simple solution measuring mV

[Efried]

For WC8 this is easy to do. For WC32 it is harder, because wc32 uses resistor networks, and its traces are tiny small.

 Ok, so this path is locked. Also the software path, because of the interpreter. So we need a WC shield ;-)
http://openenergymonitor.org/emon/emontxshield/smt
I assume that power monitoring is a business so I'm looking forward to some solutions for it.
20 A  delivering 1V seems a fairly good entry point if we don't need higher accuracy and somebody manages to make the use of the LTC1966  RMS to DC converter possible or alternatives using a TRMS step-up DC converter for this beast:
http://cgi.ebay.at/ws/eBayISAPI.dll?ViewItem&item=390831755305

 

[rossw]

 Ok, so this path is locked. Also the software path, because of the interpreter. So we need a WC shield ;-)
http://openenergymonitor.org/emon/emontxshield/smt
I assume that power monitoring is a business so I'm looking forward to some solutions for it.
20 A  delivering 1V seems a fairly good entry point if we don't need higher accuracy and somebody manages to make the use of the LTC1966  RMS to DC converter possible or alternatives using a TRMS step-up DC converter for this beast:
http://cgi.ebay.at/ws/eBayISAPI.dll?ViewItem&item=390831755305

 
It seems to me like trying to force a square peg into a round hole.
There are plenty of devices designed specifically for this purpose... measuring power in single and 3-phase circuits.
They are designed to do the whole job, and do it properly. The WC8 and WC32 devices are simply not the "right" animals for this job.
Even with a precision RMS to DC converter, it doesn't give you anything about the phase relationship (powerfactor). It doesn't have the resolution to measure small current or voltage changes over a wide enough range to be really useful.
There are single chip devices with amplifiers, filters and 22-bit ADCs, and enough processor power to calculate true power, powerfactor etc, in real time, across multiple phases, that can run directly from the mains with minimal input and power conditioning.
 
Yes, the WC makes a convenient board provided you don't need all the extra resolution and accuracy, but please don't try to force round pegs into square holes and complain when they don't fit.

 

[CAI_Support]

Efried,
 
Maybe you can modify a Kill-a-watt meter then interface voltage and current to WebControl ADC input, similar to this project:
 

http://www.ladyada.net/make/tweetawatt/solder.html

 
Xbee module has 10bit ADC only, which is same as WC8

 

[rossw]

max71020 - for around $3 for the single-phase version, does all the hard work for you.
Has SPI interface, so may be possible to read all the details you want directly into WC8/WC32.

 

[LarrylLix]

It seems to me like trying to force a square peg into a round hole.

There are plenty of devices designed specifically for this purpose... measuring power in single and 3-phase circuits.

They are designed to do the whole job, and do it properly. The WC8 and WC32 devices are simply not the "right" animals for this job.

Even with a precision RMS to DC converter, it doesn't give you anything about the phase relationship (powerfactor). It doesn't have the resolution to measure small current or voltage changes over a wide enough range to be really useful.

There are single chip devices with amplifiers, filters and 22-bit ADCs, and enough processor power to calculate true power, powerfactor etc, in real time, across multiple phases, that can run directly from the mains with minimal input and power conditioning.


Rossw is right. I spent 34 years in legal metrology fields and this is not the way to go for power measurement at all.
You will need power factor, phase anglesnd possibly distortion factors which you aren't going to get like this.
 
In the early days there were people that used a box full of transducers hooked into a  multichannel A/D and could do a phasor analysis of a 3 phase metering installation but it was clumbsy and cumbersome,
 
Later techniques developed that used multiple channel A/D converters to analyse at least 6 inputs (3 currents and 3 voltages) in real time. Before you get any ideas there think of this.
- You will need to sample the AC quantities at least 16 samples per cycle.
- You have six (or preferably more) channels of sampling to do
- At 60 Hz x 16 samples per cycle x 6 channels
   we have 5760 samples per second to do. That is 1736 usec to do each  sample and save it in a database.
- The samples cannot be staggered for sampling with a scanning A/D multiplexor. They need sample and hold circuitry so they are all simultaneous quantities.
 
Provided you can get those input requirements met here are some things you can do.
 
For power (W) multiply each voltage (V) sample with it's corresponding current (A) and average the totals for one or more complete cycles.
 
For RVA shift one of the quantities (V or A) 90 degrees and repeat above for power.
 
For VA take the Root( V^2 + A^2)
 
For PF take W / VA (edit corrected)
 
For three phase VA it can get more complicated than that. You need to know about arithmetic VA vs, phasor VA quantities. Phasor VA will be the most common and  easiest to calculate
 
For three phase measurements much more phasor relationship understanding is necessary for the system to connected properly and measured properly. For three wire systems phase relationships have to be measured for system unbalance in phasing and some assumptions made.
 
