I don't have a lot of experience with TRIACs, but with SCR's, you have to stay above the threshold gate current and below the maximum threshold current limit. I would imagine its similar for TRIACs.. The gate resistor is to limit the gate current to the device.
According to the datasheet here, the maximum peak gate current is +/-200mA. The peak gate trigger current with 12V at the terminals is at most 10mA. That means that the we need at least 10mA and no more than 200mA of gate current. I would say that 50mA should be a suitable value for the gate, but there is no way to tell. You also have to take into account that the maximum forward current for the opto-coupler is not exceeded.
As an example, with a dead short as the load, the gate current in that circuit would be around 170mA. That's fairly close to the limit of the device that you want to use, so you definately should increase the gate resistance before attempting to run the circuit.
EDIT: There appears to be an error in the application note schematic. I seems that the TRIAC is backwards. The load is supposed to be on M2 and not M1 as shown, see here for more info
Hello,
I want to make a dimmer for some lights and want to control the power output with the mbed. My idea was to use a transfo for the zero detection and connect that to an input of the mbed. Next I want to use an output pin to generate a trigger pulse for a TRIAC (by using a opto-coupler) so that I can dim 230V with a power rate of 6A (I still have to see whic TRIAC I will use). The general idea behind this little project is that I can control the light at my home over the internet. I already succeeded in controlling the LED's of the mbed board over the internet with a JAVA program. So now I was thinking about writing some extra code on my mbed for the zero-crossing detection en generating of the trigger pulse (because this should be 'real time') and use the RPC server code that's now already running on the mbed just to communicate with the java program to update the 'dimmer value'. This value is going from 0 to 100 and represents the output power.
Is this the best way to do this? And can somebody maybe help me with the C++ code for the dimmer itself (I'm still not so familiar with C++ and all the mbed library's) and a good schematic to realise my concept in practise (high efficiency, robuust, safe). I already found alot of schematics for this kind of application but I was wondering which is a really good one so that I can get started as soon as possible.
Already thanks in advance
Friendly greetings,
Kristof Geerts