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Triac Answered

In the circuit shown are we actually only switching one side of the 220 Vac? From L to L is only one leg of the 220?




Best Answer 6 years ago

We are switching both ways and I re-drew it for easy viewing.
The polarity of the MOC3021works both ways :-)


Answer 6 years ago

Thanks for the redraw it is better without the lines crossing.

Yes it works both ways for the AC. Here on the island 220 is the normal current line to residences.
What i make reference to is the BT136 would only be switching 1/2 of the 220 or a 110 line. If i wanted to switch both 110 lines i could use 2 BT136 and still use 1 MOC3021 to switch the both? If i used only 1 BT16 i would still have 1 Hot line attached, or not disconnected from the appliance or what ever it is attached to.
Is switching only 1 line considered a safe standard procedure and not really a problem?
And now the rest of the story!
This is a disconnect circuit for a UPS i have bread boarded. It will disconnect or connect the mains and inverter at appropriate times to backup my PC, as we have frequent brownouts here.

This is the easiest portion of the circuit that has turned out to be the hardest.


Answer 6 years ago

Safety first only one side needs to be turned off
but any semiconductor can fail due to a dv/dt spike on the line.

The ac power hand drills use a semiconductor for variable speed
and at the slowest trigger speed release there is a mechanical
electric disconnect to really protect the user when changing bits
in the chuck ! !

So a mechanical disconnect is ultimately required if you are tangled 
electrically or mechanically with the line.

On multiple triacs I use an MOC3021 for each triac and wire the
isolating LEDs together in series.



Answer 6 years ago

Switching only one side is a lot easier.

On mechanical disconnect, might i be just as well off to use a relay instead?
I need only disconnection not a variable speed.


Answer 6 years ago

The example was only to show how a large manufacturer handled
your safety question.
There is no way I know how to protect a semiconductors ability to fail_ON
without a mech sw or relay as you point out ..
What will happen if both turn on at the same tine for two half cycles ?


Answer 6 years ago

I answered my own question from your earlier post.



Answer 6 years ago

I will switch only one side, because now that i think about it pretty much else in my house only switches one side.

If you dont then you end up with a Triac like pictured in the reply to pfed2.

So i am not really clear on using a semiconductor switch. Is it acceptable practice in this case or relay only?


6 years ago

Be smart. Whatever you build that you plug in, plug it in at the end of a long extension cord on your initial test. Why you may ask? Who knows. All I know is I've blown some projects up to kingdom come over the years. I like my explosions outdoors better.

I usually wear ear plugs and a face shield too. After about the third time I've plugged a new project in I get more comfortable about it. If it hasn't blown up by then it probably won't. But those first few times, I take precautions.

I designed a circuit yesterday and it was easy. Now I have to design the "easy" part. I'm replacing the controller module for my solar pond pump kit. The one I bought is a piece of junk and it died on me. So far I've the low voltage motor shut off worked out. Now all I have to do is make the charging regulator. I've made lots of regulators so that shouldn't be too hard.

Famous last words right?

Seems i may need a longer extension cord.


Did that make a nice bang when it went? I wasn't familiar with the optical isolation device you're using so I downloaded the data sheet and there are schematics in it similar to what you are attempting to do. I snapped a screenshot of one. The uploader here isn't working so I put it here.

Until proven safe stay at the far end of en extension cord until you figure out what you're doing!

It made a POP and so did I !

I think the way to go is to switch one side like every thing else in my house.

Yes i have that one and about 20 more. I have been studding this for some time now. I hadent come across a circuit that switches both lines, maybe for for good reason. Because it does not work like that. As Iceng says "What will happen if both turn on at the same tine for two half cycles ?" I think i found out.

That circuit you show works fine like it is., The explosion came when i tried to switch 2 triacs from the same opto triac. The second one is what is in the picture.

Lucky i do not know where the piece or pieces went.
This one goes in my conversation pile along with an ATMEGA 328, those caps from our last conversation, ect.

I am still in a quandary about why triacs would be used if a relay is still needed to insure complete disconnection. If you find the answer to that, lay it on me brotha.

Some reasons a Triac out performs a mechanical relay are:
  • Faster switching speeds
  • lower switching current
  • more switching repetitions
  • more compact packaging
  • lower induction