Distortion and harmonics can be calculated with Fourier transforms from the raw samples.

 

[Efried]

It seems to me like trying to force a square peg into a round hole.

There are plenty of devices designed specifically for this purpose... measuring power in single and 3-phase circuits.

They are designed to do the whole job, and do it properly. The WC8 and WC32 devices are simply not the "right" animals for this job.

Even with a precision RMS to DC converter, it doesn't give you anything about the phase relationship (powerfactor). It doesn't have the resolution to measure small current or voltage changes over a wide enough range to be really useful.

There are single chip devices with amplifiers, filters and 22-bit ADCs, and enough processor power to calculate true power, powerfactor etc, in real time, across multiple phases, that can run directly from the mains with minimal input and power conditioning.


Rossw is right. I spent 34 years in legal metrology fields and this is not the way to go for power measurement at all.
You will need power factor, phase anglesnd possibly distortion factors which you aren't going to get like this.
 
In the early days there were people that used a box full of transducers hooked into a  multichannel A/D and could do a phasor analysis of a 3 phase metering installation but it was clumbsy and cumbersome,
 
Later techniques developed that used multiple channel A/D converters to analyse at least 6 inputs (3 currents and 3 voltages) in real time. Before you get any ideas there think of this.
- You will need to sample the AC quantities at least 16 samples per cycle.
- You have six (or preferably more) channels of sampling to do
- At 60 Hz x 16 samples per cycle x 6 channels
   we have 5760 samples per second to do. That is 1736 usec to do each  sample and save it in a database.
- The samples cannot be staggered for sampling with a scanning A/D multiplexor. They need sample and hold circuitry so they are all simultaneous quantities.
 
Provided you can get those input requirements met here are some things you can do.
 
For power (W) multiply each voltage (V) sample with it's corresponding current (A) and average the totals for one or more complete cycles.
 
For RVA shift one of the quantities (V or A) 90 degrees and repeat above for power.
 
For VA take the Root( V^2 + A^2)
 
For PF take VA /  W
 
For three phase VA it can get more complicated than that. You need to know about arithmetic VA vs, phasor VA quantities. Phasor VA will be the most common and  easiest to calculate
 
For three phase measurements much more phasor relationship understanding is necessary for the system to connected properly and measured properly. For three wire systems phase relationships have to be measured for system unbalance in phasing and some assumptions made.
 
Distortion and harmonics can be calculated with Fourier transforms from the raw samples.

 
Thanks so much Larry for pointing out in great detail, this is indeed the tricky part. I guess RISC based 32 bit µC may do the job without a problem, even for 6 measurements having phase correction. The WC based solution has the advantage that the WC may act as data concentrator and controller in one piece. But of course interpreted code may be to slow, so using an integrated energy meter on a chip like the the MAX71020 (or others, as below) may be a must for metering. I even have an idea for non intrusive measuring connecting the voltage at the plug and using a current transformer. But the voltage signal should also be equal to all metered devices in the area and only 120° staggered  on all three phases, so may be a way for savings on the input side to multiple single chip electricity meters at one location.
 
Annex:
http://www.ti.com/lit/ml/slat132/slat132.pdf
http://www.analog.com/en/press-release/Oct_10_2006_ADIs_SingleChip_Energy_Meter/press.html
http://www.st.com/web/en/resource/technical/document/datasheet/CD00044094.pdf
http://ww1.microchip.com/downloads/en/DeviceDoc/51931a.pdf and http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en520376

 

[Efried]

Efried,
 
Maybe you can modify a Kill-a-watt meter then interface voltage and current to WebControl ADC input, similar to this project:
 

http://www.ladyada.net/make/tweetawatt/solder.html

 
Xbee module has 10bit ADC only, which is same as WC8

 
 
Ok thats a good solution, for Europeans who may live with server side data aggregation avoiding soldering may be this one is more suitable:
http://www.overmetering.com/watto/?view=featured

 

[rossw]

But the voltage signal should also be equal to all metered devices in the area and only 120° staggered  on all three phases, so may be a way for savings on the input side to multiple single chip electricity meters at one location.

 
Please be very careful making such dangerously WRONG assumptions.
You CANNOT and MUST NOT ASSUME that the voltage on one phase is representative of the voltage on the other phases.

 

[LarrylLix]

Efried, on 15 Sept 2014 - 03:29, said:

But the voltage signal should also be equal to all metered devices in the area and only 120° staggered on all three phases, so may be a way for savings on the input side to multiple single chip electricity meters at one location.



