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clarification with TRIAC and opti-couplers for A/C circuit

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peppy2day

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Hello I am working on a few hobby electronic projects... My background is in software development, but I have taken a few electronics & hardware courses as part of my Computer Science degree.

I am working on a circuit to switch a/c from a low voltage (dc) logic circuit. I have already built a parallel interface board with led indicators as well as appropriate control/timer software that controls each output line (on or off). It's been working fine with LED's at the end of each line. That was my first step, and now it's time to make each output line switch an a/c source. I am just trying to get a grasp on the basics on how to control the flow/switching of a/c current.

My initial research pointed towards using a solid state relays. But then after reading a bit more, it seemed to be that I could use an TRIAC and triac driver (moc303. I read through several schematics and figured it would be easy enough to implement, so I ordered the parts:

my triac: http://search.digikey.com/scripts/D...g=en&site=ca&keywords=MAC15SMGOS-ND&x=21&y=17

my opto-coupler: http://search.digikey.com/scripts/DkSearch/dksus.dll?lang=en&site=CA&KeyWords=160-1722-5-ND&x=0&y=0

FIRST: I'm stating to think that the opto-coupler i ordered isn't exactly the one I was supposed to order? When I wire up just the opto-coupler to the a/c lines, the device switches ON (when supplied with 2v dc to the input). BUT when I try to wire the output of the opto-coupler to the TRIACs that I have, nothing happens (or the device turns on steady and seems to ignore the switch) I also noticed that when I connected M1 and M2 without wiring the GATE pin.. the device turned on steady -- this is not what I expected (to clarify this - I cut the a/c supply line and connected one end to M1 and 'continued the circuit' through to M2 - so basically no voltage input on the gate - and the device turns on steady when I plug it into the wall). is this supposed to be the case? i thought the two leads would be isolated?

SECONDLY, I noticed that even if i was able to get the TRIAC to function the way I think it should... I don't think I will be able to shut it OFF? (the opto-coupler turns ON the output device, but doesn't shut it off when supplied with 0... my readings of this point out that the triac will only 'un-latch' when the ac current drops)

SO.

I'm looking for some clarification on my situation. If I want to continue trying with the TRIAC method, what should my next steps be? I'm starting to think that I might need to create a 'reset' loop for the latch? um, something that will drop the a/c current enough that the TRIAC will shut off... I haven't researched this aspect of things because I'm not sure if it was in the right direction.

--OR-- should I just scrap the whole TRIAC methods and skip to using a relay? I initially ruled out wanted to use relays because of their size and unit cost. BUT it might just end up saving headaches down the road?


I would also like to point out that I am only in testing stages with these components and have not given any consideration to capacitor or resistor values as of yet and haven't used any in the tests i described. should this matter to the basic function of the devices? I could see the cap on the a/c side being useful if i was using a pulsed dc current to control the opto-coupler. But at this point, that's not in the plans.

Thanks for any input you can give.
 
With the gate open the TRIAC should not conduct AC. And if you have the TRIAC turned on and remove the gate current then the TRIAC should turn off at the first zero crossing of the AC current through the TRIAC.

If this is not happening them something is wrong.

What is the TRIAC load?

Can you post a schematic of what you are doing?
 
The load that I am testing with is just a 30w bulb. (a lava lamp plugged into an outlet that i mounted in a plastic box)

I haven't drawn a schematic yet, but I will attempt that later and post it hopefully for tomorrow. At the moment I didn't really pay attention too much to the logic circuit, because I'm just testing, i figured that that source of the voltage didn't matter much, so I just setup a toggle switch to supply 3v to the opto-isolator.

BUT even before trying to use the opto-isolator, I wanted to test something...

Basically on the a/c side on my circuit the first thing I did was wire up a 2-gang box with an outlet and a regular 'light switch' to switch the hot (black) from the main supply which then supplied the outlet. I grounded the outlet to the box and main supply ground (green wire). Also connected the white wire from the supply to where it goes on the outlet. Then I plugged in the lava lamp (30 w halogen) to the outlet. Then plugged the supply line to my box into the wall. Tested it, and the light switch turned the outlet (and lamp) on/off as expected.

So from my understanding of TRIACs, I reasoned that I should be able to essentially replace the light switch with a triac. So I took the switch out, and soldered leads onto the M1 and M2 pins of the TRIAC and then used marrets to join it back into the circuit. So without no voltage applied to the gate, I was surprised that the lamp turned on as soon as i plugged the supply line into the wall. If i break the connection at one of the marrets, the lamp shuts off.

It seems like the TRIAC is conducting current from M1 to M2 (without gate activation). I didn't think this was supposed to be the case, so I really want to guess I'm doing something wrong? Is there a chance that I heat damaged the leads of the TRIAC? (at first I thought this may have been the case so I tried again with a new TRIAC. same thing. I even tried without solder, and just pushing it into a breadboard and using smaller wires.

-- That's another thing altogether (but perhaps for a different thread) I'm also slightly concerned about the guage of wires and the size of the leads on these small chips. I'm not sure how much I like the idea of a large load (15 amp, 1000w, etc) having to pass thru the tiny little leads of these components. For this reason, I started leaning back towards the idea of using solid state relays. But maybe it's not a concern?
 
