 09:57:38 am
content: Quote from: scott216 on March 27, 2014, 12:54:09 amFor me this looks simple - just add an optotransistor and drive it's LED with an arduino. Connect emitter of optotransistor to the base resistor R2 and collector of opto to GND of LM7905. As far as I understand, you can't use the same transformer to power arduino as -5V is connected to 120V hot via triac gate, R1 and T8050, so you would have to add another transformer or SMPS for arduino. The reason is safety - you usually don't want to work with mains referenced circuits, so be aware that everything after optotransistor is potentially dangerous. Or maybe there is another winding on the onboard transformer?You could trace the original controller board (if you still have it) to get ideas how they were doing it. Maybe there is a separate winding of transformer which was used to power controll MCU, detect ZC and controll the phase?

author: scott216
author_rank: Regular Contributor
timestamp: March 27, 2014, 12:10:04 pm
content: Quote from: miceuz on March 27, 2014, 09:57:38 amIs there an easy way to see if there is another winding on the transformer by checking the resistance between a couple of points? If so, what should I measure?

author: miceuz
author_rank: Frequent Contributor
timestamp: March 27, 2014, 12:22:11 pm
content: Quote from: scott216 on March 27, 2014, 12:10:04 pmYou just have to have a look at the board, no need to measure - you will clearly see the primary winding and one or two secondaries. I can see 4 diodes arranged in full bridge configuration - those clearly are connected to one of the secondary winding which powers 7905. There is another diode, maybe that one is used as half-wave rectifier from another winding.TL;DR; if there are 4 contacts on the secondary side of trafo, you have two secondary windings. If you have 3 connections on secondary side, you have a tranformer with middle-tapped secondary and that will be no good to power arduino as one side of tap is already "poisoned" by connecting to it mains.

author: miceuz
author_rank: Frequent Contributor
timestamp: March 27, 2014, 12:24:43 pm
content: ok, I had a look at your photos myself, it seems there is only one secondary in the transformer. so, you need to power your arduino separately.

author: scott216
author_rank: Regular Contributor
timestamp: March 27, 2014, 12:38:18 pm
content: To power the Arduino could I just use an isolated DC-DC converter, something like this (Digikey 945-1655-5-ND):http://www.digikey.com/product-detail/en/ROE-0505S/945-1655-5-ND/3461632

author: scott216
author_rank: Regular Contributor
timestamp: March 28, 2014, 12:04:21 pm
content: I took a shot at drawing a schematic of the toaster's circuitry. See attached.

author: Everton
author_rank: Contributor
timestamp: March 28, 2014, 08:46:45 pm
content: There is an error on your schematic because you have ground tied to fuse. This will obviously blow the fuse every time.

author: scott216
author_rank: Regular Contributor
timestamp: March 28, 2014, 09:01:44 pm
content: Quote from: Everton on March 28, 2014, 08:46:45 pmI'm petty sure the schematic is correct. Ground in this schematic is not connected to an earth ground or anything. If it was you'd be right.

author: David_AVD
author_rank: Super Contributor
timestamp: March 28, 2014, 09:58:09 pm
content: Quote from: Everton on March 28, 2014, 08:46:45 pmIn this case "GND" is simple the circuit's 0V reference.

author: scott216
author_rank: Regular Contributor
timestamp: March 29, 2014, 11:38:41 pm
content: I changed the circuit so Vcc is not a 5 volt offset from mains anymore (just had to remove one jumper). I also removed diodes and resistors that tapped off the high side of the transformer. I added an Arduino and a couple opto-couplers. The Arduino works fine, but unfortunately, when the opto-coupler turns on, the triac does not turn on. Attached is my revised circuit.In the original circuit, if I applied -5v to the triac (actually to the 91 ohm resistor in front of the triac), it would turn on. But with the revisions I made that doesn't work anymore. I even bypassed the opto-coupler and put the -5v on the 91 ohm resistor, but that didn't work either.Any suggestions as to what's wrong?

author: madires
author_rank: Super Contributor
timestamp: March 29, 2014, 11:56:24 pm
content: Reversed MT1 and MT2?

author: scott216
author_rank: Regular Contributor
timestamp: March 30, 2014, 12:01:28 am
content: Quote from: madires on March 29, 2014, 11:56:24 pmAre you asking if I tried reversing MT1 and MT2 on the triacs? I have not. I don't think it matters, does it?

author: miceuz
author_rank: Frequent Contributor
timestamp: March 30, 2014, 12:04:21 am
content: Quote from: scott216 on March 29, 2014, 11:38:41 pmCurrent can't flow via tracs gate as -5V is not mains referenced.

