[SOLVED] Opto coupler for energy measurment ?

[ADGAN]

Is it possible to use an Opto coupler to measure the power consumption of an AC load? or what are the other easy methods I can use to measure the energy consumption? This for an energy meter that I have to design for my diploma project.

 

[KerimF]

The response of a standard opto coupler is sensitive to temperature.
And the measurement of power consumption in AC needs the multiplication of v(t) and i(t) of the load. I mean |V| x |I| gives the VA power instead, not the effective one.

Added:
You may be interested of using the Linear Optocoupler from Vishay (series IL350).

Added:
For instance, 30 years ago, I had MC1496 (Double Balanced Modulator/Demodulator). I used it as a mutiplier and got the load power on a small magnetic needle display. I designed this power meter on paper which is lost with many other things including the meter

 

[schmitt trigger]

As Kerim mentions, you need to measure the voltage, the current and the power factor of the load.

I have also used the MC1496 many years ago... A much better choice today would be to use an IC from Analog Devices like these:

for single phase;
https://www.analog.com/ps/psthandler.aspx?pstid=10210&la=en
for three phase;
https://www.analog.com/ps/psthandler.aspx?pstid=10211&la=en

I've personally used the AD7757.

 

[ADGAN]

Thank you. Actually I forgot to mention that I was planning to use MCP3901. Since the IC is not available in my country I have to import it from the US which will cost me a lot(because of the shipping cost). I was wondering whether there is any other cost effective method to implement this.

 

[KerimF]

I remember, since I also cannot get special ICs, I built a multiplier using standard opamp (as LM324) and CMOS switch (as CD4066) for low frequencies (as the 50Hz of mains). Could you find them in your local market?

 

[ADGAN]

Yes it is available

 

[KerimF]

I remember I draw this multiplier on LTspice soon after I install the software. But I changed 3 computers since then. So I try looking for its schematic in my backup CDs. I forgot about this multiplier because it is not needed in my products I work on. I hope I can find it very soon... otherwise I will need re-inventing the wheel

 

[KerimF]

Hi again Adgan,

I wondered if your diplomat project should be a suitable one commercially or just a good low-cost AC power meter using available ICs (hence it won't be compact)?
If the latter case is ok, PWM could be used to build its crucial part which is the multiplier.

Since the fundamental frequency of the input signals (voltage and current) is 50 Hz only, popular ICs could be used:

(1) LM339 to generate a stable triangular wave (if the voltage supply is regulated) and, as a comparator, it provides the pulse-width modulated signal.
(2) LM324 to amplify the voltage across the current shunt (of an ampere meter for example) and also to add DC offsets when necessary.
(3) CD4066 to multiply two signals by switching (with a simple low pass filter).

Practically, this multiplier would be on the side of the mains high voltage. And since the supply current of this interface circuit is relatively low, a capacitor could also replace a bulky transformer (in any case, this part of the meter should be made out-of-touch). To get the reading, there are, as usual, two options; analog and digital.

Analog: A suitable needle display could be made to read the AC power. In this case, the interface circuit also becomes the power meter.

Digital: A typical opto coupler could be used to transmit the multiplier analog output to a controller board, after converting its voltage to pulses (likely using PWM too).

What do you think?

Do you like working on this project this way?

If you do, we may build and discuss together every part of it, step by step.

For instance, I usually use LTspice (a free professional simulator from Linear Technology).
If you have it, I will also attach the appropriate circuit files so you too can analyze each part (then the whole project at the end) and even update it anytime you find necessary.

Kerim

 

[ADGAN]

Thanx for the help. In my design I need to display the consumed amount on a LCD display and also send it to the service providers end using a SMS. It is not really necessary to design for commercial usage but I'm hoping to build a compact model.

I'm not much familiar with those components but if you could attach a circuit diagram or the circuit files for LTSpice it could be great.

 

[KerimF]

Sorry, but do I hear from you that the branch of electronics that is related to the main part of your project (AC power meter) is not among your studies?

 

[ADGAN]

My diploma is telecommunication enginnering. We do have electronics. But not to a very deep level.

 

[KerimF]

I see... but I hope you are familiar with the basics concerning opamps and comparators. For instance, could you get the LM339 IC (or an equivalent comparator)?

 

[ADGAN]

Yes I have studied about Op-amps. Yes I can get the LM339 IC also. I would like to see the circuit diagram that you mentioned.

 

[KerimF]

I didn't find the old one so I thought we can design it step by step. I guess this helps you understand every corner in it
For instance, as I have it in mind, its block diagram consists of:
(1) A triangular oscillator, by using two comparators at a high frequency as 10 KHz
(2) A preamplifier, for the AC current
(3) A comparator, it combines the outputs of (1) and (2) and produce a PWM signal

Since the input of (2) is AC, we can start using a dual voltage supply but we can decide if it is better to alter the circuit to work with a positive one only.

