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watt sensor? — Parallax Forums

watt sensor?

mctriviamctrivia Posts: 3,772
edited 2009-12-07 09:41 in General Discussion
I need to measure both current and voltage for 20A 120V circuit.

I could use a 20A hall effect IC and a 64 to 1 voltage divider(150V RMS to 3.3Vpeak) and use 2 analog pins on the prop 2 to get my readings but what I was hoping for was a cool IC that could read both voltage and current with preferably a digital output so I could use the prop1 and not have to wait.

Anyone know of anything?

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24 bit LCD Breakout Board now in. $21.99 has backlight driver and touch sensitive decoder.

Comments

  • PJAllenPJAllen Banned Posts: 5,065
    edited 2009-12-05 14:33

    Watt?· turn.gif

    You're not asking for too much are you?

    Without an output/signal to determine the phase relationship of·e and·i it wouldn't be much good.
  • davejamesdavejames Posts: 4,047
    edited 2009-12-05 16:03
    mc,

    Pretty much what Mr. Allen said.

    I found an example that might explain in more detail...

    http://www.ece.unh.edu/courses/ece401/Referernce/AC%20Analysis.pdf

    On page 4 is a graph that represents the phase relationship in an AC circuit when driving a capacitive load.· So one guestion up front would be:· what type of load are you intending to have?

    The AC current detected would be the result of the two instantaneous values of E and I at the time of measurement.· Maybe knowing ("assuming") that there would a 90 degree phase-shift between the two would help in the calculation.· But again, that all depends on what type of load is involved.· I would suspect that the phase shift varies under certain load types or conditions.

    BTW - in capacitive load situations, the current leads the voltage by 90 degrees; with inductive, the voltage leads the current by 90 degrees.· There was a phrase that we used in school to help remember that, but it escapes me at the moment.· In purely resistive loads, there is no phase shift.


    Later,

    DJ




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    Instead of:

    "Those who can, do.· Those who can't, teach." (Shaw)
    I prefer:
    "Those who know, do.· Those who understand, teach." (Aristotle)
    ·
  • mctriviamctrivia Posts: 3,772
    edited 2009-12-05 16:53
    yes i do remember this from school. however there are laws that restrict your imaginary part of any load planning to go on the grid. if you have a completely inductive load you need to use capacitors to alter the power factor to a value closer to 1. I forget the max allowed but there is one.

    A hall effect IC and voltage divider give sin waves out so you can compute both real and imaginary power usage. for my case though just taking the RMS value of each will give me a close enough estimate. I just woundered if there was an IC that could give me both the current and voltage.

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    24 bit LCD Breakout Board now in. $21.99 has backlight driver and touch sensitive decoder.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2009-12-05 17:16
    The phrase that pays: ELI ICE (Eli the Ice Man.)

    "Close enough estimate"?· No way.· But, have fun.· Ever the lone voice crying in the wilderness, I just didn't want anybody to get the wrong idea.

    Post Edit -- Residential users don't have capacitor huts, but·commercial users·do.· The power company would love it if we had to.· They calculate for lost revenue given certain models in determining rates, so that they aren't giving power away, but, oh!,·the current spikes.

    Post Edited (PJ Allen) : 12/5/2009 5:28:04 PM GMT
  • davejamesdavejames Posts: 4,047
    edited 2009-12-06 03:05
    @mc - sorry, I digressed. You asked a simple question and I responded with a lengthy, useless answer. Personally, I have not seen an IC that does what you require...but I have a limited view of what's current in large power managment IC technology.

    @Mr. Allen - that's it! Thanks.

    DJ

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    Instead of:

    "Those who can, do.· Those who can't, teach." (Shaw)
    I prefer:
    "Those who know, do.· Those who understand, teach." (Aristotle)
    ·
  • mctriviamctrivia Posts: 3,772
    edited 2009-12-06 03:09
    I could just do current times 120 and be close enough. Would give me real power only. However I will be able to compute both real and imaginary once prop2 is out.

