Help interfacing Current Transformers to ADC

[beananimal]

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.

https://www.ohiosemitronics.com/pdf/catalog/hall_effect_current_transducer_modelCTU.pdf
or
http://www.lemusa.com/product/37.html (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!

 

[Joerg]

beananimal wrote:


A real name would be kind of nice...

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.

$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.

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

https://www.ohiosemitronics.com/pdf/catalog/hall_effect_current_transducer_modelCTU.pdf
or
http://www.lemusa.com/product/37.html (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.

Look up "ideal rectifiers" and stuff like that. Example:
http://www.analog-innovations.com/SED/FullWaveRectifier.pdf

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.

But you do need a power supply.

I apologize if this is the wrong group.

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.

 

[John Larkin]

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.

https://www.ohiosemitronics.com/pdf/catalog/hall_effect_current_transducer_modelCTU.pdf
or
http://www.lemusa.com/product/37.html (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!

The low end here...

http://www.sentrancorp.com/

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

 

[Tim Wescott]

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.

https://www.ohiosemitronics.com/pdf/catalog/hall_effect_current_transducer_modelCTU.pdf
or
http://www.lemusa.com/product/37.html (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!

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Jim Thompson]

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.

https://www.ohiosemitronics.com/pdf/catalog/hall_effect_current_transducer_modelCTU.pdf
or
http://www.lemusa.com/product/37.html (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!

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

...Jim Thompson

 

[Homer J Simpson]

(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 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.

 

[DJ Delorie]

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.

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.

 

[beananimal]

$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.

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.

Look up "ideal rectifiers" and stuff like that. Example:
http://www.analog-innovations.com/SED/FullWaveRectifier.pdf

I will look into this.

But you do need a power supply.

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)

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

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.

 

[beananimal]

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

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.

 

[beananimal]

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.

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?

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.

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

 

[beananimal]

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.

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!!!

 

[[email protected]]

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

...Jim Thompson

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.

 

[Homer J Simpson]

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.

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

 

[John Perry]

beananimal wrote:


Look up "ideal rectifiers" and stuff like that. Example:
http://www.analog-innovations.com/SED/FullWaveRectifier.pdf

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.

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

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

John Perry

 

[John Larkin]

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.

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

 

[beananimal]

Since a CT is a pretty good current source for reasonable burden

resistances, you really don't need precision rectifiers and all their
baggage.

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?

A simple bridge will do fine -- especially since you say
you're not doing measurement.

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.

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

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?

 

[Homer J Simpson]

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?

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.

 

[Paul E. Schoen]

CSE187L 20 pcs @ $2.34 each @ Digikey - $46.8 + S/H.
Datasheet is included with the actual device.
0.1 - 30A. Add a load resistor and cheap rectifier circuitry and you can
get a channel under $5 in components cost.

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:

http://www.st.com/stonline/products/literature/ds/10853/stpm01.pdf.

Paul

 

[Tim Wescott]

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?

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?

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

"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
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Joerg]

beananimal wrote:

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?

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.

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.

Then a passive solution might suffice.

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?

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.