author: madires
author_rank: Super Contributor
timestamp: March 30, 2014, 12:12:32 am
content: Quote from: scott216 on March 30, 2014, 12:01:28 amPlease cross check with the original circuit diagram! Triacs don't work exaclty the same in all 4 operation quadrants. In Q4 the triggering is least sensitive. Some triacs won't trigger in Q4 at all.

author: scott216
author_rank: Regular Contributor
timestamp: March 30, 2014, 12:13:39 am
content: So do I need something like this circuit:http://www.flickr.com/photos/38917611@N08/4886233493/#sizes/lI don't have a zero-crossover optocoupler triac like in the schematic above, but I do have an APT1221 phototriac coupler (data sheet). Would that work?

author: madires
author_rank: Super Contributor
timestamp: March 30, 2014, 12:22:18 am
content: Just have a look at the original design. Is MT1 or MT2 connected to 120V?

author: David_AVD
author_rank: Super Contributor
timestamp: March 30, 2014, 12:29:26 am
content: Quote from: madires on March 30, 2014, 12:12:32 amThat's exactly why I bought up the quadrant issue early on. I suspect the OP didn't pay attention to it though.

author: scott216
author_rank: Regular Contributor
timestamp: March 30, 2014, 12:42:59 am
content: Quote from: madires on March 30, 2014, 12:22:18 amMT1 (aka A1 on data sheet) is connected to 120 volt

author: scott216
author_rank: Regular Contributor
timestamp: March 30, 2014, 01:19:22 am
content: Quote from: David_AVD on March 30, 2014, 12:29:26 amThe whole quadrant aspect of triacs is new to me and I didn't really grasp the importance of it. So I need to find some info on it. I took a quick look atwikipediaand it seems like they have some good info. If you know of any other good sources for a beginner please pass them on.

author: madires
author_rank: Super Contributor
timestamp: March 30, 2014, 11:11:48 am
content: There'shttp://www.onsemi.com/pub/Collateral/HBD855-D.PDForhttp://www.nxp.com/documents/application_note/APPCHP6.pdf.

author: Fank1
author_rank: Regular Contributor
timestamp: March 30, 2014, 12:56:33 pm
content: A triac is nothing more than 2 SCR's back to back with a common gate.You don't control the amount of current flowing thru them you only control when it turns on.Each SCR turns off when the phase of the ac supply reverses.You could control the on time and the off time to control the average power.It's only a switch. ON or OFF.

author: madires
author_rank: Super Contributor
timestamp: March 30, 2014, 04:52:51 pm
content: Quote from: Fank1 on March 30, 2014, 12:56:33 pmThat's just a simple model to explain the basic idea behind it. The reality is more complex, as asual. A TRIAC can be triggered by a negative gate voltage. Can you do that with a SCR?

author: max666
author_rank: Frequent Contributor
timestamp: March 30, 2014, 10:40:46 pm
content: Quote from: miceuz on March 30, 2014, 12:04:21 amI agree with miceuz, by isolating the -5V from mains you also made it unusable as trigger voltage.I think there is a crude way to return from pin 3 on the opto-couplers to neutral/mains with the proper resistance and somehow get the right trigger voltage/current. And there are also refined circuits with a capacitor and a Diac to actually trigger with a proper fast rising trigger current (as it should be), but I'm no expert on hot to properly trigger a Triac, so I would have actually hoped someone here would explain the Triac triggering 101 (or refer some read up).

author: scott216
author_rank: Regular Contributor
timestamp: March 30, 2014, 10:44:54 pm
content: Ok, so I replaced myLTV-816 optocouplerwith aAPT1221 phototriacI had on hand. I wired it between the triac and neutral - see attached schematic. I found anexample circuitusing an Arduino to control a triac and I usedthis schematicas a reference. Well, it blew the 91 ohm resistor in front of the triac. I was worried about this before I powered it up because I didn't know what the current would be going into this resistor, I though that maybe the gate would limit it. Also, I found a couple similar circuits like the one in the above example so I figured I'd give it a try. My theory to this not working is that my phototriac is not a zero crossing, so the full 120 volts is going into the resistor. Compared to a zero-crossing phototriac that's only on when the voltage is around zero and thus the current through the resistor is low. Am I right about that?Could I just replace the 91 ohm resistor with a bigger one? If I had a 2400 ohm, then the current would be 50mA at 120 volts. But this would be 6 watts of power, so I'd need a big resistor. I don't really want to do that, but I want to know if that would theoretically work.If this circuit will work with a zero-crossing phototriac instead of the APT1221, then that seems like the way to go.

