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Can somebody clarify the following points for the MOC3041 sample circuit given at their datasheet:

  1. What are the wattages of resistors at the HOT side
  2. What is the difference between 360Ω and just 330
  3. The capacitor 0.01, is it micro farad or something else and what voltage rating
  4. Is this circuit safe for inductive as well as resistive loads
  5. Is this a practical circuit which will not cause any false trigger
  6. Can any triac be used?

Datasheet MOC3041

enter image description here

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  • \$\begingroup\$ Can you provide a link to the datasheet? \$\endgroup\$ – SolveEtCoagula07 Oct 3 '18 at 18:36
  • \$\begingroup\$ @SolveEtCoagula07, updated the question with datasheet \$\endgroup\$ – Babu James Oct 3 '18 at 18:43
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    \$\begingroup\$ the text on page 7 of datasheet does mention that the capacitor is in microfarads \$\endgroup\$ – mith Oct 3 '18 at 18:44
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For what it's worth, I've used the exact same part using the exact same reference circuit from the datasheet. It works very well.

  1. The HOT side resistor wattage values do not have to be very high. Either the MOC3041 will be off and no current will be passing through the resistors, or the MOC3041 and the TRIAC will be on and there will be minimal voltage difference across the TRIAC and those resistors. There is a small delay between the time when the MOC3041 turns on before the TRIAC turns on (up to 1/mains_freqency), so make sure the resistor you choose can handle the current draw for that time period. Most resistor datasheets will include a "safe operating area" plot that says what current draw is safe for a given time duration. I used 1/4 W through-hole resistors for my particular application, but you could calculate other appropriate values.
  2. The 360 vs 300Ω resistors is just setting an appropriate voltage level and current draw to trigger the TRIAC. Use what they have in the example.
  3. Capacitor is 0.01 uF, and it needs to be rated for the entire 240 VAC (plus margin) mains.
  4. The asterisk in the image in your post does say that this is appropriate for inductive loads, but that the 39Ω resistor should be changed in that case.
  5. I have not had any false triggers when using this circuit in my application.
  6. Any TRIAC that is rated for the 240 VAC (plus margin) and load current draw that you are expecting. Note that you might need a heat sink if you are drawing amps-worth of current.
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    \$\begingroup\$ The crucial thing about this circuit is the main TRIAC absolutely has to trigger. If it doesn't, the optotriac and/or resistors burn out during the attempt. It's a standard circuit nevertheless. \$\endgroup\$ – Janka Oct 3 '18 at 18:57
  • \$\begingroup\$ I find that pulse transformers with ZCS pulse currents generate the most reliable method to overcome reverse biased gates due to reactive loads. A large voltage is created until the gate conducts then a 10 Ohm or so gate impedance together with mutual coupling limits the current. The magnet wire tape gaps over an SMD pulse transformer this works over a wide range of load and trigger current levels and can be gated with a logic signal ZCS pulse. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 4 '18 at 5:19
  • \$\begingroup\$ I once did a chaser for Canadian rock band in the 70’s my buddy was in. He wanted the band’s name with 8 letters made of Fat Albert Lights for each letter. . Using simple CMOS logic and 8 Triacs with XOR ZCS pulse a Johnson counter sequenced the letters. The rate was controlled by a Schmitt Trigger clock with pot slider pot 1 to 20 Hz or alternatively a piezo mic all taped up tossed inside the kick drum on a long cable to sync the light sequence. It was simple yet effective and just ok 1 weekend at work to put into a box. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 4 '18 at 5:39
  • \$\begingroup\$ Then I heard it failed years later when the Soundboard guy modified the load to 1000W Stage lights and had them flashing at high rates with low duty cycle 1/8 enough to create a repetitive surge power of about 4kW per Triac. Poof. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 4 '18 at 5:42
  • \$\begingroup\$ I built the circuit on a perf-board with 1/4 watt resistors and 0.01uf400v capacitor marked "2G103K". I was testing it so far and last night this capacitor failed, effectively shorted it. Can anyone tell me the reason? \$\endgroup\$ – Babu James Oct 19 '18 at 7:40
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  1. What are the wattages of resistors at the HOT side?

They can be 1/4 W 5%. Only the Triac sees the load current.

  1. What is the difference between 360Ω and just 330?

The 360 ohm limits current into the opto-isolator and triac gate. The 330 ohm acts as a pull-down resistor to make sure the triac if OFF when there is not drive current from the opto-coupler. It keep the triac trigger input from "floating". NOTE: The triac MUST trigger ON as expected or the resistors and the opto-coupler may burn up. The resistors should be the flame-proof type.

Note that the 'R' values are chosen to match the triac gate drive needs without burning up the opto-coupler. Over a wide range of triacs both resistors can vary a great deal as long as the triac ALWAYS turns ON when expected and OFF when the opto-coupler is OFF. There are sensitive gate triacs used in motion-detect nightlights that have a 100 K resistor in series with the gate driven by an op-amp or mini MPU with just 5 to 10 volts.

  1. The capacitor 0.01, is it micro farad or something else?

When you see a decimal point in a capacitor value, it is normally in uF. If in Farads or pico farads it should state so. Nano farads are often written as xxnF. It should have an X2 rating, or twice the AC line voltage.

  1. Is this circuit safe for inductive as well as resistive loads?

It is as long as the triac is rated for the start-up current of a motor, or 3 times the amp rating of an electromagnet. Note that for heavy inductive loads a 40mm MOV across the triac will act as a snubber. It should be rated 50% above the AC line voltage.

  1. Is this a practical circuit which will not cause any false trigger?

This is as simple as it gets. If there is a false trigger it is coming from the logic driving it. If driven by software it much be clean and checked for any chance of errors.

  1. Can any triac be used?

That is a loaded question because the triac needs to be rated for the voltage of the AC line plus 50% for a safety margin. It needs to be rated for 3 times the maximum continuous load current. This affects the package it is in as well. To drive a tiny night light a tiny 3 pin triac will work. To drive a 10 horse power motor it will be large and need to be bolted to a large metal heatsink of aluminum.

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  • \$\begingroup\$ 2. Is incorrect, both R’s are in series and limit current, prior to latching when the voltage drops to <2V. the Gate current is limited by CTR of opto \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 3 '18 at 20:37
  • \$\begingroup\$ @TonyEErocketscientist. I disagree as I have built many triac circuits, with those resistors in place for the reasons I mentioned. I would say the value of both resistors could vary a great deal (upward) and the circuit would be just fine. \$\endgroup\$ – Sparky256 Oct 3 '18 at 20:42
  • \$\begingroup\$ I don’t disagree with this but I did not say that. Both affect ext gate current but the Gate R alone does not limit current rather creates the gate voltage from the opto output ZCS resistance being much lower. Thus it becomes a voltage divider with say 5V AC after the ZCS triggers. Your answer is still incorrect. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 3 '18 at 20:45
  • \$\begingroup\$ @TonyEErocketscientist. After 40 years of working with triacs I am not changing my answer to suite your twisted opinion. If it was wrong it would have been down voted already. \$\endgroup\$ – Sparky256 Oct 3 '18 at 20:51
  • \$\begingroup\$ I doubt anyone else has the same experience as you or I that voted. If you say twisted , I say your answer violates Ohms Law when the opto triac latches ON (during an interval called ZCS) it conducts ONLY 360 + 300ohm in series current limit to generate the voltage drops with the gate voltage being divided by (330/330+360) minus the latch voltage of ZCS Triac . It is obvious to me , but I am not sure my explanation makes sense to you. Would you like a discrete analysis to understand? I never had a product that failed or not meet spec in my career. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 3 '18 at 22:27

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