Your circuit should look similar to this one: http://www.simpleio.com/design/triacout/images/TriacOut4Sch_L.gif


Triacs are often misunderstood.

gate current turns it on, but not off.

To turn off triac, the gate current must be zero and the triac will turn off when the load current passes through zero.

You can turn a triac off if you short the triac out with a gate current of zero.

They have a minimum load requirement, otherwise they appear stuck on if you use a voltmeter. They also may never turn on.

They have Quadrants of operation: Polarity of the gate can influence operation.

They can turn on because of excessive dv/dt. That's why most designs have an RC network (snubber) across MT! and MT2.

The TAB may or may not be isolated from the line.

They usually have to be heat sinked.

If you did not use a resistor in the gate lead, the triac may be toast.
 
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Hello there,

Some good points brought up by the other posters here, and the schematic KeepIt posted looks good except for one little detail.
You also need a resistor from the gate terminal to what is commonly called MT1 (main terminal one) of the triac. MT1 is the terminal closest to the gate when drawn schematically. The value is 1k or less. This resistor ensures the gate leakage does not hold the triac 'on' when it is supposed to turn 'off'.
Also as KeepIt mentioned, there will be some 'normal' conduction current when the device is deemed to be 'off'. If your load draws a very light current (such as a 30 watt bulb) relative to the rating of the triac the triac may appear to be 'on' all the time. Just to note, a 30 watt bulb may work ok with a triac rated for 1 amp but for a triac rated for 25 amps it may appear to be on all the time, so you'd have to use a bigger load to test that circuit.

So you should try this again with a load that draws more current and also add a gate to MT1 resistor. The other resistor goes from MT2 of the triac to MT2 of the opto driver.
 
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Original schematic (first attempt)

Can you post a schematic of what you are doing?

I've attached a drawing of what I was doing. (I think I messed up drawing some of the symbols for the a/c lines)

So now my next step: I am going to try to integrate resistors. As well as increase the load on the TRIAC.

I think my resistors are only rated 1/4w though... Do I need a different style resistor on the a/c side of things?
 

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Hi peppy,

You need to read more posts since your last one. You would have found out already that you need two gate resistors...one in series and one in parallel.
Also, your upper connection should be going to the other triac terminal (on the bottom) which is MT2.
So you have three things you have to do to get this working right.
You may also need a capacitor in series with a small value resistor for snubbing MT1 to MT2.
See attached diagram.
 

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Thanks MrAl for your revised drawing - It works great now!

You said that I might also need a cap from MT1 to MT2? I haven't tested a larger load yet, as the 30w lamp gave the results I was after.

What are the cases that would require this cap needing to be used?

How concerned should I be about the current passing through these small components? With everything I've done with a/c in the past, I've always tried to use the lowest guage wire that was suitable - I'm currently using stranded 14/3 for the a/c side of things.

I'm only testing and building these circuits as an experiment for now, but it would be good to know the limitations of these devices/(triac circuits) in real world applications. It's partly because of this uncertainty that the solid state relays looked better (imo) because of the size of the terminal connections that they'd be able to deal with higher loads? My terminology might be a bit off, but hopefully my concern is understandable...although perhaps invalid?
 
It is usual to have a current-limiting resistor in series with the LED of the opto-driver. Depending on the characteristics of your driver you may get away without one, but the LED life will be shortened if it's drawing current at or above the rated maximum.
 
Thanks MrAl for your revised drawing - It works great now!

You said that I might also need a cap from MT1 to MT2? I haven't tested a larger load yet, as the 30w lamp gave the results I was after.

What are the cases that would require this cap needing to be used?

How concerned should I be about the current passing through these small components? With everything I've done with a/c in the past, I've always tried to use the lowest guage wire that was suitable - I'm currently using stranded 14/3 for the a/c side of things.

I'm only testing and building these circuits as an experiment for now, but it would be good to know the limitations of these devices/(triac circuits) in real world applications. It's partly because of this uncertainty that the solid state relays looked better (imo) because of the size of the terminal connections that they'd be able to deal with higher loads? My terminology might be a bit off, but hopefully my concern is understandable...although perhaps invalid?

Hi again,

You might need a cap and resistor as a snubber if the triac seems to turn on by itself sometimes. I've done circuits with out it though.

The current through the main terminals will be whatever the load is, and 14 gauge wire is considered good for up to 15 amps, 12 gauge for 20 amps. For the gate, 22 gauge would be fine.

The limitations of the triac are found on the data sheet for that part number triac. The same for a solid state relay. If you have trouble interpreting these data sheets, simply post the data sheets or a web link to them. They often have pdf files that show lots of info about the devices.
You can get triacs that work up to 25 amps almost everywhere.
 
Be aware that the TRIAC will dissipate more than 1W per ampere of current through it so you need to put it on a heat sink. If the TRIAC heat sink tab is not isolated (as is true of your selected device), then you will need to isolate it with an isolation washer or isolate the heat sink from the chassis as it will have the mains voltage appearing on it.
 
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