Protection circuitry for opto-triacs - Page 1
Forum section: electronics / beginners

author: vector
author_rank: Newbie
timestamp: June 06, 2018, 07:50:33 pm
content: Hi!I'm designing a board to control the heaters and the boiler in my apartment with a Raspberry and I'd need some advice related to safety and protection circuitry.The boiler is already controlled by a relay (coil working at 230 V), so the circuit I came up with, after looking a bit online, is the following (pin 1 is for the mains line, pin 2 goes to the relay):The MOV is supposed to protect the opto-triac from the voltage spikes due to the relay coil, but I have some doubts:Is this necessary if there is already an RC filter on the relay? And if there is no RC filter?Is it better to connect the MOV between pin 1 of J3 and ground?In the latter case, should I put one MOV also on pin 2?While writing this I realized that might be a good idea to put a fuse on the mains line hereThen the heaters are controlled with the so-called "pilot wire": you set their mode (comfort, eco, nofrost, off) depending on the type of signal you feed here (no signal, full 220 V sine wave, positive/negative half sine). The circuit I developed is the following:Here I get the mains lines in J1, then with two opto-triacs I select the half-sine I want to send to the heater (the MOSFET is there only because I want the negative half sine to be on if the raspberry is off). Here I put MOVs to ground and a PTC fuse (the smallest I found, since the pilot wire pulls a very small current). In this case I'm more confident about the protections.Then, for the layout, I'm keeping 0.8 mm between tracks and 2.5mm (where possible) between exposed pads. I will also cover everything with insulating lacquer. Of course the mains part is completely separated from the low voltage part.Do you think this is enough?If someone is interest in the full project, I can share it, of course.Thank you!EDIT: I should have fixed the images now...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2018, 08:58:04 pm
content: Put the fuses on the other side of the MOV, just in case. The MOVs are correct, and not a bad idea. Make sure the peak MOV voltage drop is below the TRIAC's rating, at the load current (so, 1A or thereabouts). MOVs are pretty sloppy limiters, but I think the voltage ratings should work out okay here.Why diodes for half cycles? Can get a zero-crossing MOCxxxx part, or just time it yourself on the rPi (needs a line frequency/phase detect signal from somewhere).Tim

author: wraper
author_rank: Unknown
timestamp: June 06, 2018, 09:15:52 pm
content: These optotriacs are not intended to drive any load directly, even if it's something like relay coil. They are only suited for triggering triac/SCR, nothing else. And when you use triac, snubber should be placed to protect triac, not optoisolator.http://www.onsemi.com/pub/Collateral/MOC3163M-D.pdfQuote

author: james_s
author_rank: Super Contributor
timestamp: June 06, 2018, 09:25:33 pm
content: There's a Big Clive video discussing design of robust opto triac circuits. He was at the time designing fairground lighting controllers but the same principals will apply.

author: vector
author_rank: Newbie
timestamp: June 07, 2018, 06:17:07 am
content: Quote from: T3sl4co1l on June 06, 2018, 08:58:04 pmOk, I see. Thanks for the suggestion. So, you would put the fuses before the MOV so it can limit the MOV current in case of a surge? Is there the possibility that the PTC would fail short in this case?I used diodes because they are simpler than a zero-crossing circuit, and also because I'm not sure if I can get such a precise timing with the RPi. Using diodes allows me to write a much simpler code.Also, I wouldn't know how to get the half-wave output if the raspberry is off/rebooting.Quote from: wraper on June 06, 2018, 09:15:52 pmI assumed that since they have a max current of 1A they would have been suitable for such a small load. What can be the differences between using just the optotriac and using a second triac (let's say a Z0103, that has the same max currenthttps://datasheet.lcsc.com/szlcsc/STMicroelectronics-Z0103MA-5AL2_C58459.pdf)? (this is the first time I try to use triacs, so it might be a stupid question)Quote from: james_s on June 06, 2018, 09:25:33 pmI will look for it, thanks!

author: Zero999
author_rank: Super Contributor
timestamp: June 07, 2018, 08:39:58 am
content: Yes, the opto-coupler is not designed to switch the load directly. Interestingly the MOC3063 data sheet doesn't specify a continuous current rating. It only gives a peak rating of 1A for 100μs, at a repeat rate of 120pps (120Hz). Presumably this is just long enough to trigger a TRIAC. The output stage is only rated to dissipate 150mW and the peak on state voltage is 3V at 100mA, which implies that the continuous current rating will be much lower.http://www.mouser.com/ds/2/348/rb168l150-774152.pdfThe Z0103 appears to be suitable for driving the relays.http://www.st.com/content/ccc/resource/technical/document/datasheet/af/ee/c1/7b/df/8a/46/d7/CD00002268.pdf/files/CD00002268.pdf/jcr:content/translations/en.CD00002268.pdfA PTC resistor, i.e. a resettable fuse which, should be designed not to fail short, but open circuit, if it conforms to the relevant safety standards, which in my experience is what happens when the component reaches the end of its lifespan.

