Small Triac Switch





Introduction: Small Triac Switch

In order to switch some floodlights by my Arduino, the small switch circuits I built around a 39mf22 were just not man enough to handle the current needed, so I decided to build a switch with discrete components around a TRIAC.

Now these circuits are very easy and come a dime a dozen on the internet: just google for TIC206 or MOC3041 and you have lots of choice. The reason I make an instructable for such a simple circuit is to also provide a print design for those people to lazy or unable to make a design themselves :-)

But let me start with a disclaimer/warning:
This circuit is connected to a mains voltage that can kill you. If you do not know what you are doing then do not build this circuit.. Do not touch it when it is connected to the mains. The metal plate of the Triac is connected to it's Terminal 2 and thus to the mains voltage. When in use, do not touch that plate to feel how warm it gets.
Put this circuit in a proper container.

Materials List
Triac TIC206 €1.20
MOC3041 €0.65
DIL 6 IC foot €0.15
LED €0.15
Resistor 220 Ohm 1/4 Watt €0.10
Resistor 470 Ohm 1/4 Watt €0.10
Connector €0.20
Piece of scrap PCB 23x43 mm

That is a total of €2.55
The links are just there to illustrate what I mean and to give a price indication. I have no links to that webshop other than that i use it and am satisfied with it.
It is also possible to use the considerably cheaper BT136-600, that like the TIC206 can deliver 4Amps, but it's gate current is a hefty 35-70mA as compared to the TIC206's 5mA. If a BT136 is used, the 470Ohm resistor may need to be a bit smaller (150 Ohm). The BTA16-600 is also a good choice

additionally you may need:
capacitor 10-100nF 400V
resistor 39-100 Ohm
resistor 330 Ohm

The PCB is here.
and mirorred here.

The circuit
The main components in this circuit are the MOC3041 optocoupler and the TIC206 TRIAC (M or D version). I have chosen for a MOC3041 because it has a zero crossover circuit in the chip, ensuring smooth switching. The TIC 206 is a triac that has a relatively sensitive gate.

The output from an Arduino or other uP is led to the MOC3041 via a 22o Ohm resistor and an LED. This way one can see for sure if there is current flowing through the optocoupler.
The gate of the thyristor is fed from a 470 Ohm resistor that connects through the MOC3041 optocoupler
On the printed circuit board there is room for a 330 Ohm resistor (labelled with a '*'). If one uses the TIC206 TRiac this resistor is not necessary. Some other thyristors however may need that resistor in order to start.

Triacs are not really suitable for inductive loads. As I am switching a lamp, I have no use for a snubber circuit (formed by the 100R resistor and C1). However, if you switch inductive loads you will need one. The values of the resistor and capacitor may vary depending on your situation but a proper way to start would be a 100 Ohm resistor and a 100nF capacitor. The PCB offers holes at various distances, suitable for different sizes of capacitors.

Download the PCB design(s) above and use your favorite method to etch your PCB. With regard to the design: the pcb picture in this ibble is seen from the component side: The copper traces thus are actually seen as if you are looking through the board. That is why the writing is in reverse. This design is suitable for a direct toner transfer

If you use the mirrorred design, the writing is in fact not mirrorred.

I have used the non mirrorred design for a direct toner transfer (printing it on glossy paper and ironing it on a copperplate) and that works well. Etching in Hydrochloric acid and Hydroyperoxide.

There is only a two pole connector on this PCB that can be thought of as a switch, interrupting the 'Live' line of a connection, just like a regular switch. The figure shows the proper way to connect it according to EU codes: Blue is neutral, brown is live black is switched and yellow green is earth. Outside the EU there are different colour codes.



  • Epilog Challenge 9

    Epilog Challenge 9
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    Pocket-Sized Contest
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    Paper Contest 2018

We have a be nice policy.
Please be positive and constructive.





I want to operate simple LED Lights of maximum 20W which are not dimmable.

Can I use the same circuit for switching purpose of LED Lamps or any lighting appliances?

Yes, this is basically just a switch so i see no reason why and LED lamp couldnt be switched by it

Sorry, but I am getting flicker for LED Light if I am using it in '0' Logic also. It works fine for Logic '1'.

I want to specify one thing that I am using SMPS based LED Driver for that appliance. What should I do for proper operation? Should I modify the circuit? If yes, Plz suggest...

Well Triacs are very suitable for a pure ohm resistance, they do not do very well if electronic circuits are behind it.

A conventional SMPS provides an output that is regulated to give a constant voltage.

That explains why it works in the on position. In the OFFposition, meaning with no voltage on the gate, a Triac switches off at the next zerocrossing. Sometimes that doesn't work well if the load is too small,which one would expect with LEDs.

I am afraid that 'modifying' the circuit in this case would mean a completely different circuit, SMPS driven LED's are usually regulated by resistors

I want to ask one question...can you explain the significance of 100R Resistance and 100nF Capacitor?

Because, if I remove those components, the same circuit works very well for lower load resistance like LED lamps. So if those two components are used for protection purpose then my circuit may damage... otherwise it is working fine.. and we can say it as so called 'Modified' circuit.

Those 2 are there mainly for suppression of noise on the mains line and they are especially important if you switch an inductive load.
But yes you are right, sometimes, with a lower load they can keep the TRIAC open. So in your case, justremove them, no harm done

One more difficulty I am facing that if I am using output of Arduino Promini which is operating on 3.3V, then I am not able to get switching of Appliance connected at Load side. Please suggest me suitable changes for the same.

I think initial resistance of 220 ohm and indicator LED is consuming most of the output voltages so if I remove both these components, will MOC 3043/3063 (I am using in my case) work properly or because of absence of current limiting resistor it might get burnt?

The switch was built in the 5 Volt era with a red LED in mind. With 3 Volt, the LED will already consume much of the voltage.
The Forward Voltage of the MOC is about 1.25-1.5 Volt. An LED can be between 1.8 and 2.5 Volt forward voltage, so obviously there is not much room to move. Depending on the LED you use, you could do 2 things
1 remove the Resistor out of the equation and feed your 3.3 Volt directly to the LED in series with the MOC3041. Again, depending on the LED (say we take a red one), that will have a combined forward voltage of 3.2 Volt, so shld be OK.
2 a safer option is to remove the LED out of the equation and just use the series resistor. Suppose you want to feed the MOC 10 mA, you shld have a resistor of (3.3-1.8)/10= 1.5/10=0.15kOhm=150 ohm

Can we connect 230V AC to power the infrared LED inside the Opto coupler with suitable resistant in series. Or we have to convert AC to DC then feed the the optocoupler LED. Then the value of resistance in serier (230-1.8) /10 = 23 K resisitant.

Yaa.. I also used same red LED.. That's why I also think to bypass the LED and using a 150 ohm resistor. Thank you for your response...