Some news about my christmas lights

So today I was able to ask to a couple of friends who knows electronic way better than I do, and they confirmed what I was already thinking. The IN4007 are actually 1N4007 diodes, simple 1000V-rated diodes, and the way they are used is just a standard Graetz bridge (a rectifier bridge).

For those who want to see what I’m talking about, I uploaded the photos on Flickr, see the set here .

After passing through the diodes, the 220V AC wave is replaced by a (very bad) approximation of (about) a 220V DC supply. I wonder if it was the unstable supply (there is no capacitor to smooth the wave) that blew up the SCR.

The 180KΩ resistor is part of the CR/RC net (not sure what it is yet, I haven’t dug that up, as I don’t much care), together with the capacitor; they are used as a timered trigger for one port of the actual controller, so I’m counting them off, as I need to replace the thing anyway.

The 240KΩ resistor is used to reduce the voltage that powers the controller, not knowing the current absorbed by the controller, it’s difficult for me to guess which voltage was given to the controller. I could probably just plug the thing in and take a look with a voltmetre. It’s anyway superfluous, as the controller is, once again, something I want to get rid of.

Now what I know is that the lights are powered by DC, there’s no resistor, so the direct output of the bridge is used, they probably don’t need the supply to be stable at all.

So now I know something more about the thing I’m working on, which is an interesting thing. Now I just need two ideas to complete the basic design of the thing, so that I can move to a proper design with microcontroller and code.

The first idea is how to reduce 220V DC to something usable for the microcontroller supply; for smaller voltages I usually used LM7805, but the LM78 series does not support voltages higher than 40V; a quick search around suggested that the LM78S series could actually support 220V voltage, but I can’t find any datasheet of that series, nor I can find the ICs anywhere around.

The second idea is how to dim the lights; what I was thinking of was using a digital potentiometer, I know I seen some of those around, but I’m not sure if they work well with a 220V passing voltage, and how easy are to actually program.

I’m also wondering if it’s easier to get an IC for the diode bridge or just using the four diodes as the current PCB does (adding a capacitor to smooth the output); I seen Texas Instruments have DIP packages with two or four Schottky diodes, but those are rated only up to 50V, I need at least 240V.

Any suggestion is welcome 🙂