Understanding Zero Cross Triac operation - Page 1
Forum section: electronics / beginners

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: December 24, 2017, 06:49:52 am
content: 1. I have a application in which need to turn on & off a load, resistive in nature.mains is 220va/50Hz & load is 750ohms approx. MCu is +5V operated.2. I was looking at MOC3063M (https://www.fairchildsemi.com/datasheets/MO/MOC3061M.pdf),As it only needs 5mA forward current for tun on & off its diode at input.3. Does zero cross triac works as :a)If I send +5V input on diode (Pin 1 & 2), with minimum 5mA of current, then the load will be turned on whenever zero cross happens& load will be on as long as +5V is there on 1 & 2 pin.b) And If I send zero input on diode (Pin 1 & 2),then the load will be turned off whenever zero cross happens& load will be on aff long as zero is there on 1 & 2 pin.c) is this how this triac will work?d) Will use Figure 12 ckt as datasheet link provided above except the snubber circuit.4. is there any other zero cross triac to use ?

author: james_s
author_rank: Super Contributor
timestamp: December 24, 2017, 07:25:46 am
content: Yes that's how zero cross switching works. It's handled normally not by the triac but by the opto-coupler that drives the triac. When you apply power to the LED side of the opto coupler the triac will switch on at the next zero cross, continue switching on at every half cycle, and when the signal is removed it will stay on until the next half cycle.

author: Ian.M
author_rank: Super Contributor
timestamp: December 24, 2017, 07:44:23 am
content: The input side is a LED. From datasheet fig.3 its Vf will be about 1.2V @10mA. 5mA If is the *MIMIMUM* to guarantee a new MOC3063M will turn on at 25 deg C, but the LED optical output will fall as it ages + it requires more current at lower temperatures, so it would be prudent to design for double that - 10mA. If you are stuck for drive current capability, you can squeeze the margin a bit but I wouldn't go below 6mA.As the input side is a LED, it needs a current limiting resistor. Assuming 5V rail to rail logic levels, the resistor must drop 3.8V, and pass 10mA. 390R is probably near enough, though you should also allow for the voltage drop in the MCU's output pin driver, so 330R or even 270R may be a better choice - check your MCU's data sheet for output voltage vs load current.It will turn on whenever the input side LED is on *AND* the voltage across MT1, MT2 is less than the inhibit voltage (typically 12V). This effectively prevents turn-on before the voltage zero crossing. It then latches on until the current through is output side falls below 500uA - that's the current zero crossing, which will only be the same as the voltage zero crossing if the load is purely resistive. If the LED is off, it will then turn off.I wouldn't recommend totally omitting the snubber as even with a resistive load, wiring inductance can be enough to be problematic. With an ordinary TRIAC, you'll probably need a low capacitance snubber network. For a so-called snubberless TRIAC, I'd lay out a snubber network, then leave it unpopulated so its possible to retro-fit it. SeeST's AN437for snubber network design.

author: David Hess
author_rank: Super Contributor
timestamp: December 24, 2017, 06:47:13 pm
content: Quote from: Vindhyachal.takniki on December 24, 2017, 06:49:52 amIt is actually current controlled so when you apply 5 milliamps, the forward voltage drop across the LED will be about 1.15 volts nominal. Applying a full 5 volts without current limiting will likely destroy the LED.Otherwise it works exactly as you describe. While the LED is powered, the TRIAC will be triggered during every zero crossing and conduct for at least half of a line cycle.QuoteThat is exactly how thyristors like TRIACs and SCRs work. They shut off via commutation which means during the time that their forward current drops to zero.QuoteThe snubber circuit is probably not required but is still a good idea to prevent false triggering under adverse conditions.QuoteI am not sure what you mean. There are solid state relay modules which include the zero-crossing optocoupler and TRIAC as part of one module which may be easier to use.