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[Ideas For Design]
Easy-To-Build AC Power Meter

D. Prabakaran  |   ED Online ID #3378  |   May 12, 2003


If your goal is to save on electricity, you must first measure the ac current flowing through your appliances. The formula for power is the product of the ac line voltage and the measured current. Presented here is a demonstration of how to build an ac power meter using a current transformer (see the figure).

The current transformer is similar to an ordinary voltage transformer, except that a primary wire is generally absent around the core. The primary coil is constructed by winding either one of the wires connecting an ac plug and an outlet around the core (two turns, in this case).

Current ratio flowing through the primary and secondary wires is inversely proportional to the turns ratio of the transformer. The secondary ac current is rectified and smoothed by D1 and C. The R1 and R2 shunt resistors yield a correct full scale for the meter. You may need to combine a few standard value resistors to create the odd-valued R1 and R2. D2 is used to bypass the opposite-phase current.

The transformer may be broken if the secondary circuit is open with a high current on the primary wire. Of course, a voltage transformer doesn't fire when the secondary is short-circuited.


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    Reader Comments

    How can the electricity meter be slow down using an external device ? Is there any circuit or socket like thing that can be fixed into the power plug to slow down the electricity meters?

    eagle -November 03, 2008

    Do those values for R and C hold if you are using a different turns ratio?

    Anonymous -October 13, 2008

    Hi,i am to design an energy/power meter,but i have a problem,which is the schematic,i need you to help me with the schematic alright?plzzzzzzzzzzz.

    David -September 21, 2008

    hi,

    I need 3 phase AC current meter schematic. Is there anyone who could sugeest/or help me?

    Thanks

    Ulambayar -September 09, 2008

    hi, i am a final year student of eng. i am going to build a digital energy meter for design project. plese send me the information how to build that with 8051. and an ideal circuit

    Anonymous -August 21, 2008

    hi, can anyone help me know how an electric meter can be slowed down using any remote sensing device? also can any one send me a circuit of how a crystal oscillator works inside an electric meter ?and how can one disturbs its frequency? and

    can there be power theft by disturbing harmonics of three phase supply?

    kindly try to send solutions if u know? thanks

    vaibhav -August 10, 2008

    i don't know what is it but this is very simple power meter. how about tehe secure of this power?

    agus -April 22, 2008

    While many commenters are correctly concerned about power factor (phase) there is another even more complex consideration that if ignored results in serious errors: RMS conversion. Again, except for resistive loads, many AC loads can have very odd current waveforms that must be converted to their true RMS values.

    Anonymous -March 11, 2008

    There seems to be considerable confusion among the readers about power, energy, and power factor. The circuit shown will measure apparent power, or VA only. It will not read real or true power (watts) unless the load is resistive and the current is in phase with the voltage. For loads that have both an R and X component, the circuit would need to resolve the effect of phase angle. That is why the antique power meters had both current and voltage connections. Utility company mechanical kilowatt-hour meters resolve the phase angle mechanically by the way the motor is constructed with two sets of windings determining the torque on the disk. Kilowatt-hours are a measurement of energy, not power. If you know the power, just multiply it by the time the power is on to get kilowatt-hours.

    Jim Cook -February 28, 2008

    It's very easy to slow down a power meter. I'm surprised all these engineers haven't mentioned it yet actually. You need to get a special tool though. It's pretty big, but not that expensive. Called a sledgehammer.

    Good luck with it!

    Anonymous -January 11, 2008

    Z

    Anonymous -December 07, 2007

    good

    Hassan Hammad -December 04, 2007

    im looking for the circuit diagram of an eletronic prepaid energy meter

    Anonymous -December 02, 2007

    im looking for the circuit diagram of an eletronic prepaid energy meter

    Anonymous -December 02, 2007

    ??? ???? ?????

    abed -November 21, 2007

    Super Prabakaran

    RAJESH -September 20, 2007

    im looking for a circuit which can measure the power factor....can anyone suggest me any circuit

    tejas -September 19, 2007

    I need some kind of Indicator(LED?)that lights up when my Solar System is pushing back more than I use at the Moment,my Power Co installed digi Meters,that CHARGE you for the Power I push back!

