How to switch triacs @ 240Vac

[kybert]

Hi,

Im trying to design a simple control On/Off for a light.

It is to be driven from a PIC controller, and the lamp switching rate is about 4Hz so too fast for a relay.

Im going to use a triac, i have looked at other posts arround here and found this circuit:

(I will be replacing the transformer with a lamp)



Could someone explain the circuit to me please?

Why use a 390R resistor - how was this calculated?

Is the MOC3041 absolutly required?


Does it matter which side (MT1 or MT2) is live (i think not because this is AC, right?)

 

[Nigel Goodwin]

Could someone explain the circuit to me please?

Why use a 390R resistor - how was this calculated?

Presumably from the datasheet?.

Is the MOC3041 absolutly required?

Yes, it provides isolation from the mains, and also zero crossing point switching.

Does it matter which side (MT1 or MT2) is live (i think not because this is AC, right?)

No, it doesn't matter.

 

[kybert]

Found another circuit using this triac driver, and they are using a 22R resistor...

Nothing in the datasheets, unless im missing summut?

 

[Sebi]

I've posted this sematic few months ago. I always use this circuit with sensitive gate triacs e.g. TICxxx. The 22ohm maybe enough for BTxxx series (max. gate current 50mA) The TIC serie triggerable with 5mA.

 

[bmcculla]

The resistor is to limit the current going through the Isolators triac. Check so see if there is a max current listed on the MOC3041 part. Also check the latching current on the main triac you have to be sure you will exceed this current fairly early in the AC cycle to make sure the triac will turn on for most of the cycle. Either resistor will probably work though. The easiest thing would be to start with the 390R and if the triac isn't turning on use a smaller resistor.

Brent

 

[kybert]

OK i understand now, but no matter how i try to calculate it, i cannot reach a value of 390R


Any chance of a formula of some sort?

 

[IRQ57]

Triac Control

Hi Kybert,

This resistor is used to limit the max current allowed in the high side of the optocoupler and it is, in the case of the MOC3041, 1 A.

The max current will be present when the AC wave is at the peak, it is 1.41 times the RMS value, if in this point the opto conducts, the current will be aprox. (240 x 1.41) / R, so R must be at least 338 ohms, a safer value would be between 10 to 20% higher, for 15% : 390 ohm.

BUT, your circuit is using a MOC3041, a Zero Cross driver, it means that inside the optocoupler there is a ZC detector that will only fire the triac when the voltage between MT! and MT2 be zero or very near to zero, in this point the gate current will start increasing up to the point when the triac starts conducting, this collapses the difference of voltage between MT1 and MT2 near to zero, and the high side of the opto will not have more work to do up to the next half cycle.

Due to the possibility of an inductive load, at ZC voltage point there is some lag current that may cause damage to the gate if it is not protected by a limiting resistor. This is the reason because a resistor is included in a ZC driver circuit.

Using a phase control opto ( the same but without the ZCD), it is possible to fire the triac in any point of the AC wave from 0° to 180°, and in this case the resistor is mandatory.

Instead of using an opto and a triac, you may use a solid state relay, which is the same, but in a plastic block with screws for high and low sides, and the resistor included! . It has a higher price of course and there are Phase control and Zero Cross types.

Regards.

 

[kybert]

Yep... I understand, and the formulas add up correctly now too.



Cheers dude.

 

[Exo]

Re: Triac Control

Instead of using an opto and a triac, you may use a solid state relay, which is the same, but in a plastic block with screws for high and low sides, and the resistor included! . It has a higher price of course and there are Phase control and Zero Cross types..

Many companies now offer small solid state relays for pcb mounting. It's an excellent and easy alternative...

 

[kybert]

Is the MOC3041 absolutly required?

Yes, it provides isolation from the mains, and also zero crossing point switching.

I've just found this circuit that switches triacs at 240VAC without the opto.

https://www.electro-tech-online.com/threads/convert.360/

What are the pros and cons of using transistors to switch instead of the opto (except cost) and what is zero-crossing?

Is mains isolation really required?

 

[zevon8]

I would absolutely isolate the mains from the input controller. It's a safety issue. It protects you and your accessable circuitry from contact with the mains voltage.

in general terms, Zero crossing refers to the "firing point" of the triac. The idea is to turn on the device when the AC is crossing the zero point, ie no voltage peak-to-peak. This is to reduce stress and inrush currents, as well as to reduce possible dv/dt issues with the power device. This means you won't be turning on the triac at the peak of the AC waveform, when there could be a large current demand.

Technically, it could be said this works with purely resistive loads, given the current waveforms of a mainly inductive or capacitive load not directly corresponding to the voltage. Current is out of phase with voltage ( 90 degrees ) in non-resistive loads, leading in Inductive loads, trailing in Capacitive loads. You can remeber the relationship with the saying:

ELI the ICE man ( E ) being voltage, leading current ( I ) in Inductive ( L ) loads, and Current leading voltage in Capacitive ( C ) loads.