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Haplo
Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia
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Power Meter |
Posted: Tue Sep 16, 2003 6:15 pm |
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Hello everyone,
I'm trying to build an 'intelligent' power board. I need to sense the change in the current being drawn when the various devices connected to the board turn on. Has anyone done this before? Can anyone point me to a circuit that does a similar thing? Thanks. |
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Kenny
Joined: 07 Sep 2003 Posts: 173 Location: Australia
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Posted: Tue Sep 16, 2003 7:54 pm |
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You will need a current transducer suitable for mains power measurement.
LEM have a range of current and voltage tranducers. I have used them in the past, but now there are new ones available.
http://www.lem.com/
and go to Transducers/Sensors. The agent in Australia now is Fastron.
HTH
Kenny |
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Pete Smith
Joined: 17 Sep 2003 Posts: 55 Location: Chester, UK
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Re: Power Meter |
Posted: Wed Sep 17, 2003 1:15 am |
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Haplo wrote: | Hello everyone,
I'm trying to build an 'intelligent' power board. I need to sense the change in the current being drawn when the various devices connected to the board turn on. Has anyone done this before? Can anyone point me to a circuit that does a similar thing? Thanks. |
It depends on the amount of power you're talking about really.
I've just finished (hopefully!) a project where I had to determine whether or not power was being used in a home appliance, so I build a device that plugged into a power socket, that the appliance plugged into.
Because I had to measure very small power usage (less than 40 watts), but still be able to measure 13A, I looked into the problem in great depth, and decided that the best solution for <u>me</u> was to use a Current Sense Resistor (25mOhms) and to measure the voltage across it. This then gave me a measure of the resistive current (as opposed to the inductive current, which can lead/lag the resistive current) flowing through the resistor.
The problem I had with the LEM current transformers at the time, were that they had a cutoff point where they wouldn't pick up small currents, so I'd either not be able to detect what I was looking for, or they'd burn out if you plugged a kettle into it.
HTH
Pete. |
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Haplo
Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia
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Posted: Wed Sep 17, 2003 2:48 am |
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Kenny, Pete, thanks for your reply. I have to detect a TV coming on. Is the LEM device sensitive enough for that?
Pete, did you just put the 25ohm resistor in series with the power line? Or did you use a current transformer?
Thanks,
Ali |
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alan jones
Joined: 17 Sep 2003 Posts: 1
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Posted: Wed Sep 17, 2003 5:11 am |
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Have a look at the analog devices web site they sell a range of chips and provide advice on suitable sensors. |
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Pete Smith
Joined: 17 Sep 2003 Posts: 55 Location: Chester, UK
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Posted: Wed Sep 17, 2003 5:48 am |
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Haplo wrote: | Kenny, Pete, thanks for your reply. I have to detect a TV coming on. Is the LEM device sensitive enough for that?
Pete, did you just put the 25ohm resistor in series with the power line? Or did you use a current transformer?
Thanks,
Ali |
LEM sell a _huge_ range of sensors, some of which may be capable of detecting a TV coming on, depending on the TV.
FWIW, both of the Sony TVs I have here take less than 40W when operational, and ~0.1W when in standby.
When I looked, IMO, none of their transformers were sensitive enough to measure 40W (160mA in the UK), and at the same time, allow 13A to pass, without melting. If you're only ever going to want to measure the TV, you can get transformers to do the job perfectly.
The 25mOhm resistor was placed in the power line, and the voltage across it measured.
I got my initial design from here...
http://www.circuitcellar.com/pastissues/articles/May96/rick-96.pdf
or
http://www.canton.edu/employee/hartle/elec243/pic_power_meter_rick-96.pdf
This article isolates the measuring equipment from the mains supply.
It was then decided a transformer was too heavy, so I designed a transformerless PSU, and then just used a comparator to compare the voltage across the CSR (Current Sense Resistor) with a fixed level, which then triggered the PIC. Because there was no transformer, the circuitry ran at mains voltage anyway.
I could have amplified the signal by 9 times, and done an A/D conversion to measure the exact current, but that wasn't required.
