Triac seems to be always turned on

• Triac seems to be always turned on
  Posted by sujeshm3 on 04 Apr, 2015 19:58
• I've made a small Triac (BT136) based switching circuit, using MOC 3021 Opto Triac as the driver.
  The circuit seems to br working perfect with a 5W bulb as load, but when I tried connecting a LED bulb at the load, it was turned ON continuously even with no gate current.
• #1 Reply
  Posted by wraper on 04 Apr, 2015 20:14
• First of all, LED bulbs usually are not dimmable or are cheap Chinese crap containing just a capacitor as a ballast.
  Secondly if the load is reactive and mains voltage is 230V AC, MOC3021 is a very poor choice. It often triggers by by itself in such cases. MOC3051/3052 is much better. Thirdly, to cope with reactive load you need to use snubber circuit consisting of resistor and capacitor.
  And lastly, post the schematic and what LED bulb you are using.
• #2 Reply
  Posted by wraper on 04 Apr, 2015 20:20
• If you are not using this circuit as a dimmer, then you need different optocoupler with zero crossing.
• #3 Reply
  Posted by sujeshm3 on 04 Apr, 2015 20:30
• Thanks for the reply,
  Yes, they are not cheap for sure.But as of now Im not planning to dim the bulb but just basic ON OFF control.
  I will do consider replacing the MOC3021.
  I'm not from a power electronics background so don't know much about TRIAC and other thyristor devices, so could you please let me know about the advantages of having a Triac based switch, does it make sense to have one in my project which for now is controlling home appliances. Does it have any substantial advantage over a relay...?
• #4 Reply
  Posted by sujeshm3 on 04 Apr, 2015 20:32
• And yes my circuit does have a RC snubber.
• #5 Reply
  Posted by Zero999 on 04 Apr, 2015 20:53
• Does the LED turn off if it's connected in parallel with a 5W bulb?
  
  Perhaps it's the leakage current lighting the LED?
• #6 Reply
  Posted by wraper on 04 Apr, 2015 21:06
• Quote from: Hero999 on 04 Apr, 2015 20:53

  Does the LED turn off if it's connected in parallel with a 5W bulb?
  
  Perhaps it's the leakage current lighting the LED?
  

  BTW yes, what is the power of this led bulb, does it light with the full brightness? Have you measured the voltage over the LED bulb? What is the value of the capacitor used in the snubber circuit?
  
  If you are going to just turn on/off once it a while, then relay would be a better choice. Triac will give any advantage only if you need to control power on something by dimming or need to switch very often, like every few seconds. Otherwise there will be mainly disadvantages over the simple relay, like 1-1.5V voltage drop across triac and therefore heating. If any significant power, you need to put it on the heatsink, additional parts which will give increased size and cost.
  
  If using Triac just for switching, then better to use something like MOC3061/2/3 https://www.fairchildsemi.com/datasheets/MO/MOC3061M.pdf
  This will give more robustness and less interference in the switching moment.