Light Dimming Using Ardiuno with Inverters

Since there’s not a lot of information about it out there in one place, I thought I’d share some insight into a problem I’ve been grappling with – dimming 110V LED lights run off an inverter using Arduino-driven triacs.

Recently, I’ve been experimenting with dimming strings of LED lights using an Arduino and similar devices. My goal is to kick my Light Suit up a notch by going from simple on-off flashing patterns to dimming intensities, ultimately driven by sound spectrum intensities – so that the strings throb in sync with music sources.

There is a trick to dimming 110V AC strings – since you’re typically using triacs to control hihg-voltage AC power, you have to worry about “Zero Crossing”. The problem is that, once you turn a triac on, it stays on until the voltage falls to the “zero crossing” point – the point at which the voltage is at (actually near) zero, even if you turn off the controlling voltage. So, to dim 110V strings, you actually have to do some timing – where you wait a small amount of time into the phase of the AC sine wave before turning the triac on. That way, you only voltage during a portion of the since wave – reducing the overall power. There’s a good article on the technique here: http://playground.arduino.cc/Main/ACPhaseControl. That is basically the setup I’m using.

A problem I’ve been encountering though, is that I’m running these strings off of a 12V-powered inverter. That was the easy way to take commercial string of LED lights attached to the suit and still be mobile. The problem I’m running into is that, at least in the case of small, inexpensive inverters, they use a modified sine wave, not a true sine wave. Google it for more details, but this is basically a notched square wave which emulates a sine wave in terms of total power delivered. But since the beginning and end of each phase of a modified square wave are at zero voltage, if you try to turn the triac ON at the beginning or end of the phase, it doesn’t happen, since the voltage is at zero.

Image1

You have to turn in on at some point during the “notch”, which is a narrower window than you have with a full sine wave.

Image2

What was happening for me was that, at low or high dim values, the lights would start flickering erratically or not coming on at all. Limiting the triac firing to the window of the actually square “modified sine” wave helps tremendously. It’s also worth noting that my two inverters behave differently – one flickers quite a bit even when I limit the firing window. I’m guessing it must be a “dirty” square wave or something, but can’t really be sure without a oscilloscope (this is one of those occasions where I really consider plunking down some $$ for one).

Also, I’m not sure why, but I still get some flickering when I drive the triacs from an Arduino Uno. When I use a chipKIT uC32 (an 80Mhz pic-32 “Arduino clone”), it works smoothly. I have know idea what’s going on here, but I’m kind of guessing that the speed with which the Arduino responds to a hardware interrupt can very a bit based on how busy the processor is or something.

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