Power Meter

Haplo · Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia

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.

Kenny · Joined: 07 Sep 2003 Posts: 173 Location: Australia

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

Pete Smith · Joined: 17 Sep 2003 Posts: 55 Location: Chester, UK

Haplo · Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia

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

alan jones · Joined: 17 Sep 2003 Posts: 1

Have a look at the analog devices web site they sell a range of chips and provide advice on suitable sensors.

Pete Smith · Joined: 17 Sep 2003 Posts: 55 Location: Chester, UK

TSchultz · Joined: 08 Sep 2003 Posts: 66 Location: Toronto, Canada

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

Ttelmah · Guest

burnsy · Joined: 18 Oct 2003 Posts: 35 Location: Brisbane, Australia

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....

asmallri · Joined: 12 Aug 2004 Posts: 1635 Location: Perth, Australia

_________________
Regards, Andrew

http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!!

Haplo · Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia

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.

Robert Burns · Guest

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.

Tom-H-PIC · Joined: 08 Sep 2003 Posts: 105 Location: New Castle, DE

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

Robert Burns · Guest

Hi,
You can start here.

http://home.earthlink.net/~jimlux/hv/varvom.htm

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA