INTRODUCTIONMy first semester at Cornell University ended late December (2013). I went back home to Dhaka,Bangladesh for my winter break. During this period of time (late December to mid January), there was a lot of political turmoil in the country due to which I could not leave the house a lot to spend time withfriends and family. So I ended up spending a lot of time in the house with electronics specifically on two things: making some small projects with the PIC32MX250F128B (Microchip PIC32 series), and,making an automatic voltage stabilizer circuit. Ill talk about the automatic voltage stabilizer here. First Ill give a short introduction as to the motivation behind me working on it before I go on to talk about the operating mechanism of the voltage stabilizer and then the circuit diagram and source file. At the end of the article, youll find the links to download all the files. Also do check out the Youtubevideos where I demonstrate the voltage stabilizer and its operating mechanism.

MOTIVATIONMy dad knows a man named Kamruzzaman who worked under my dad (in electronics) for a very shortamount of time, doing stuff like soldering boards, etc. A few days after I went back to Dhaka, Kamruzzaman called my dad and mentioned that he wanted to talk to my dad about something. We invited him home, where he showed us a nice Chinese-made automatic voltage stabilizer circuit he was trying to replicate albeit unsuccessfully. At the same time, he mentioned about his financial hardship and asked for our help with designing the automatic voltage stabilizer so that he could have some goodfinancial support from this product. In Bangladesh, the automatic voltage stabilizer (AC-AC) is a ubiquitous little piece of hardware that is used to somewhat compensate for the varying line voltages (which while being advertised as 220V, canon a given day vary between 170V and 240V in Dhaka and can vary over a larger range in other parts ofthe country, due to the unreliable electrical grid).This was a good learning opportunity, a great opportunity to gain some experience and mostimportantly, a great way to help someone in need through doing something I truly love.3And I got to it. I spent about somewhere between a day and a half, and two days thinking about howbest to go about designing this voltage stabilizer circuit, while maximizing performance and minimizingbuild hassle. Then I built the test prototype on verroboard and tested it out. Kamruzzaman and I testedthe entire product through a long eight-hour testing process where I kept on refining and improving thecircuit until I achieved what I wanted a blend of the right amount of performance and a minimalamount of build effort/hassle. After that, I designed the PCB for the board; it was a long night designingthe PCB fuelled by coffee (=P), I started at around 12 AM and finished at around 9.30AM after whichKamruzzaman got the PCB made (that very day) and we performed the final testing of the product thatnight. The circuit worked as expected and the project was complete.

SPECIFICATIONS

Now, lets go on to talk about the technical part of the project. For this automatic voltage stabilizer, theparameters were decided initially: Output voltage must lie between 200V and 240V for all input voltages above 150V and upto260V. Input voltage range must be 150V to 260V, preferably wider. Output frequency and waveform should be unchanged from the input frequency and waveform. The voltage stabilizer must be inexpensive. There should be no variable resistors in the final finished product. This was somethingrecommended by Kamruzzaman, as he said that sometimes, some of the variable resistors heuses tend to drift in resistance slightly and this causes the circuit to become less reliable overtime. Although this seemed quite challenging (due to resistor tolerances in the voltage sensesection, tolerances in the diode forward voltages in the AC-DC rectification section, etc), I quiteliked the idea.

Based on the above initial design decisions, the final parameters/specifications are as follows: Input voltage: 125V/135V (Ill explain this later) to 270V Output voltage: >=200V and