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    24 bit LCD Breakout Board now in. $21.99 has backlight driver and touch sensitive decoder.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2009-12-06 12:46
    mctrivia said...
    I could just do current times 120 and be close enough. Would give me real power only.
    No X ∞.· You will not be anywhere near·"close enough."
    Phasors on stun.· turn.gif
    e·X i would not result power in any (AC) context other than light bulbs.· If it's about·fans, or refrigerators, or air conditioners, then you need to know the phase angle.
    It's not because I say so.· I'm not cherry-picking data and burying nuggets·like cats in a sandbox, I·don't work for the University of East Anglia, NASA, or the UCLA History Dept.· It's the physics, the truth.·

    Post Edit -- Added hot-link (not the La. variety)
    http://en.wikipedia.org/wiki/Power_factor
    http://en.wikipedia.org/wiki/AC_power#Real.2C_reactive.2C_and_apparent_power

    Post Edited (PJ Allen) : 12/6/2009 1:35:17 PM GMT
  • kwinnkwinn Posts: 8,697
    edited 2009-12-06 14:48
    Use of a moderate speed ADC and a microprocessor would allow sampling the current and voltage several times per cycle and from that you could calculate the actual power. The simplest calculation would be the sum of the instantaneous power.
  • mctriviamctrivia Posts: 3,772
    edited 2009-12-06 17:07
    ok I think we have some misunderstanding here.

    1) I have a diploma in Electronics Enginering and understand phasors.
    2) Accuracy is not of any importance. I really only need the current draw and the voltage to make sure things are working right.
    3) Power is made up of real and imaginary power. multiplying the rms values of e and i will give you the real power you need to know the phase angle or instintatnius values to calculate imaginary.
    4) Once the prop 2 is out I will have enough pins to compute both real and imaginary power.

    was just woundering if there was an ic i could use for e and i or if i need to make my own circuit.

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    24 bit LCD Breakout Board now in. $21.99 has backlight driver and touch sensitive decoder.
  • ercoerco Posts: 20,260
    edited 2009-12-06 17:57
    I·recently got one of these Kill-a-Watt meters for $16 from Ebay:

    http://cgi.ebay.com/P3-KILL-A-WATT-LCD-ELECTRICITY-MONITOR-METER-P4400-NEW_W0QQitemZ200413874341QQ

    Simple to use, compact·and ready to go. Gives volts, amps, VA, watts, time, hz, power factor, & cumulative·KWH. Lets you easily experience the "Prius Effect" on your home appliances.

    It verified my suspicion that·my neighbor's·highly-touted and overpriced ($400) Edenpure quartz/copper electric heater wasn't·noticeably more efficient than a cheap·electric coil heater.

    P4400__1.jpg

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    ·"If you build it, they will come."
  • mctriviamctrivia Posts: 3,772
    edited 2009-12-06 18:25
    Nope electric heat is 100% efficient. Watts used is heat created. I need to build in but those things are good.

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    24 bit LCD Breakout Board now in. $21.99 has backlight driver and touch sensitive decoder.
  • mctriviamctrivia Posts: 3,772
    edited 2009-12-06 18:30
    There are tones of hall effect ics that measure current. often people want to measure voltage also so why not add the voltage divider into the ic.

    I will just have to layout the circuitry on my PCB. not a problem.

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    24 bit LCD Breakout Board now in. $21.99 has backlight driver and touch sensitive decoder.
  • FranklinFranklin Posts: 4,747
    edited 2009-12-06 18:38
    Speaking of the Kill-A-Watt here is a nice project and you may find some info on reading both current and voltage. www.ladyada.net/make/tweetawatt also there is this www.allegromicro.com/en/Products/Part_Numbers/0750

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    - Stephen
  • dMajodMajo Posts: 855
    edited 2009-12-07 09:41
    mctrivia: I have enclosed just the links to two recent posts. On the first you will found an energy meter IC from ST and a simultaneous sampling adc from TI (really good for this kind of applications since you need current and voltage sampled the same time). The application notes are for the ST energy meter (spi i/f). This IC is used in half of the European·digital energy counters in home environments (for billing pourposes) IIRC beside the one referred (just metering part) there is a second one from ST with an integrated 8052 core (for autonomous standalone applications).

    On the·second some hall effect current sensors from AllegroMicro

    http://forums.parallax.com/showthread.php?p=857153

    http://forums.parallax.com/showthread.php?p=856024

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