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Problem reading multiple mains voltages with microcontroller.

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microherb:
Hi,
I'm having some difficulties with a circuit to measure more than one mains voltage inputs.
Attached is a simplified circuit diagram of what I am trying to achieve.

There is a fly-back switch mode power supply to generate the 5V for the micro-controller and power other parts of the circuit (not shown).
The incoming mains voltage is divided down with resistive divider and passed through RC filter, buffered and input to ADC of micro-controller to read an average AC mains voltage, this section of the circuitry works fine.
The lower part of the circuit is to read a variable AC mains control voltage.  The voltage is divided down and passed through RC filter, buffered and input to micro-controller as above.  There is something going on that I can't get my head around just yet which I believe is related to the single ground point for the 2 rectified inputs.  When observing output of the second bridge rectifier only one half cycle is varying as the the variac is turned, the other half cycle is always maximum (see scope screen shot attached).

Any advice on how to correct problem with current circuit or alternative circuit much appreciated.

Dean.

Marco:
A bridge rectifier is not a galvanic isolation, you can't just connect circuit ground to the two neg outputs like that.

Use halfwave rectification instead.

microherb:
Thanks Marco,
I realise there is no galvanic isolation through the bridge rectifier, I tried just half wave rectified on the 2nd read channel with similar result.  I need full wave rectified from the main 240VAC for the fly-back power supply.
I thought there would be a simple way to do this, I don't want to use transformer due to size and cost, similar for isolation amplifier.  I'm tossing around ideas using differential amplifier or maybe current mirror.

Chriss:
Hi!
From several my projects where I dealed with mains I learned a really good stuff.
Always separate galvanic the two circuits.
I had so many problems before I decided to put a transformer,the worst case scenario was when I missed and used a ground where I shouldn't have.
The magic smoke will be your best friend trust me.

However, I would separate the two part completely from each other.
1. The power source for the other parts
2. The measurement lines
3. Separate galvanic the measurement lines

How much current will use the other- not mentioned part?

Maybe you can put a small size and low power transformer for the measuring line and to power the uC stuff.

For the load you can use an smps in your design.

Sent from my SM-J500F using Tapatalk

ali_asadzadeh:
There is no need for the bridge rectifier to make the signal DC! :) you can add some DC bias to the scaled signal to scale it to the 0-3.3V ADC range of the micro.

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