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21st January 2016, 07:04 #1
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AC power meter dsign with power factor information
hi
I want to design a energy meter using 8 bit Pic microcontroller.
I want to know the following
1)How voltage is measured.
2)How current is measured.
3)How Power factor can be measured.

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21st January 2016, 08:03 #2
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Re: AC power meter dsign with power factor information
Hi,
There are dedicated "energy meter chips". Do you want to use one of them?
> if yes, then read their datasheet and application notes.
If you want to use the PIC inside ADC:
* there are a lot of examples and hobby solutions in the internet. Read through them.
* in general you need a fixed and stable sample rate to read both voltage and current continously.
* then take the samples of one (or integer multiples) full wave (line frequency)
* calculate the RMS voltage
* calculate the RMS current
* multiply both to get apparent power
* Calculate active power
* calculate: active_power / apparent_power = power_factor
There are other(less reliable) solutions by just using zero cross timings
Klaus
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21st January 2016, 08:34 #3
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Re: AC power meter dsign with power factor information
Thanks for reply.
kindly explain this
"then take the samples of one (or integer multiples) full wave (line frequency)"
What should be the adc sampling frequency?
What about negative cycle (rectifier required?) or generate rms by using diode opamp circuit?

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21st January 2016, 12:04 #4
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Re: AC power meter dsign with power factor information
Hi,
"then take the samples of one (or integer multiples) full wave (line frequency)"
What should be the adc sampling frequency?
You can choose 3, 4, 5, 6, 7, .. time the line frequency)
I recommend to use a sample frequency of (about) 32 x lines frequency. Use an appropriate analog low pass filter to avoide alias frequencies.
With 50Hz line frequency it means 1600 Hz sampling frequency (for each channel) .
What about negative cycle (rectifier required?) or generate rms by using diode opamp circuit?
If you are not interested in DC, then no OPAMP is needed. some Rs and Cs do the job.
Klaus
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21st January 2016, 12:19 #5
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Re: AC power meter dsign with power factor information
Why alias filter is required? Are there high frequency harmonics in Main?
I need reference design for filter + ads side.

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21st January 2016, 12:29 #6
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Re: AC power meter dsign with power factor information
Hi,
Why alias filter is required? Are there high frequency harmonics in Main?
Consider all the small power supplies without PFC.
The input current waveform is far away from sine.
I need reference design for filter + ads side.
A simple LPF is just an R and a C.
Klaus
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21st January 2016, 12:38 #7
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Re: AC power meter dsign with power factor information
Ok.
Sampling the mains at 1600 Hz and then apply RMS formula to get the value?
Are you talking about ADE7751 type chips?

21st January 2016, 13:22 #8
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Re: AC power meter dsign with power factor information
Hi Klauss,
There are some regulatory specifications for measuring mains voltage that define the sampling frequency as being not exactly the multiple of the wave period. The reason for that is due to the risk of happening synchronized noises occurring between samples, and therefore would be not detected, dont you agree ?
An example of that was illustrated here:
https://www.edaboard.com/thread207641.html#post885041
Doing that, at least part of the artifact could be accounted by integrating the average over time.
Part of the world that you live in, You are the part that you're giving ( Renaissance )

21st January 2016, 13:41 #9
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Re: AC power meter dsign with power factor information
Hi Andre,
yes, sampling a 20ms fullwave with a sample rate of 19ms is undersamling.
While it has itīs benefits, it also has itīs drawback.
I didnīt mention it because it makes things (sampling theory) more complicated.
I did this with an 8 channels (lines frequency) true RMS voltmeter.
Continous sampling (If I remember right then it was 48 samples in 100ms, this makes it usefull for 50Hz as well as 60Hz)
a total sample rate (480Hz x 8 channels = 3840Samples/s) and an output every 100ms for all 8 channels, all within a ATMEGA32.
Very precise and stable readings.
Klaus

21st January 2016, 13:47 #10
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Re: AC power meter dsign with power factor information
How accurate you are going to aim? There are short time frequency and phase errors on the line frequency. Also you need a switch to select the mains frequency, 50/60 Hz. You should also ensure that the PIC clock is TXCO giving a good stability. If you just want an (approximate) idea of the power consumed, then you can assume the line voltage is a sine wave and the load current is also a sine wave. Then you will get the power consumed at the fundamental frequency. Measuring phase accurately with a 8 bit accuracy is not easy because of the conversions involved (sine and cosine functions). If you want to include harmonics in the overall calculation, you will need to average over 2345 cycles and also sample over 1/2, 1/3, 1/4, 1/5 the line period (20 ms for 50Hz). As KlausST has pointed out, you also need to take care of the Nyquist theorem.
It will be a fun project to do.
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21st January 2016, 19:25 #11
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Re: AC power meter dsign with power factor information
Perhaps this DIY project can be of some help: https://www.edaboard.com/thread343354.html
Kind Regards,
DanyR

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28th January 2016, 06:54 #12
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Re: AC power meter dsign with power factor information
hi,
I have just implemented this circuit for current sensing. For this circuit i am using MCP6022 rail to rail op amp with large rc constant out put filter as described in the document.
Current transformer rating is 1:1000 load and resistor is 130 ohm.Here are some readings
(Load) (Clamp meter current readings ) (Voltage at opamp output)
(Watts) (A)
200 0.72 220mv
400 1.57 400mv
600 2.38 580mv
800 3.23 760mv
1000 4.07 940mv
1200 4.89 1.14v
Now i am looking for voltage sensing circuit. Do i need zero cross detect circuit?
   Updated   
In a current transformer with bridge rectifier , how voltage drop 0f 1.4 v is compensated?
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