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Aubrey Kagan

RMS Measurement

Aubrey Kagan
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Maciel
Maciel
8/31/2013 2:38:22 PM
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Newbie
Energy METER
My experience with good current measurement is to use a shunt amplifying the voltage as a function of the current that was flowing over it. I used a comparator circuit to detect the level of tension that needed ... It worked, but the accuracy was not very good, since the signal varied. much.

I've used also cis ACS Allegro, a good perfomace but requires some external components, and a more precise calculations and use the microcontroller.

Microchip offers very interesting material on medidore energy:

the link below:

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en557090

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
8/29/2013 2:49:18 AM
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Master
UPS
Aubrey,

We designed a display unit to measure 3- phase RMS values of input and output voltage values of an UPS. We used similar technique used in the blog. Our MCU was R8C 25 which has a hardware multiplier.

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Doug21201
Doug21201
8/28/2013 6:00:23 PM
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Newbie
Re: Thermoelectric vs. thermistor
Aha! The LT1088. There is a story behind that product involving more than a little JIm WIlliams mischief...chekc my blog...I'll write it up and post it there.

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antedeluvian2
antedeluvian2
8/28/2013 4:17:38 PM
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Newbie
Re: Thermoelectric vs. thermistor
WtR,RtW

I did use a tiny board mount transformer to scale down the voltage but more for isolation safety concerns than anything else.

You are right- isolation concerns are very important. The circuit in Figure 1 used an external current transformer B5303 from Bicron. Figure 2 used the Allegro ACS712 hall effect device. Of course the frequency reponse of these devices limit the universality of any design. For better or worse my designs almost always revolve around 50/60Hz.

 

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Work to Ride comma Ride to Work
Work to Ride comma Ride to Work
8/28/2013 3:19:18 PM
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Newbie
Re: Thermoelectric vs. thermistor
I was very happy with the results I received when I used the AD8436.  I implemented it with capacitive coupling and a single supply rail.  I was using it to measure 115 VAC at 400 Hz for monitoring the voltage of three-phase aircraft power.  Careful selection of the averaging capacitor and output filtering capacitor can make for a fast response.  Certainly more than fast enough for humans to monitor.  I did use a tiny board mount transformer to scale down the voltage but more for isolation safety concerns than anything else.  Otherwise a plain old voltage divider would have worked just fine.

I might be a little circumspect about using them for fault isolation if you needed something that would react in half-cycle. 

I also used the same device to monitor current on a high voltage (>1kV) by using a small board mount wideband current transformer and an appropriate burden resistor.  That worked well too.  (I know, the power guys will snicker if you call 1kV high voltage.  :-)

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antedeluvian2
antedeluvian2
8/28/2013 2:49:42 PM
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Newbie
Re: Thermoelectric vs. thermistor
Tim

My favorite wideband approach was Jim Williams' development of the LT1088 RMS -DC conversion.  Find it in app note AN22.

It is a very interesting document. Thanks for the pointer

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ncallen
ncallen
8/28/2013 2:37:38 PM
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Newbie
measure power
Better yet.....sense voltage and current (instantaneous), multiply, filter.  Gives you real power, regardless of load variations and reactive loading.

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Tim W
Tim W
8/28/2013 2:26:19 PM
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Newbie
Re: Thermoelectric vs. thermistor
The app which I remember used a resistor thermally coupled to a transistor which was one half of a current mirror.  Another aspect of the thermal approach is to consider the history of RF power meters - they were THERMISTOR based.  

My favorite wideband approach was Jim Williams' development of the LT1088 RMS -DC conversion.  Find it in app note AN22. ( http://cds.linear.com/docs/en/application-note/an22.pdf)

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samicksha
samicksha
8/27/2013 6:03:54 PM
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Student
Re: Thermoelectric vs. thermistor
I am sure we are considering the fact that it is used for temperature measurement, to compensate for temperature variations in a circuit.

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antedeluvian2
antedeluvian2
8/26/2013 6:40:03 AM
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Newbie
Re: Thermoelectric vs. thermistor
eafpres

 Considering the thermistor design, I wonder if it would be possible to design an RMS measuring circuit using a resistor and two thermoelectric devices. 

The concept does raise a bunch of ideas on how to tackle the measurement. Years ago I came a across a National Semi app note (which was even older) by Bob Pease where he had some other idea on measuring RMS. At the time I dismissed it as impractical in a production product and I wasn't even sure if Mr Pease wasn't talking with a little tongue in his cheek. I tried to find it for this blog, but without luck. Perhaps someone else remembers the app note?

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Page 1 / 2   >   >>
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