    Anonymous -August 21, 2007

    hi can usay me how prepaid energy meter work &which is best controoler for design thanks

    Awate -August 13, 2007

    i need some circuits for 12-48 v dc or ac input and 15 v Dc output if you can help me thankfull

    aygun -August 08, 2007

    let us c what u have done . Show the progress. Give ur college,university details

    ravindrra -July 06, 2007

    hi there pls help on my research work: how prepaid electric meter works?

    SPIRAL -June 28, 2007

    Hey, if you're a final year engineering student and you can't convert power in KWH, nor get datasheets on your own, you should spend the time to get the routine down as this is not an unusual thing. What're you going to do when someone wants to pay you for the work? Can't keep being handed answers forever, that's what your education was supposed to do for you.

    Anonymous -March 22, 2007

    Hello Guy I'm a final yr student in Electrical Eng'g. I propose Digital KWH Meter Design for my project but I can't get the reading material for it. so pls help me how i can design it using a microcontroller for remote control.

    urs

    Anonymous -March 10, 2007

    hi, i am a final year student of eng. iam going to build a prepaid digital energy meter but i have no idea how to convert power in KWH units. plese send me the information how to build that with HT1380 and 8051.

    Anonymous -March 07, 2007

    Slow down a meter? Well as a matter of fact, you can do this with a simple single transistor circuit. Simply pulse a ferrite rod (find that inside an AM radio) at high frequency and hold the end of the rod near the aluminum disk. Better still, thunp the meter around 3 hz quite hard. It will stop and you will get a sore hand.

    Alan -December 24, 2006

    Seems to me that D2 will short out the negative half of the sine wave. You can't connect a diode across a transformer secondary like that I don' think.

    Alan -December 24, 2006

    Very Simple and nice kwh meter circuit

    raj -December 20, 2006

    How can the digital electricity meter be slow down using an external device ? Is there any circuit or socket like thing that can be fixed into the power plug to slow down the electricity meters?

    raj -December 20, 2006

    HY!!

    No i'm Harkov Sergey -September 14, 2006

    ????? ?????(hellove people Howe are you?^) I'm engeneer from Ukraine Do you know thi country)

    Anonymous -September 14, 2006

    nice circut, i want to ask if any1 can help to make integrate circut to measure the power factor ?

    Mobashir -August 24, 2006

    hi, i am a final year student of eng. iam going to build a prepaid digital energy meter but i have no idea how to convert power in KWH units. plese send me the information how to build that with ADC804 and 8051.

    kamAL -May 15, 2006

    "How can the digital electricity meter be slow down using an external device ?"

    This is easy. Get a big generator, put some fuel in it, and hook it up to your house's power lines. Running the generator will supply power to your appliances, and any left over will go back to the power company, and actually turn the meter backwards! This will save you lots of money on your power bill. (However, it'll also cost you even more money in gasoline.)

    Probably not the answer this moron was looking for, but "ask a dumb question..."

    Anonymous -May 02, 2006

    I need to learn how to make a digital power factor meter. Please if you know,let it know to me. Thanks very much.

    HERNANDO ROJAS -April 21, 2006

    we need the IC use full to measure MVAR & we need the complete circuitry with design such that i can procure the IC.name the ic & give details availability in india

    K.Srinivas reddy -January 10, 2006

    Very Simple and nice circuit

    Talat Sahim -December 05, 2005   (Article Rating: )

    How can the digital electricity meter be slow down using an external device ? Is there any circuit or socket like thing that can be fixed into the power plug to slow down the electricity meters?

    Anonymous -November 05, 2005

    Nice circuit. Assembled works fine!

    Aias Marques -October 17, 2005

    I have once read about a circuit that could be built to slow down a power meter. Does anybody have any idea what this circuit may be called or when to find more information about it.