author: wraper
author_rank: Unknown
timestamp: June 07, 2018, 09:17:18 am
content: Quote from: vector on June 07, 2018, 06:17:07 amFor Liteon, 1A is peak current for 1ms @ 120 times per second (which is firing triac with twice of 60Hz mains frequency). For Fairchild it's only 1A for 0.1ms @ 120 times per second. ON semi basically says 1A peak current for firing triac at 60Hz mains frequency, no explicit duration given. You even won't find RMS curent rating in datasheet from ON semi and Fairchild as once triac is fired, there is no current through optocoupler. In datasheet from liteon, RMS current is 100mA. As it's peak current duration is higher as well, I'd assume liteon has either beefier detector or is less conservative with their specs.QuoteZ0103 has 1A RMS current rating, while many MOC3063 don't even have RMS current rating, or if they have, it's rather small. If say we take Fairchild MOC3063 with no explicit RMS current rating and use specs we have, 1A peak for 0.1ms @ 120 times per second, would be 12mA RMS.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2018, 09:28:39 am
content: Quote from: vector on June 07, 2018, 06:17:07 amPTC..?At mains voltages?It's not like you're going to protect anything, even though it's a TRIAC, I doubt it's anywhere near capable of fusing even a fast-blow type. Let alone a PTC that takes an eternity to go open (~100ms).On the upside, AFAIK, high voltage PTCs are ceramic based types, much higher reliability than polymer types. Downside: R25 and resistance ratio suck.QuoteYeah, using an outboard TRIAC is best, even a small one.Alternately, just buy a prepackaged SSR -- they're not at all pricey, and already have everything solved for you (including zero-crossing if you like). Just add MOV and fuse!Incidentally, if the SSR comes with a I^2*t fusing rating, you can try to find a fuse with the same or lower rating. That gives some hope that the TRIAC can actually clear the fault, without failing shorted in the process. The fuse can also be pretty small, like one of those SMT ceramic types, 2410 or even smaller. Although that's not exactly replaceable. Well, sort of -- you can get 2410 fuse holders, interestingly enough. But, hey, for one-offs, who cares, you'll always have your soldering iron handy right?Tim

author: Zero999
author_rank: Super Contributor
timestamp: June 07, 2018, 10:10:50 am
content: Yes, a PTC resistor is no good at protecting semiconductors such as TRIACs. To be honest, I didn't even consider that the original power was intending to to that.A PTC resistor is only there to protect the cable from melting down, causing an even bigger short circuit and fire.An alternative to using a really small fuse, is to use a much larger TRIAC, with a much higher fusing than a fast blow cartridge fuse. The downsides are more cost and board space.Another thing to watch out for is the breaking capacity of the fuse, which is the maximum current it can safely interrupt, without blowing up, overheating or arcing over. It needs to be higher than the maximum short circuit current in the circuit, which is only limited by the impedance of the wiring. A small, possibly fusible resistor could be used to limit the short circuit current, as long as it's a much lower value, than the load impedance.

author: Circlotron
author_rank: Super Contributor
timestamp: June 07, 2018, 11:51:46 am
content: Quote from: vector on June 06, 2018, 07:50:33 pmWhen the LED drive stops, the triac will continue conducting until the relay coil current pases through zero. That being the case, there will be no energy left in the relay coil and consequently no voltage spike. A small RC snubber across the triac to slow down the sudden jump in voltage as the triac turns off might be nice though. Others can advise of the value of R and C.

author: Zero999
author_rank: Super Contributor
timestamp: June 07, 2018, 12:36:48 pm
content: Quote from: Circlotron on June 07, 2018, 11:51:46 amOne of the problems is it's important to know the inductance and resistance of coil being switched, which is often unknown. If you know the inductance and resistance, then all that's needed is a capacitor and series resistor to make the RLC circuit critically damped, so there's no overshoot in voltage.https://en.wikipedia.org/wiki/RLC_circuit

author: vector
author_rank: Newbie
timestamp: June 08, 2018, 06:38:53 am
content: Quote from: Hero999 on June 07, 2018, 08:39:58 amOk, thanks. I have to be careful when reading the datasheetsQuote from: T3sl4co1l on June 07, 2018, 09:28:39 amQuote from: Hero999 on June 07, 2018, 10:10:50 amWell, the point of the PTC fuse would have been more a protection in case of shorts to avoid fires... even if it blows the optocoupler, I can always change it(probably I was not clear in the beginning about why I put the PTC fuses)Quote from: Circlotron on June 07, 2018, 11:51:46 amQuote from: Hero999 on June 07, 2018, 12:36:48 pmOk, goot to know. I will look at the specifications of the relay to see if I find anything useful.