# flashingLEDs

Posted on Feb 6, 2014

A few people have requested some clarification about calculating resistor values for the triac circuit. If I`m doing this correctly it might prove helpful in adapting the Sous Vader for custom builds. If I`m wrong, hopefully somebody will point out my mistake(s). Win win, right The circuit in question is reproduced below, and we`re going to talk

about the size of R1 and R2 for this particular optoisolator+triac+mains configuration. It should be straightforward to adapt this discussion to 120V mains or to a different set of components. When I first put the page up, I incorrectly listed R1 as 10kΩ. I`ve actually been using 1k ©, which certainly works but as we`re about to see could be improved. The purpose of R1 is to limit the current going into the LED side of the optocoupler. To absolutely guarantee you turn it on, the LED current should be greater than the maximum value of "IFT" as quoted in the datasheet. For the MOC3023, this is 5mA. Given a typical voltage drop across the LED of 1. 2V, and assuming you`re using a 5V control line, R1 should ideally be: We should choose R1 to be no larger than this, but at the same time it`s not good to go too small - your microcontroller might not like sourcing tens of mA, and it will decrease the life of the LED. In future builds I`ll be using 680Ω. There was no error on the other side, I`ve definitely been using 680Ω. This resistor serves two purposes. It limits the current through the `switch` side of the optocoupler to a safe value, while at the same time allowing enough current into the triac gate to switch it on. From the Fairchild datasheet, the current limit for the MOC3023 ("ITSM") is 1A. The peak current I can expect from 240V mains through a 680ohm resistor is (240V*1. 414)/680 © = 500mA, so we`re safe....

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