Please be very careful making such dangerously WRONG assumptions.

You CANNOT and MUST NOT ASSUME that the voltage on one phase is representative of the voltage on the other phases.


 
...or assume the voltages are 120 degrees apart but the phase relationship assumption will usually get you within about 2-3% accuracy. With 3 wire systems analysis, the voltages associated (delta system) with the same phase line currents are not available from measurements and this is always the case (2-3%) without some fancy trig calculations to emulate them.

 

[CAI_Support]

max71020 is a single phase monitoring chip.  If you have 3 phase, the relationship between phases will not be perfect.
In the US, we have two phase power to most home and office, 3 phase to most factories.
Tracking single phase is a lot easier than tracking two or three phase. 
 
Larry and Ross must have a lot of experience on electrical power meters.
Most US home and business now have digital power meter.  Inside it must have really fast CPU to figure out if power flow is from or to grid.
It is easier to measure power through the wire, but it is hard to detect AC power flow from grid or to grid.

 

[Efried]

Efried, on 15 Sept 2014 - 03:29, said:

But the voltage signal should also be equal to all metered devices in the area and only 120° staggered  on all three phases, so may be a way for savings on the input side to multiple single chip electricity meters at one location.

 

Please be very careful making such dangerously WRONG assumptions.

You CANNOT and MUST NOT ASSUME that the voltage on one phase is representative of the voltage on the other phases.

 
...or assume the voltages are 120 degrees apart but the phase relationship assumption will usually get you within about 2-3% accuracy. With 3 wire systems analysis, the voltages associated (delta system) with the same phase line currents are not available from measurements and this is always the case (2-3%) without some fancy trig calculations to emulate them.

Ok, I don't want to build a calibrated meter but only create insight into domestic power demand over time and use that for incrasing own PV consumption in case. Plug meters cost 11 EUR, so I hope we may have 30 EUR for the translating system plus the WC to do the job...

 

[Tschmidt]

Ok, I don't want to build a calibrated meter but only create insight into domestic power demand over time and use that for incrasing own PV consumption in case. ..

Not to take business away from our friends at CAI but have you looked at the TED Energy detective. They have ones with multiple sets of sensors so you are able to monitor utility power and PV. Comes with spiffy software. I assume you are in Europe. These are designed for US 120/240v power system, not sure if there have a version available for European 240V 50Hz.
 
http://www.theenergydetective.com/

 

[CAI_Support]

TED devices are cheap, in the US, most smart meter allows directly read back through Zigbee, if your TED device is registered with your local power company.
 
However, most TED devices do not display power generation. If you have PV panels generate power with grid tie inverter and try to figure out how much power generated vs how much power used, it probably does not tell you that much details.

 

[LarrylLix]

Ok, I don't want to build a calibrated meter but only create insight into domestic power demand over time and use that for incrasing own PV consumption in case. Plug meters cost 11 EUR, so I hope we may have 30 EUR for the translating system plus the WC to do the job...

If you want a more crude setup to monitor your power consumption there are devices in N.America that stick onto the front of a utility digital energy meter and transmit the pulses to a remote box. They sense the iR pulses from the meter's optical ports. Most of N.America is going through a "smart meter" fad but I am not sure if this is happening elsewhere to give you access to these "smart meters" in the homes in Europe.
 
There are also wireless pickups that listen to the RF transmissions from these wireless meters, using zigBee systems mostly. Many have used Brultech systems for their home automation systems. IIRC they come with clamp-on current devices and make it quite easy. Not sure about output features.
 
I am not sure of the details of connections or prices but it may be a start. Building your own current transducers could also be an option to get a handle on things by current values alone.
 
Sorry for complicating your thread  as I misread your application somewhat.

 

[rossw]

max71020 is a single phase monitoring chip.  If you have 3 phase, the relationship between phases will not be perfect.
In the US, we have two phase power to most home and office, 3 phase to most factories.
Tracking single phase is a lot easier than tracking two or three phase. 
 
Larry and Ross must have a lot of experience on electrical power meters.
Most US home and business now have digital power meter.  Inside it must have really fast CPU to figure out if power flow is from or to grid.
It is easier to measure power through the wire, but it is hard to detect AC power flow from grid or to grid.

 
I said at the time that the Max71020 is a single-phase chip.
78M6631 is a far more sophisticated chip designed for 3-phase.
It has a 32-bit compute engine to process data from the 22-bit delta-sigma ADC, and an 8051-equivalent core for the general hack work.
Has I2C and SPI interfaces, works with 3 and 4-wire (star and delta) configurations.
Measures all 3 phase voltages, and 3 currents via differential inputs.
Around $5-$6 in 1K quantities.