author: David_AVD
author_rank: Super Contributor
timestamp: March 31, 2014, 06:27:02 am
content: Read up on the MOC302x and MOC304x series opto app notes.

author: miceuz
author_rank: Frequent Contributor
timestamp: March 31, 2014, 07:59:16 am
content: Quote from: scott216 on March 30, 2014, 10:44:54 pmNow your optotriac might be dead and the triac too. Maybe not, but have this in mind.You have to limit the current thru the optotriac to account for the optotriac max current, but keep in mind the minimum latching current for the main triac. I've seen some advices on the net to make it worst case absolute maximum for optotriac. Usually, when everything is working, optotriac sees this current for a very brief moment - until the main triac is on, then the main triac shorts it thru itself. Plase note, that you have to calculate peak current for peak-to-peak mains voltage - 120V * sqrt(2) = 170V.You have correctly noticed taht when using optotirac with ZC detection you would not have this problem as current would flow thru the resistor and opto until the main triac latches, you just would have to keep the opto ON for long enought for the current thru the gate to reach the latching current. Note that in this case you loose the possibility to do phase control.

author: scott216
author_rank: Regular Contributor
timestamp: March 31, 2014, 11:44:15 am
content: Quote from: miceuz on March 31, 2014, 07:59:16 amoptotriac still works, I haven't tested the triac.QuoteI'm not going to do phase control at this point, just on-off.

author: max666
author_rank: Frequent Contributor
timestamp: March 31, 2014, 12:24:32 pm
content: Quote from: miceuz on March 31, 2014, 07:59:16 amPhase controlling a heating element is a very bad idea anyway, at this high currents you'll create huge amounts of EMF and without filters it will all bleed back into the mains.And it's completely useless anyway, since the heating element is slow in the order of seconds, switching at phase level is ridiculous.

author: miceuz
author_rank: Frequent Contributor
timestamp: March 31, 2014, 12:27:25 pm
content: Quote from: max666 on March 31, 2014, 12:24:32 pmI agree, but sometimes it might make sense if the heater element is more capable than your mains circuit breaker.

author: ConKbot
author_rank: Super Contributor
timestamp: March 31, 2014, 01:07:54 pm
content: Quote from: miceuz on March 31, 2014, 12:27:25 pmUse zero crossing switching for turn on and turn off, no harsh EMI from turn-on mid cycle, and you can still control the element on time with 8ms resolution. If youre popping breakers in under a single cycle, then getting it working on a 15A supply might be a bit on the challenging side.

author: scott216
author_rank: Regular Contributor
timestamp: March 31, 2014, 03:49:26 pm
content: I'm looking at the MOC3031 and MOC3032 zero crossing optocouplers. I have a question about the gate trigger current (Igt). For the 3031 it's 15mA and for the 3032 it's 10mA. The only gate I know of is internal to the part and is used by the internal zero-crossing circuit. So, should I care about what the gate current is if I'm driving a BTA26-600B triac?

author: David_AVD
author_rank: Super Contributor
timestamp: March 31, 2014, 11:34:27 pm
content: Quote from: scott216 on March 31, 2014, 03:49:26 pmI can't see those figures (Igt) in the data sheet I have. Maybe link to the sheet you're reading?

author: hamster_nz
author_rank: Super Contributor
timestamp: April 01, 2014, 12:43:53 am
content: Quote from: scott216 on March 18, 2014, 06:43:07 pmWow! Is that a really big oven, or are you sensing the temperature wrong? My el-cheapo 1.5KW oven takes under 5 minutes (http://www.thewarehouse.co.nz/red/catalog/product/Necessities-Toaster-Oven-White-9L?SKU=1689799). No wiring changes were needed - just a SSR on an extension lead.Mike

author: scott216
author_rank: Regular Contributor
timestamp: April 01, 2014, 01:18:13 am
content: Quote from: hamster_nz on April 01, 2014, 12:43:53 amThe reason it took 10 minutes is I rewired the heating elements so some were in series instead of all parallel. The result was they elements didn't get as hot, but this didn't work out. So I wired them back, but I can't have the top and bottom elements on at the same time, this draws about 30 amps (120 VAC). My solution will be to just alternate between the top and bottom heating elements. I think I'll switch once a second or something like that.