(4) A voltage divider with DC offset, by an opamp for the measured AC voltage
(5) Analog switch, as of CD4066 to be triggered by (3) to switch on/off the second signal from the output of (4)
(6) A passive or active low pass filter, it follows (5) so that its filtered output voltage will be proportional to the instantaneous AC power, the product v(t)*i(t), hence its fundamental frequency is 100Hz (for 50Hz mains).

For instance, what is the remaining time for your project?

 

[ADGAN]

I have less than 3 months to finish the project. But I'm hoping to finish this part by next month. I have already ordered the MCP3901 energy measurement IC but have not yet confirmed it. Before confirming I felt that it is bit expensive even though it is still within our planned budget. I found many datasheets & reference guides to implement this system given by the Microchip. So initially we thought to implement using that IC. I was just wondering whether there is any other feasible method to implement this project after looking at similar projects done by university students. Do you know where I can find a similar circuit diagram on the Internet since I need the circuit diagram ASAP to determine whether it is feasible to implement or not & need to discuss it with other project members?

 

[KerimF]

I think if you can get MCP3901, your work would be reduced a lot.

You are fortunate that you can access Microchip webpages. Since about 10 years, Microchip (and Texas Instruments) don't allow any of their pages be opened from Syria (at least by an ordinary citizen as I). In general, when I do a search, I can't access most sites. I mean you should have a better chance to find application circuits than I. So I used designing anything I may need (or my local market needs) from scratch and by using the modest available components only.

For instance, in year 1979 (spring term) when I was at the American University of Beirut (AUB), I designed and built as a master degree thesis, a simple analog demodulator (having no coils or sensitive filters) for DSB-SC signal (Double Sideband Suppressed Carrier signal) using CD4046 (a conventional PLL IC), LM339 (or LM324) and CD4066 only as the main ICs. But I didn't have time to prepare and submit the papers of my work (I am a bit lazy doing paper work). After all, I didn't need higher degrees than my BS to start my private business as a designer of electronic circuits So I used the novel concept of demodulator, for a few years, in my private RF links (between my house and work) since, at that time, I didn't have a phone line at house In case of an AM link, I let the suppressed carrier swing +/- 50KHz at about 6 Hz so that during the talk, the ordinary AM receivers (MW) detected my signal as noise. In case of an FM link, I modulated the voice signal on an ultrasonic frequency as in stereo FM (also DSB-SC) but without adding a pilot since my demodulator doesn't need it. Also the ordinary FM receivers heard my link during a talk as a silent one Now, even after more than 30 years, I don't think we can find a source that made a DSB-SC demodulator by using these simple 3 ICs or an IC in which their functions are integrated. I mean even your professors in communications cannot imagine by a conventional PLL this could be done though the trick is simple and even obvious when known

About your project, I will likely try uploading the circuits of the blocks above, one after another, even they won't be important for you. Perhaps some others may like seeing them

 

[KerimF]

The attached files are for block (1); a triangular oscillator at a frequency about 9 KHz.
I was able uploading an image only (the attachment manager here is not working right now).

https://obrazki.elektroda.pl/3877525100_1382124964.png

I prepared its LTspice files in case I can attach them.

 

[KerimF]

Hi again,

To be honest, your project stimulated me personally to build a multiplier without using a readymade IC for this function.

The switching method is one of the methods that are used for AM modulators and demodulators (the synchronous ones). So I used seeing the switching process as a sort of multiplication. But when I think here how to use this technique to multiply two analog signals of low frequency (not just one and a high frequency carrier), I find out it is not simple (4-quadrant multiplier). And I realized that, at many times in the past, I tried doing it but I couldn't get a reliable and precise result (in comparison to an analog multiplier IC) and this explains why I wasn't interested to save carefully any of my work on it.

Though I am a bit busy at work, designing a good practical multiplier using standard ICs by chopping one of the two signals became like a personal challenge to me now It is much like an exercise at school to build a circuit for a complex function by using one or two basic logic gates only I mean I will work on it and even upload it here (if I do it well) but not necessarily for you. For instance, to save time, I wish you will get soon a suitable IC that has an analog four-quadrant multiplier or two fast A/D converters.

Added:
I noticed, in the series MCP3900, that MCP3909 is an "Energy Metering IC with SPI Interface and Active Power Pulse Output". Its ouput can be used for the opto isolator.

 

[ADGAN]

Thanks again. As I can remember MCP3901 consist of two A/D converters. So I'm going to use it for my project. But I'm definitely going to keep your method for an alternative. Really appreciate your help.

 

[KerimF]

I hope that soon I can upload files, not just images. Since yesterday, I was surprised that I got only a blank gray window anytime I run "Manage Attachments"!

Extra note:
The beauty of using MCP 3909 is that the MCU will need just doing a simple substraction of two values of energy read (via an opto isolator) at the end of every fixed time interval.