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Forum Index : Electronics : Various aspects of home brew inverters
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Revlac Guru  Joined: 31/12/2016 Location: AustraliaPosts: 1214 |
It would e easy to keep the output stepdown xformer.  Cheers Aaron Off The Grid |
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poida Guru Joined: 02/02/2017 Location: AustraliaPosts: 1447 |
this note is examining the Vfb circuit that TinyT designed, that makes up part of the nanoverter he created for us. When I first looked at it, it seemed a bit too much for the job.  But it is all needed, to get the job done well and with protection which is: - take 12V AC and rectify it, via a full bridge diode layout - smooth the result via a 4.7uF cap - limit high voltage via a 5.1V zener - give the user facility to adjust output voltage via a trimpot - again smooth output, via a RC low pass filter - limit low voltages to about -0.2V using a low forward voltage drop schottky diode - feed this into a second order low pass active filter - but first buffer it's input with a simple voltage follower - the output is now cleaned of all nasty high frequency stuff that would upset the closed loop control loop that manages the inverter output. All this was easy to see back when I first saw it. But I was always wanting to see how that active filter worked... The filter is a Sallen-Key second order low pass filter. It uses 2 caps and 2 resistors to define the filter. In this case, both caps are 1uF and both resistors are 15K 3db cutoff is 1/(2xPIxCxR) or 10.6 Hz The second order filter is nice, in that it attenuates output frequencies 10x for each doubling of frequency. or it's 20db per octave. same thing. Once input noise that gets to 1kH the attenuation is very strong indeed. It's perfect to get rid of the 20kH FET switching, and it's higher harmonics. I made a simulation of the Vfb circuit. It runs in a free simulator you can download. the simulator here is the file you need to open from with the simulator app. vfb sim.zip unzip the app folder, run the program "circuitjs1.exe" I have a simple input, 50Hz, and it's voltage can be abruptly changed via a switch. (just click on the switch) The traces at the bottom show input into the active filter and it's output.  Red is the filter input, Green is it's output. Technical analysis (way beyond my brain..) of the filter has shown it's transient response to be about 0.1 second and we can see the output get very close to the input voltage after about 0.1 seconds. This speed of responding to input is fine. It permits me to design a closed loop control that will not go haywire and blow it's self to bits because it will always have smoothly changing inputs thanks to the filter. But it still has fast enough response too. It has taken me a lot of words to say the thing I wanted to say in the first place, which is, TinyT you did a fantastic job designing the nanoverter and I remain grateful for your time and effort you put into this project. wronger than a phone book full of wrong phone numbers |
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