I need to mod this thing...

[tomy983]

Hi all!

A friend ask me help to mod his mains 8000VA ac voltage regulator.

The unit is quite simple, one large autotransformer, 4 relays to select the output, one relay as main switch.
Also a dual display indicating input and output voltage, a ledbar indicating power output, a switch to select the delay time on power on, a couple of contactors and some caps on the outputs.
Link of the unit(similar):http://www.yiyen.com/portfolio/avr%E2%85%A2-rce-series-ac-voltage-stabilizer-avr/

There is a small board controlling the unit. The mController is a Freescale MC9S08.
On the board there are 2 7812 regulators in parallel after rectification to power the relays coils, and a 7805 for the logic.

Now, it all works and is also within specs.

The problem is the following: At my friends home, the mains are almost always outputting more voltage than it should be (very rare the voltage goes below 230V). The regulator works up to 260V, so it gets the job done.

When the mains voltage is actually right within the 230 - 240 range, which is fine, the unit senses few extra volts, and switches the coils to output more voltage. Its display would read 240V, but in reality the output of the unit is 245V or more.

So, the actual problem, I think, is in the voltage sense circuit. Both the output and the input readings are lower than they should be.

It took me a wile to do this, also is my first time, but I reverse engineered the relevant part of the board. There might be errors, but I checked many times...

I am asking you for help with 2 things.

The first is help understanding how the voltage sense circuit works.

The main thing I need help for is to mod the unit to read the correct voltages. Ideally, what I would like to do is add two trimpot to calibrate the readings of input and output voltages.

I attach a couple of pics of the board, a pic of the schematics and the KiCad schematic file I drawn (schematics might contain errors...)

[tomy983]

I took some more measures.

The output to the micro looks like it is around 1.8 / 2 volts (cannot be more accurate...) when input voltage is 232Vac

The connection marked as "to other components" in the schematics is actually coming from a winding. My dmm measures around 9.4Vrms.

I started to play with LTspice and I drawn the schematics there.

It looks like changing the non regulated voltage input changes to some extend both the pulse width and amplitude.

Here is the ugly made LT spice schematics, if someone wants to give it a look.
Again, I have no means of measure the smd caps values, so I just guessed them looking at the LTspice output waveform.

please guys, give me any idea on how to make the voltage sensing circuit trimmable, I really don't know how this things works...

[tomy983]

Well, I figured it out...
Trimming the value of R52 (KiCad schematics) I could modify the sense value of the input voltage. Luckily, the thing is stupid enough not use feedback from the output voltage to select the proper winding.