[SOLVED] Connecting load on MT1 terminal of Triac.

[bharat bhushan]

Triac is generally operated in Mode 1 & Mode3 modes of operation. And when we connect the gate from MT2 terminal of triac through a resistance (and maybe through optocoupler diac which works as a switch), as current flows in from MT2 to MT1 in say first half cycle of ac, and then in opposite direction in second half cycle, triac get automatically operated in mode 1 & mode 3. Its very much clear till here . But as it is an ac current which will flow in both direction alternatively , so I think it should not matter where the load is connected, on MT1 terminal or MT2 terminal. After all triac working as a switch which should be in series with the load. What else should be needed?
Yet generally in all circuits available on net , it is shown that load is connected with MT2 terminal ,whereas line in is connected on MT1 terminal.
One may say that , it is for hot line switching , so that when triac is switched off , load should not be connected with hot line, but same may be achieved by connecting hot line/phase at MT2 terminal , gate connected to MT2 through resistance and optocoupler (optionally) , and connecting load to MT1 at one side and neutral line at other side. In this way load will be at same potential as neutral or earth when triac will be off.
But still such circuit is rarely seen on net. I have tested this circuit, it works .
Is there any problem in this circuit? Please clarify .

Thanks

 

[BradtheRad]

I made a project around a triac. I tried a few different hookup methods. I seem to remember that the only arrangement which worked was the schematic on the package (it was from Radio Shack).

Two lines of thought seem to apply here.

(a)

Normally the load is put in the collector leg. Thus it does not interfere with bias current. You get maximum sensitivity of transistor response.

Although it may be possible to put the load in the emitter leg, and still have the transistor turn on, it is easier if the load is low resistance.

However suppose the load has some complex behavior. Suppose it is high impedance sometimes. Then the transistor is not properly biased. At such times it cannot turn on fully.

(b)

The transistor (and extending this to the thyristor) by custom has its bias referenced to the emitter. You can operate it upside-down (that is, switch the emitter and collector). However it does not work as well. The reason has to do with the size of the B-E junction compared to the B-C junction.

 

[bharat bhushan]

I made a project around a triac. I tried a few different hookup methods. I seem to remember that the only arrangement which worked was the schematic on the package (it was from Radio Shack).

Two lines of thought seem to apply here.

(a)

Normally the load is put in the collector leg. Thus it does not interfere with bias current. You get maximum sensitivity of transistor response.

Although it may be possible to put the load in the emitter leg, and still have the transistor turn on, it is easier if the load is low resistance.

However suppose the load has some complex behavior. Suppose it is high impedance sometimes. Then the transistor is not properly biased. At such times it cannot turn on fully.

(b)

The transistor (and extending this to the thyristor) by custom has its bias referenced to the emitter. You can operate it upside-down (that is, switch the emitter and collector). However it does not work as well. The reason has to do with the size of the B-E junction compared to the B-C junction.

Thanks BradtheRad for your reply. But if the analogy of transistor works in Triac also, I wonder how certain alternistor/snuberless triac based ssr doesnt make any difference there terminal for load or line in . I mean you can connect load to any side.

 

[FvM]

You are right, as long as the control circuit is floating (isolated by opto triac or similar), there's no difference.

You should also consider that most 203V connectors are unpolarized, you can't know which line is hot.

 

[bharat bhushan]

You are right, as long as the control circuit is floating (isolated by opto triac or similar), there's no difference.

You should also consider that most 203V connectors are unpolarized, you can't know which line is hot.

I was talking about changing the load from MT2 to MT1 terminal, not about changing changing the hot & neutral line. Hot & neutral line are interchangeable for sure. What about load ? Can we put load on any of the side of triac ( means on MT1 or MT2 , doesnt matter) ?

 

[D.A.(Tony)Stewart]

Triacs are at least 2 quadrant types. Some are 3 quadrant and options for 4 quadrant triggering.
**broken link removed**

https://www.digikey.com/product-detail/en/MAC15A6G/MAC15A6GOS-ND/918540

 

[KlausST]

Hi,

Triacs differ. Not all work in all four modes.
But datasheets will show you.
Also take care that the trigger current may differ from mode to mode.

Klaus

 

[bharat bhushan]

Hi,

Triacs differ. Not all work in all four modes.
But datasheets will show you.
Also take care that the trigger current may differ from mode to mode.

Klaus

Hello,

My question is , if we are ope rating in mode 1 & mode 3 by connecting gate to MT2 through a resistor and optodiac and say voltage across MT1 & MT2 is alternating 220V AC , can we put load on any side(either on MT1 side or on MT2 side) as current is flowing in both directions.

 

[bharat bhushan]

[Moved]On which terminal(MT1/MT2) should we connect load to a triac ?

Hi,

In case of 220V ac switching control application if we want to operate triac in mode 1 & mode 3, we can connect the gate to terminal MT2 through a resistor Rg and optocoupler/optodiac like MOC3021. Now since it is ac it will flow in both direction alternatively. My question is there any difference in two terminal of triac which are MT1 & MT2 for connecting 220V line in and load . Can I connect load to either side of the triac ( ie. MT1 or MT2)? I will connect the other side with 220v hot line in, for hot line switching.

Thanks .

 

[FvM]

I was under the impression that you did already answer the question yourself.

Why should it make a difference where you connect the load?

 

[bharat bhushan]

Hello FvM,

There are two reasons because of which I am in doubt about the flexibility to connect load to any side.

1) Certain SSR and Dimmer which I bought from the market specify explicitly the terminal where load should be connected and where line in should be connected . And they are triac based.

2) BradtheRad suggested that load should be connnected to MT2 terminal giving the analogy that in transistor we generally connect load to the transistor sothat if the load resistance is high it should not effect the base emitter bias current and hence doesnt affect the transitor to get fully switch on. So as in transistor load is connected to collector and emitter is connected with ground for proper operation of transistor switching independent of the value of load, hence MT1 should be grounded ( in this case neutral line) & load should be connected to MT2.

So what do you think?

 

[bharat bhushan]

Thanks FvM for your quick reply. Yeah I too agree with that transistor analogy is flawed and reason is triac needs just a trigger at gate and only requirement is it should be wrt . MT1 terminal while transistor needs continuous current for biasing between base and emitter.

Yes dimmer and ssr works even if we reverse the load and line in side.

Regarding your explanation on switching noise on load side , which should be avoided to opto-triac. So does that mean preferred connection will be load on MT1 side as optotriac is connected with MT2?

 

[FvM]

2. The transistor switch analogy refers to a circuit where control voltage and load power supply are both emitter connected. The comparison is flawed when discussing a triac switch with "floating" control, e.g. by an opto triac between MT2 and G.

1. A dimmer is a two terminal device. It might have a suggested connection but you can connect it reversed as well.

A possible reason for preferred connection could be generated electrical noise, in an opto-triac controlled circuit also crosstalk to the low voltage side. You have switching transients on the load connected triac terminal, you don't want to put it across the opto-triac if avoidable.

- - - Updated - - -

Regarding your explanation on switching noise on load side , which should be avoided to opto-triac. So does that mean preferred connection will be load on MT1 side as optotriac is connected with MT2?

No. I was thinking of opto-triac circuit with an additional RC filter and the capacitor connected to MT1. In this case connecting MT1 to neutral and MT2 to the load would be the preferred circuit. See e.g. https://www.edaboard.com/threads/162207/#post686270