author: scott216
author_rank: Regular Contributor
timestamp: April 01, 2014, 01:27:51 am
content: Quote from: David_AVD on March 31, 2014, 11:34:27 pmI think I figured it out. On Digikey's site, they call it Igt (see here). But on thedata sheetthey call it IFTwhich is the LED trigger current. It seems to just be an error with Digikey.

author: David_AVD
author_rank: Super Contributor
timestamp: April 01, 2014, 02:01:29 am
content: Quote from: scott216 on April 01, 2014, 01:27:51 amThat's why it pays to read the actual data sheet and not rely too heavily on the supplier's page.

author: scott216
author_rank: Regular Contributor
timestamp: April 08, 2014, 02:39:42 am
content: I'm still struggling with this. I got a MOC3032M zero-crossing opto-coupler and wired it up as shown in the attached schematic. If I enable the Arduino output, my heater comes on, but I'm still getting too much current flowing into the triac's gate. I measured about 220mA. I only kept the heater on for a couple seconds at a time. The resistors between the opto-coupler and triac get warm pretty quick. The reason I have 3 resistors like this is I just had these laying around. I thought the opto-coupler would just turn on for short pulses when the voltage was zero and thus there wouldn't be much current flowing into the gate. That doesn't seem to be what's happening. What am I doing wrong?

author: David_AVD
author_rank: Super Contributor
timestamp: April 08, 2014, 03:11:06 am
content: Here's something I built a while back. Take note of where the opto output is wired to. Hope that helps.

author: scott216
author_rank: Regular Contributor
timestamp: April 08, 2014, 03:13:13 am
content: What is R6 doing in the circuit?

author: David_AVD
author_rank: Super Contributor
timestamp: April 08, 2014, 03:15:53 am
content: Quote from: scott216 on April 08, 2014, 03:13:13 amR6 reduces the gate's sensitivity to leakage in the opto and stray crap that may otherwise turn it on unexpectedly.

author: scott216
author_rank: Regular Contributor
timestamp: April 08, 2014, 03:17:20 am
content: Thanks. I'll add that to my circuit and see if it helps.

author: David_AVD
author_rank: Super Contributor
timestamp: April 08, 2014, 03:37:02 am
content: Go back and see how my circuit is different to yours. There is one important difference to do with the opto output top connection.

author: scott216
author_rank: Regular Contributor
timestamp: April 08, 2014, 04:02:53 am
content: Well, you have R5 on pin 6, where I have it on pin 4. I guess in my circuit it doesn't matter, but when you add R6 it changes things a bit. These resistors just limit the current going through the opto, so I don't think it matters if it's on pin 6 or pin 4, but maybe I'm wrong about that.The other difference is I have pin 6 connected to neutral, you've got it going to the MT2 (via R5). So if you had the load connected to pin 1 on the connector and 120 hot to pin 2, then there would be a voltage drop across the load, so the voltage seen by pin 6 would be less in your circuit. But wouldn't you basically have a voltage divider, something like: Neutral---Load---R5---120 VAC. The resistance of the load is going to be pretty small (heating coils in my case), so I can see how with your circuit the voltage going to the opto is less, but it doesn't seem to me it would be a huge difference. You also have the snubber circuit, but I have a resistive load, so I don't think I need that. So, I see some differences, but I don't really understand to root cause of my problem - though I'm trying hard too.

author: David_AVD
author_rank: Super Contributor
timestamp: April 08, 2014, 04:12:02 am
content: The circuit I posted works. Please try it and see how you get on. Leave the snubber off if you like.

author: scott216
author_rank: Regular Contributor
timestamp: April 08, 2014, 04:13:05 am
content: I'm going to give it a try tomorrow when I'm back at my workbench.

author: scott216
author_rank: Regular Contributor
timestamp: April 08, 2014, 03:33:40 pm
content: Quote from: David_AVD on April 08, 2014, 04:12:02 amI tried your circuit (without snubber) and it works. Thanks!! The triac gate was only drawing a couple mA. I also tried it without R6 and it still worked. And I tried putting R5 on pin 4 where I originally had it and it still worked. So the real difference is how pin 4 is wired. I attached a simplified schematic of the working and non-working circuits. In my non-working circuit, I'm assuming the problem is that the zero-crossover circuit isn't triggering at the right time, thus letting too much current and voltage through and into the triac gate. But I don't understand why.

author: David_AVD
author_rank: Super Contributor
timestamp: April 08, 2014, 09:06:42 pm
content: My circuit followed the exampled in various application notes. Google "MOC3021 application note" and you'll get tons of hits. They explain the operation in there.