Triac hot line switching circuit issue - Page 1
Forum section: electronics / beginners

author: PaulM
author_rank: Newbie
timestamp: December 19, 2015, 04:29:55 pm
content: When I breadboard the attached circuit, the load is always on:- I'm using 120v, not 240v.- For testing, I'm just using a clock radio drawing 25ma as the load.- When I remove everything from pin 4 of the moc3041, I do measure 120v only when I supply 3.3v as vc -- as expected.- With pin 4 of moc3041 disconnected from gate of triac (and the 330 resistor disconnected from pin 4 to MT2), the gate voltage on the triac measures 120v which puzzles me.- I have tried with and without the snubbing portion of the circuit (39R + 0.01uF) -- mostly without.For the optocoupler and triac, I'm using:MOC3041https://www.fairchildsemi.com/datasheets/MO/MOC3041M.pdfBTA12-600Bhttp://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00002267.pdfConnecting pin 4 of the moc3041 to MT2 of the triac makes the circuit turn on/off as expected but I believe all the current is going through the optocoupler instead of the triac. If I'm understanding the datasheet correctly, the max we would want to go through pins 4 and 6 of the moc3014 is 60mA.What am I missing? I'm hoping to get this going while learning something along the way. Any and all help is appreciated.

author: mij59
author_rank: Frequent Contributor
timestamp: December 19, 2015, 04:42:14 pm
content: The 330k resistor should be 330 Ohm, what your measuring is just a leakage current.

author: PaulM
author_rank: Newbie
timestamp: December 19, 2015, 04:46:40 pm
content: Quote from: mij59 on December 19, 2015, 04:42:14 pmI corrected my typo. I am using a 330 ohm resistor.

author: kripton2035
author_rank: Super Contributor
timestamp: December 19, 2015, 04:48:30 pm
content: invert mt1 and mt2 on your breadboardthe 330 resistor and the moc3041 must be between MT2 and G of the triac

author: PaulM
author_rank: Newbie
timestamp: December 19, 2015, 04:56:39 pm
content: Quote from: kripton2035 on December 19, 2015, 04:48:30 pmWinner, winner, chicken dinner! Thank you!!!

author: kripton2035
author_rank: Super Contributor
timestamp: December 19, 2015, 09:46:47 pm
content: also for safety reasons I would NOT use mains voltages on a breadboard ...

author: Zero999
author_rank: Super Contributor
timestamp: December 19, 2015, 09:51:25 pm
content: Quote from: kripton2035 on December 19, 2015, 09:46:47 pmI was just about to say that.If you do decide to use mains on a breadboard, then treat everything connected to the circuit as though it's at mains potential. Breadboard cannot be relied on to provide isolation from the mains and therefore adequate protection against electric shock.

author: PaulM
author_rank: Newbie
timestamp: December 20, 2015, 12:32:29 pm
content: Thanks for the suggestions.I have been careful to unplug the mains prior to fiddling with or changing anything on the breadboard. Good thing because I did let the magic smoke out of a triac once.Planning ahead for something heftier than a clock radio, would it be advisable to upgrade from 1/4W resistors?

author: ornea
author_rank: Unknown
timestamp: August 04, 2017, 11:24:51 am
content: Quote from: kripton2035 on December 19, 2015, 04:48:30 pmThe MOC3041 data sheet does not label the Pins of the triac in its typical circuit.But if you over lay the symbol and corresponding pinouts from the BTA16-600B data sheet then you would be fooled into thinking that the 330 ohm resistor is between MT1 (A1) and G.My circuit is currently wired between A1 and G and think this is the reason why it fires the triac at any point in the phase (see attachment) rather than only after a zero crossing as expected with the MOC3041. I Will swap A1 and A2and see if resolves the issue. .