Fuzzy Urn

Posted on Feb 4, 2014

Accurate control of water temperature, this got me thinking and I figured it was a good opportunity to have a play. To control the temperature I decided to try and put some of the theory I was taught at uni into practice. I had it in my head even though it is probably unnecessarily complicated to

Fuzzy Urn
Click here to download the full size of the above Circuit.

use a variable AC chopper to vary the power and Fuzzy Logic to manage the control. To vary the power I decided to use a variable AC chopper, basically a light dimmer I could control with a micro controller. It works by chopping up the ac signal like you can see in the image below, where alpha a is the triggering angle. As alpha increases the signal is off for longer therefore the voltage is lower, it seems simple really. The equation below calculates the chopped RMS voltage where Vp is 240V in Australia and alpha ranges from 0 to pi radians. To trigger the load I have decided to use a TRIAC, there are probably other ways of doing this but power electronics was not one of my best subjects so I just picked something I recognised the name of. We were taught about these clever sounding semiconductors at uni, what they seemed to forget was how you actually use one. My first attempt left me quite frustrated when I tried to trigger the TRIAC directly from my microcontroller and somehow all I managed was to trip the safety switch. After poking around on the Internet turns out you need to drive the TRIAC using some sort of driver such as an optocoupler. After digging through data sheets I ended up with the circuit below, I trigger the optocoupler using an NPN transistor, even though my micro controller could probably have supplied sufficient current it`s nice to have some isolation. The chip I picked also has a maximum input voltage...

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