problem in AC Voltage sensing through resistor divider

[nishu's]

Hi All

I am building an ac voltage sensing circuit for sensing two different ac voltages w.r.t. same neutral line. Please refer to circuit attached, when i turn switch s2 to on position
and s3 to off , assuming AC_IN port is connected to 260 V RMS and AC_OUT is connected to 220 V RMS, I get 2.5 V across C5 and 1.2V across C6. According to what i knew (which obviously is wrong, somehow) voltage across c5 is ok but voltage across c6 would have been 0V. Similar is the case when i turn s3 to on and s2 to off position, i figured out that voltage across caps of off terminal is approx half of the 'on' terminals voltage....

any leads would be helpful....


Thanks
Abhi

 

[pinout]

Interesting observation and question.
First we need to determine the voltage on the Neutral pin wrt to your ground.

I think it assumed that Neutral is at ground potential but this can vary enormously according to the distribution configuration, local earthing and soil impedance for the ground return impedance.

If there is a voltage present on the neutral wrt Gnd, then I suggest you may need to isolate the voltages by input transformers which will have the added advantage in that you can use the transformer to reduce the volts to be measured.

Or you will have to change the circuit to measure wrt to Neutral,(not ground) and use an isolation technique at the uC input.

I would be tempted to investigate the transformer option for safety and isolation.

 

[nishu's]

@pinout

neutral and ground are not at same potential (no physical connection), potential difference between them is approx 110V dc for AC_IN voltage 260 V rms and AC_OUT voltage of 0 V rms. Isolation transformer is not an option as my design cannot afford it (price and form factor).

 

[pinout]

What is happening then, is even with S3 open the top right diode in D10 is rectifying (half wave) the Neutral voltage with respect to ground, hence your output voltage on C6.

I'll try to think of alternative circuits, but the issue is you need to measure the AC lines wrt to Neutral, but your circuit is referencing them to Gnd.

 

[nishu's]

What is happening then, is even with S3 open the top right diode in D10 is rectifying (half wave) the Neutral voltage with respect to ground, hence your output voltage on C6.

I'll try to think of alternative circuits, but the issue is you need to measure the AC lines wrt to Neutral, but your circuit is referencing them to Gnd.

it makes sense now the off terminal is conducting for half cycle using returning diodes of on terminal thats why we have half the voltage of on terminal at sense points, what would you suggest as a remedy of this...

 

[betwixt]

I would have to spend time working out the various current paths but my immediate thought is that you might be able to measure the neutral to ground voltage as well and subtract it from the other measurments. At the moment you have diodes linking neutral and ground and they clamp one half cycle to Vf of the diode but the other half cycle may be measurable and assuming the AC is a sine wave, would be proportional to the N-E difference.

If your 'GND' really is ground you ave a far more serious problems though as you have tied it to within Vf of the neutral wire and that is potentially very dangerous. If you have done that, I suggest rearranging the components so the resistors are in the AC measurement points (before the switch for safety reasons). In other words, drop the AC before rectifying it rather than dropping the rectified output. This will also let you use smaller diodes, possibly Schottky types which will allow lower voltages to be measured. You should also add a Zener diode across the capacitors to clip any over-voltage spikes that may come from the AC wires.

Brian.

 

[Venkadesh_M]

No doubt your both phases coming into the mc ground........ Try a half wave rectifier or,

You can use two different small transformers for Ac in and ac out.

 

[nishu's]

@betwixt


yes AC is sinewave, PCB GND is not tied to actual ground line. only connections on PCB is of Phase In, Phase Out and Neutral, considering components whatever is in schematic are the only parts mounted till now even controller is not on the board (so other parts interference is ruled out).

to all:
meanwhile i changed the circuit a bit, replaced first 330k res (in contact with bridge) from both AC_IN and AC_OUT section with a diode which will allow current flow towards ground but not in opposite direction. but still no change except the voltage levels at sensing point are raised.

 

[betwixt]

I think I would be inclined to link neutral and GND then use a simple half wave rectifier and resistive divider from each AC signal.

Are you actually measuring the AC or just sensing it's presence?

Brian.

 

[pinout]

From your additional information, I would suggest trying the circuit with the grounds in the top circuit and bottom circuits isolated from each other, i.e. straight from the bridge rectifier to their respective RC connection point.

So for now just keep them separated and measure your outputs with an isolated scope or meter.
If this gives the voltages you expect, it shifts the problem to how do you interface these two 'floating' measurements to the uC.

 

[nishu's]

@betwixt

was measuring AC, earlier I did used half wave based circuit and was working fine only issue was sensing voltage not 1:1 related to AC (47uF/450V capacitor at diode's output would have done the same but since i was redesigning the pcb and form form was critical so i jumped to full bridge).

to All: I simulated the design in multisim and it had same result as my circuit, Now if isolate both bridges negative end (which were earlier attached to ground) and measure voltage between brides output results are as expected. so yes leakage is via ground port as suspected. so for now i think only option would be to go with single diode (half wave) unless you guys suggest something better.

 

[Venkadesh_M]

Why do you want to go from half to full rectifier??

 

[pinout]

If you can accept "Neutral" with the voltages you state are present on it as the Gnd reference for your (future) uC, then yes halfwave diodes and Neutral as the common reference.

I would prefer some isolation, so could you put capacitors in series with the 'new' common neutral reference line and between the (new) diode and first resistor?
You could model the effect of these on the circuit measurement and estimate the risk to the user at the uC if a fault occurs.

Also don't forget lightning! Perhaps a MOV across each supply input?

 

[nishu's]

half will reduce 50% of the wave and requires a bigger capacitor just after the diode rated for 200V AC, and full will require small capacitor for same result except this capacitor would be placed across sensing resistor.

 

[Venkadesh_M]

I got an idea but don know It was efficient or not.........
put the NC contact of the top push button to open the bottom rectifiers output and vice versa...

I don know the availability of double contact push button...

 

[FvM]

If ground to neutral isolation is required and a measurement transformer isn't an option, symmetrical voltage dividers and differential amplifiers are probably the next best variant, as used in oscilloscope high voltage differential probes.