Thread: Using an opto-triac (MOC3020) to control a mains lamp

I'm trying to design a circuit to control a lamp using a triac and MCU.

Ignoring the zero-cross / timing issues at the moment, I have a couple basic queries about the use of an opto-triac (MOC3020).

Taking the circuit in Figure 5 of the datasheet PDF (http://www.ohm.wz.cz/PDF/moc3020.pdf) as an example.

1. Which lamps are classed a 'resistive' and which are classed as 'inductive'? I'm assuming a standard incandescent lamp is a 'resistive' load.

2. For the resistor that sits on pin 6 - what wattage does this need to be to safely control an 80W lamp from a 220Vac UK (50Hz) supply? (If there is any maths involved I'd appreciate the equations to understand for the future)

3. Would a BTA26-600B be suitable triac (http://www.datasheetcatalog.org/data...onics/7470.pdf)?

Thanks in advance.

1. Incandescent lamps are resistive but don't try using it with low energy CFL lamps unless they are 'dimmable'. The only loads you have to be careful with are things like motors and transformers. You can still sometimes control these but you may need additional components around the triac.

2. I couldn't open your link but my datasheet for the MOC3020 (Fairchild) is probably the same. The resistor can be low wattage (0.5W) as it either isn't conducting when the MOC is off and only has the voltage drop across the triac across it when switched on.

3. Should be OK, I used a very similar one to remotely switch a DVD recorder on. I used PIC --> MOC3020 --> triac --> DVD. I've also used them for switching 1.5H.P. motors without problems.

Brian.

In addition, there's of course a difference between dimming and simple on/off control of a CFL lamp or other load. In my opinion, on/off control with an opto-triac or electronic relay is possible in most cases.

There's no harm in providing the suggested "inductive load" RC circuit with resistive loads, too. I even experencied a case, where heavy harmonic power supply distortions caused false triggering of an opto triac that could be eliminated by adding a RC circuit.

The selection of a sufficient series resistor power rating is a bit tricky. 0.5 or even 0.25 W axial standard resistors are usually regarded as suitable. I don't want to contradict, but you should be aware of the worst case pulse power of about 500 W in a 230 V mains application, that last for the e.g. 10 us trigger delay of the main triac. Usual axial resistors can stand it, other types (e.g. SMD chips) most likely can't.

Thanks, it's worth while knowing that the circuits for an inductive load (i.e. Figures 6 & 7 on the MOC3020 datasheet PDF) will work for resistive loads too.

What's the difference between the "nonsensitive-gate" and "sensitive-gate" setups given in these examples?

The only difference (apart from the values of one of the resistors and the capacitor) is the range of IGT (i.e. <15mA for "sensitive", and 15mA to 50mA for "nonsensitive"). I can't see IGT referenced elsewhere in the document, does this refer to input diode forward current maximum?

That is the only difference, some triacs need less current to turn them on, these are the sensitive types. There isn't a definitive threshold between sensitive and non-sensitive, it is just an indication of types that need a little less push to get them going.

Brian.

I suggest to read the Microchip AN 91094A . http://ww1.microchip.com/downloads/e...tes/91094A.pdf
You can also see the AN458.
With the first note, you can do a simple and efficient dimmer with 2 buttons after removing IR input. The soft of 91094A (load it on Microchip site) will be rewritten, it can be far simple (no need to separate positive and negative pulse, and other functions to remove). I made on this basis a specific dimmer to switch off slowly the light after one hour, unless pulse on button restart timer. Second button start slow switch off.
With a small uC, costing less than a NE555 !

PLEASE BE AWARE THAT...
the moc output MUST BE between the triac pins A2 and G
it is never written in any datasheets but it MUST be so
or your triac won't commute ...

Originally Posted by betwixt

1. Incandescent lamps are resistive but don't try using it with low energy CFL lamps unless they are 'dimmable'. The only loads you have to be careful with are things like motors and transformers. You can still sometimes control these but you may need additional components around the triac.

2. I couldn't open your link but my datasheet for the MOC3020 (Fairchild) is probably the same. The resistor can be low wattage (0.5W) as it either isn't conducting when the MOC is off and only has the voltage drop across the triac across it when switched on.

3. Should be OK, I used a very similar one to remotely switch a DVD recorder on. I used PIC --> MOC3020 --> triac --> DVD. I've also used them for switching 1.5H.P. motors without problems.

Brian.

----------------------------------------------------------------------------------------

Hello!

