Help interfacing Current Transformers to ADC

The Scenario:

(20) 120VAC 5A and 10A devices. I would like to come up with a VERY CHEAP way to data log the power consumption of each device. I don't need exact power consumption with power factor correction or anything like that. I would gladly just deal with a reading of the amperage of each device and the assumed or measured voltage. I am more concerned about trends, not the real world kWH.

I have found current transducers that provide a mV output that can be fed to an ADC, but certainly can not see buying 20 of them @ $200 or more each.

I would like to use simple current transformers such as those shown in the following links: Both are Hall type sensors.

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or
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(the LTS 6 for example)

The problem (as I understand it) is that the output of these devices can not be fed straight into an OP-AMP for scalling and then to the ADC. Is there a different approach I should be looking at?

How exactly does one go about converting the AC signal from a current transformer into a DC signal usable by an ADC, and do it cheap enough to build 20 or more devices.

The plan is to use the Dallas 1-wire 4 channel ADCs (DS2450 i think) so that I can toss this right onto my 1-wire network.

I apologize if this is the wrong group.

Thanks in advance!

Reply to
beananimal
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A real name would be kind of nice...

$200? Huh? Check again. Amveco, carried by Digikey, they call the category "current sense", $6.50 a pop for the 10A version. Then CR Magnetics and lots of others. If that's still too much you'd have to wind your own.

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Look up "ideal rectifiers" and stuff like that. Example:

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But you do need a power supply.

It's the right group :-)

And never, ever, forget the burden resistor when doing this kind of stuff. I have seen lots of grief when that happened.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

The low end here...

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is about $20 or so, as I recall. The output could be high enough to rectify, then go into an ADC to indicate average current.

There are hall-effect ICs that are a couple of bucks, but need a pc board and such.

There are probably also super-cheap, not very accurate CTs somewhere, or maybe you could use come stock torroidal power inductors at a couple of bucks each.

John

Reply to
John Larkin

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You can feed them into some ADCs and sample faster than 60Hz (1200Hz, say) and do some math to find the currents. This would require an all-up sampling rate of 24000Hz, which might require an expensive processor, unless you did them each in turn.

You can feed them into ADCs and sample at a rational, but oddball fraction of 60Hz -- like 11/600ths, or 1.1Hz. Then when you've taken the right number of samples (11 in this case) you'll have a picture of the waveform and you can do some math to find the currents.

If the idea of doing some math up front bores you, you can amplify the current, run it through a precision rectifier, low pass the result and sample that with an ADC -- you'll spend more on op amps and capacitors than you will on your microprocessor, but you can do it.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

For AC why do you need a Hall device? Just use a current transformer into an OpAmp with gain and rectification.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

I was in a computer store which has a plug in meter which reads and stores all of this data and more. I'm guessing $30 - $40 each.

Reply to
Homer J Simpson

If you have access to the AC voltage, say as part of your power supply, just use an opto to trigger on the zero crossing and use a timer off that to sample the ADC right at the peak.

Reply to
DJ Delorie

I was refering to the prices I found for the DIN mount units with all the electronics built in and a mV signal ready for DAQ units.

I will look into this.

That will not be a problem, as I will have to supply power to the rest of the electronics on the project (some opamps, line drivers, and otehr stuff)

Yes, I have read quite a bit about the dangers of an open secondary. Along with the rectifer and interfacing design help, I am sure I will need some burden resistor guidance!

I will try and do some research before I ask any more questions with regards to the solution you have offered.

Reply to
beananimal

I am not opposed to doing some boards for this project (as the rest of the project needs a board anyway). My problem would be that I have no idea what particular IC to use or exactly how to use it (thus my delima in the first place).

I am also not opposed to winding my own CTs if it is worth the trouble and not to involved. The fact that I have 20-30 of these to do would seemingly mean that I could save quite a bit of money winding my own CTs. However, I would again need some basic guidance and how-to info.

Reply to
beananimal

That is an interstesting solution, however I fear that 20 micros and the supporting electroinics would get somewhat pricey and take up a lot of space. A small Atmel or PIC for each CT maybe? Or a larger multi input unit for a bank fo CTs?

The idea does not bore me, the problem is in the details and my lack of knowledge. I assume you are refering to an "ideal rectifier" followed by an op-amp and then an RC network as the low pass filter? I am game for either the micro or the second solution and would be willing to do either. The cheaper would be the best, as long as you guys can help with the details. Otherwise I may be as lost as when I started

Reply to
beananimal

Interesting, could you elaborate a little more on the details of how I would do this. I am familair with the zero crossing, and somewhat familair with using an opto to trigger on it. I am not sure how many supporting components each channel would take. Again, the key here is to get a basic power measurment for 20-30 devices in my DIY automation system and get those values to the user interface.