    Anonymous -August 12, 2005

    I need advice on how to design power factor angle meter.

    LINDA -July 25, 2005

    fine

    Anonymous -June 14, 2005   (Article Rating: )

    fine circuit

    volta -June 14, 2005

    The circuit looks fine. there seems to be a problem with the understanding of what real and reactive power is.

    If you want to measure reactive power get a Varmeter.

    The person who wants the Fp of the circuit use a volt meter and a current meter to find the apparent power because this one gives the real power.

    Obviously, an understanding on how an analog meter works is absent in all of the above comments. These meters work on a mechanical principle of Torqque and the instantaneous power which is directly related to the instantaneous torque.

    Since there is a PM meter this means that the only dependant variable is the current through the moving coil within the PM field of the meter.

    Current in an AC circuit rises to peak values and diminishes to zero many times a secong. the energy stored in the magnetic field of and inductor, or the plates of a capacitor, is returned to the source when the current changes direction.

    If the current in an AC circuit is purely inductive (the instantaneous power which is directly related to the instantaneous torque) the torque is clockwise for 1/4 cycle and reverses for the next 1/4 cycle, the net torque is zero, ergo no movement on the meter.Where as real power is not returned to the source and its instantaneous power, hence torque is the average of the min and max of the torque and power hence the Average power or real power is recorded, because of a net torque being generated in the meter movemnet.

    This is the whole problem with modern technology they don't teach the basics anymore. They give the student a digital meter and the first time they see a analog meter is in someone's basement and have no clue on how it works. Analog meters are work on an entirely differnt principle than digitals. Digitals generally use an 'algorythim' in order to measure power and current and the voltage.

    Mark Gray -May 05, 2005

    Hi there! Can you please revise the circuit so that it will take into account the reactive component of the load? As you know, AC current is the vector sum of real current (produced by purely resistive load) and reactive current. Reactive current leads real current by 90 degrees if the load is capacitive and lags it by the same degrees if the load is inductive. About 95% of consumer's load is inductive. Thus, real or true power = (V)(I)(power factor) for a single-phase circuit. Power factor is the cosine of an angle between the voltage and current. The smaller is this angle, the closer is the cosine or power factor to unity, the less you pay for your electric bills.

    May I request, if you could provide an schematic diagram for a power factor meter? Using a DSP IC would be much better since the circuit will contain less components. This DSP IC will measure the correct power factor even with the presence of harmonics in the line. Thanks.

    Pete -February 08, 2005

    Has it been built for the home consumer? By who? If not, it should be.

    Fred Browning -October 15, 2004

    artical is excellent documented well and i will visit this for futher clarification

    vibhakar shukla -September 18, 2004

    novice-like idea, but useful circuit

    thomas pillai -February 18, 2004

    The circuit is at best an "AC Current" indicator. Power in AC circuits is the product of RMS voltage and current and POWER FACTOR.

    vineet -February 01, 2004

    wow, tiny circuit but useful to me

    mattahaza -February 01, 2004

    good circuit, really i searched it for so long

    samaraweera -December 24, 2003

    very simple and interesting

    sergio scicluna -December 01, 2003

    Article is incorrect -- the circuit is at best an "AC Current" indicator. Power in AC circuits is the product of RMS voltage and current and POWER FACTOR. The latter was never even mentioned in the article!

    Joseph A. Kelemen -June 03, 2003

    Very simple circuit,and useful for beginners

    SABEER KHAN -May 23, 2003

    Good circuit, useful for many designs. Electronic Design should publish such simple DBs like this one.

    kavinayak -May 13, 2003

    While the circuit may appear to work from a novice point of view, one should be warned that true power is the product of E x I x Cos theta. Without taking into account the power factor, or phase angle, the wattage result can be drastically inaccurate. Therefore, the illustrated circuit will only be valid for purely resistive loads.

    David Green -May 12, 2003

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