HTH
Pete. |
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TSchultz
Joined: 08 Sep 2003 Posts: 66 Location: Toronto, Canada
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Posted: Wed Sep 17, 2003 6:51 am |
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A few years ago I designed a single and 3 phase power measurment unit, PIC based. It measured both current and voltage, then calculated Watts, VA, Power Factor, Peak Current, Peak Voltage, Wh, VAh. These were done on a per phase and total basis for the 3 phase version and then just the single phase for the single phase version.
There are a few issues to contend with, the biggest being how to accurately measure the current and voltage, and safety (ie. isolation). How you measure will be somewhat dependant on what you want to do with the measurements. We were calculating true RMS and wanted at least 20 measurements per cycle so had to do a lot of work to improve the caclulations and went to a multi-processor design where there was a PIC responsible for the measurements and RMS calculations for each phase, then another for summary, final calculations, and communications. All our measurements were 12 bit resolution. With the newer PICs, especialy the 18 series, the execution speeds have improved and this task becomes easier.
The design was configurable at manufacture and had an input current rating of 20 to 5000A full scale per phase, and 100 to 600 V full scale. We used custom transformers for the voltage isolation and current sense transformers (CTs) for the current and then used a shunt resistor to measure the voltage. I have since designed a new unit that uses optical isolation rather than galvanic isolation and has 16 bit resolution.
Look very carefully at what you have to measure and what you want to do with those measurments. Then you can review what is available and determine the best approach.
-Troy |
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Ttelmah Guest
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Posted: Wed Sep 17, 2003 7:58 am |
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burnsy
Joined: 18 Oct 2003 Posts: 35 Location: Brisbane, Australia
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Posted: Tue Nov 02, 2004 5:02 am |
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Hi, just to put in my two cents worth, I have been using LEM sensors for some time. I use the LTS-6NP, which measures up to 6A. In another mode, set by how you wire it up, will measure 0 - 2A, and not blow up if you plug a kettle into it. They are a simple solution.
An alternative I am looking at is a similar device by CUI www.cui.com. They have similar devices but are more compact and are cheaper. I am about to test them out. I'll post my results... _________________ This is the last code change until its ready.... |
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asmallri
Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia
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Haplo
Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia
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Posted: Tue Nov 02, 2004 6:09 am |
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Well it's been over a year since I posted the original message. I successfully finished and finalized the project a long time ago, using a 18ohm resistor along with a 5.6v zener and two 1N4004 diodes in parallel with it to limit its voltage drop on both directions. |
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Robert Burns Guest
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Resistive current! |
Posted: Tue Nov 02, 2004 6:27 am |
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Mr Smith´s measurement of the current via the voltage drop across
the low ohmage resistor doesn´t reflect the resistive portion of the
current in the circuit. I suggest that Mr. Smith get better acquainted
with phase angles,inductive and capacitive reactance etc. before attempting to call the current through a resistor as being resistive in nature. |
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Tom-H-PIC
Joined: 08 Sep 2003 Posts: 105 Location: New Castle, DE
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AC current measuring? |
Posted: Tue Nov 02, 2004 12:17 pm |
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Mr. Burns
Way don’t you explain some of this phase angle, inductive and capacitive reactance etc.
So all of us can learn why or why not and how to or how not to using a current sensing resistor in AC circuits.
All of the AC current sensing resistor that I have looked at have all most zero inductive and capacitive reactance. So I don’t understand? If using a resistor designed for AC current sensing and reading the voltage drop at the proper point in the phase angle what the issues are?
I know that this AC current sensing issues has been posted a number of times on this form.
I also know that a lot of us on this form are extremely well versed in DC circuitry but we can do just enough of the AC to DC power supply work to get us by.
I know for me learning some AC current measuring information would help out a lot.
So I think that I can say we would all be grateful of any information that you would be kind enough to share with us.
Thank Tom ![Laughing](images/smiles/icon_lol.gif) |
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Robert Burns Guest
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Mark
Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA
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Posted: Wed Nov 03, 2004 7:07 am |
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Quote: | All of the AC current sensing resistor that I have looked at have all most zero inductive and capacitive reactance. So I don’t understand? |
Has to do with the load that you are trying to measure and not really the device used to measure. If you know the type of load then you can compensate for it. |
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