Thanks again to everybody who has replied so far!!!

Reply to
beananimal

That is my thinking. I have some cheap doughnuts I got from Hosfelt or Allelectronics for a buck or so. I am not measuring but I do use them into my trusty old 324s for sensing if the motion lights are on or off. A couple amps through the doughnut will saturate the 324. I suppose you do some of that "engineering" stuff with feedback to get that down to a linear scale.

Reply to
gfretwell

For the size you need you could probably use a small, cheap audio transformer and use one piece of wire as a primary.

Reply to
Homer J Simpson

Since a CT is a pretty good current source for reasonable burden resistances, you really don't need precision rectifiers and all their baggage. A simple bridge will do fine -- especially since you say you're not doing measurement.

Let's emphasize this one -- NEVER, EVER LEAVE THE SECONDARY OPEN!

John Perry

Reply to
John Perry

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There's no need to sample fast or to alias the waveform. The object is to gather statistics on the waveform, not to reproduce it, so Nyquist doesn't apply; any old scattered bunch of samples will do. I sold many thousands of electric meters that did true RMS volts and amps, power, power factor, KWH, KVAH, all that stuff, 16 power channels with a 6803 uP, sampling each channel at about 27 Hz.

And it's easy to offset the sine waves roughly midscale on a unipolar ADC and software auto-zero the DC part out.

John

Reply to
John Larkin

I am not sure what "baggage" comes with the precision rectifier but am doing this as much for learning purposes as I am to complete my automtion project (a learning experience in its own right). That op-amp circuit looked pretty simple, though 4 diodes is MUCH simpler and cheaper I guess. I suppose a rectifer package would be much better suited than 4 discrete diodes?

I only hope to get a reasonably close picture of the power consumption of each device. I am looking to log more of a trend than the actual watts. I mean I would like the results to be resonably close... but I am nto going to lose sleep over a few percent.

I kepe seeing these warnings and will heed them. I am just curious as to the damage that can be done without the burden. The measuring device will be a permantent part of each circuit. I don't suppose the rectifier is considered safe as the burden and a burden resistor is required for safety no matter what?

Reply to
beananimal

You should carefully wire a shunt (burden) resistor across the secondary and use that to measure the current.

An open secondary will try to achieve infinite voltage (approx). This is a bad thing. It won't achieve it but it'll be a very high voltage.

Reply to
Homer J Simpson

I have used these with success as well. However, they are not very linear over a wide range with a bridge rectifier. It is actually fairly simple to shift the output AC from the burden resistor so it stays within the positive input range of an A/D on a PIC.

Select the burden resistor so it produces 5 V P-P at maximum current, and tie one end to ground. Add a resistor about 20x that value to the A/D, and another of the same value to +5VDC, and the sine wave is centered on a 2.5 VDC level. A simple software routine takes the absolute value, and you have a precision rectifier in PIC code, with only three resistors per input. Sample at about 300/sec and read average or true RMS.

Another way to do this is with an active 2.5 VDC reference for one side of all the CTs. Use a burden resistor for 2.5 V P-P max, and a high value resistor (and cap) to protect the A/D inputs from spikes. You can get a PIC with at least 8 A/Ds for under $5.

For a very sophisticated single IC solution, ST Micro's Electric Metering IC is very interesting. See:

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Paul

Reply to
Paul E. Schoen

Who said anything about 20 microprocessors? How about one, with an analog multiplexer going into one ADC? Or one ADC with a built-in multiplexer?

"Cheaper" and "best" are always judgment calls, to be sorted out by free-market forces. If you're just starting out, "best" may well be what you can understand, duplicate and maintain.

--
Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

The lower measurement range will be non-linear and somewhat temperature dependent with a plain old rectifier. Doesn't have to be a bridge BTW. As to rectifier packages I haven't used one in designs for at least ten years. Discretes are cheaper and with SMT machine stuffing being rather low cost there is typically not much of an advantage for a reduced parts count.

Then a passive solution might suffice.

A burden resistor is required for a CT. In the 50/60Hz world you might be somewhat safe if it fails because the core would saturate but it might fry the rectifier before that happens and then your electronics could think there is no current. When the core saturates it almost becomes a core-less inductor where its inductance drops to a very small value.

In RF apps I have seen things like burnt up traces, transistors and diodes that blew off their plastic parts.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

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