I am trying to switch AC motor through BTA25, I have opto-coupled it with MOC3021. I have a doubt, if the resistor of 180 ohms is sufficient to drive gate of BTA25.
In my case I have't put any load but tried to measure voltage across the output and common terminal. but both the resistors burnt. I have attached the schematics, the fashion in which I made connections.



HTML:


Normal link:
http://images.elektroda.net/62_1263475367.gif


Kindly help me out.

I'm surprised the MOC3021 survived with the LED connected like that!

Brian.

Originally Posted by betwixt

I'm surprised the MOC3021 survived with the LED connected like that!

Brian.

---------------------------------------------------------------------------------
:D I just wanted to test whether it works before connecting MCU to the circuit. It did gave me output for about 5 seconds, before the resistors got burnt. I used 1/4 w resistors. should I try higher wattage. I want my waterpump to switch on/off via mcu whenever my overhead tank runs out of water.

can you help me in this matter

try like this, check triac pins MT1 MT2 Gate


Normal link:
http://images.elektroda.net/78_1263612065.jpg

Originally Posted by rajudp

try like this, check triac pins MT1 MT2 Gate


Normal link:
http://images.elektroda.net/78_1263612065.jpg

Thanks, It did worked. I Tried 1W resistors with the values you specified. Thanks:D

you can use 1/4 w resister and it will work fine

Originally Posted by rajudp

you can use 1/4 w resister and it will work fine

Ok I'll Try

Does this / can this work with 1/4W resistors?

How do you calculate the values required?

firstly, remember that the circuit works in two conditions:

1. The MOC3020 is not conducting so although there is high voltage across it, the current is very small (leakage only) so the power dissipated (V*I) is small.

2. The MOC3020 is conducting so the voltage across it is decided by the forward drop of the triac in it's conducting state.

There is a brief middle state where the triac is partially conducting.

The resistors are calculated to allow sufficient gate current, taking into account the drop across the MOC3020 and possibly current through the capacitor from the voltage across the A1 and A2 pins. Remember that once 'fired' the triac remains conducting by itself until its current/voltage drops close to zero.

I would advise against using very small resistors although by calculation they may be adequate. There are two reasons, the most important being that most 0.25W rated resistors are also only rated at 250V and the circuit will have at least √2 times the RMS voltage across it and quite likely more due to power spikes and load switching. The other reason is that during the turn on and off periods, the dissipation could be significantly higher than in a steady condition. The sudden heat rise in small resistors can result in the carbon film cracking or rupturing. For the sake of a few pennies a larger resistor will overcome these problems.

Brian.

As usual, many thanks betwixt.

I'll heed your advice and get some larger rated resistors.

Question relating to C1 - the 100nF 500V capacitor - what type is this (difficult to tell from the ASCII art). Electrolytic? If so - which side should be -ve?

Also - I've got a BTA08-600C Triac, this is rated at 8Amps - so, what is the maximum bulb size (in Watts) I can safely dim with this triac? (equations would be useful for my understanding).

Thanks in advance.

Definitely NOT an electrolytic !!

It has the full AC supply across it when the triac is turned off and although it is possible to get high voltage non-polarized electrolytics, they are expensive and difficult to find.

You can use almost any kind of plastic film or paper dielectric capacitor with sufficient voltage rating. Ideally find one rated at the supply voltage AC (250VAC for example) but as long as the DC voltage rating is greater than √2 times the RMS voltage plus a safety margin, you should be OK.

Keep in mind that when the circuit is 'off', when the triac is not conducting, there is still a leakage path through the resistor and capacitor so some current will still flow. It will be very small and shouldn't be enough to operate your load but it will be plenty to give a nasty electric shock!! You have been warned!

Brian.

And the maximum power, does anyone know how I calculate that for an 8A triac?

Originally Posted by snaiks

Originally Posted by rajudp

you can use 1/4 w resister and it will work fine

Ok I'll Try

Hi again!

The circuit is working perfect at 230v (Single phase).

Now, I was trying to use it for 3-phase motor control. I have connected Starter Switch's Relay coil in place of load. The relay coil is 440v rated and It is conncted between two phases where potential difference is 400v. Though the triac is 600v rated, it gets on without any trigger. That means, even if I remove MOC3021 from its socket, the starter relay gets on. the 1.2K and the capacitor are still connected but the gate is hanging with a wire (5 cm) connected to socket of MOC3021. Can I use resistor across Gate and A1 to so as to stop it from false triggering? (Like we use in transistor circuit - 4K7 to 10K resistor as limiter - to keep its base low when no signal is on the base of the transistor) . And if yes, then what should be the value of the resistor?

Waiting . . .