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Calculating burden resistor

Discussion in 'General Electronics Discussion' started by Esteban, Nov 22, 2015.

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  1. Esteban

    Esteban

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    May 24, 2015
    Hi!

    I'm using a 50/1 A CT and I want to read current passing through a single phase motor with a microcontroller, I've done all the programming and rectifying stuff and It's perfectly working with a 100mV to 1V 60Hz sine wave provided by a function generator.

    I've also simulated this ammeter project and it's working (the circuit is uploaded on this post), but the problem is that I'm not sure if my CT is working.

    I think that for a 1Ω burden resistor, the voltage on the resistor should be Vo = Ip*0.02*1Ω. Where Ip is the actual current flowing through the phase and 0.02 = 1/50. So I'm trying to measure the voltage with different resistor values and check if it's the voltage expected, what resistor value do you recommend for a CT burden resistor?

    I've check information about calculating burden resistor on the browser, but it's all about increasing the voltage to a Vref+/2 level when a dc bias is used. Since I'm using a opamp base precision rectifier, I'm not strictly calculating an specific burden resistor value.

    Any suggestion will be helpful. Thanks.
     

    Attached Files:

  2. GPG

    GPG

    452
    66
    Sep 18, 2015
    What max current? does the rectifier output need filtering?
    Use a better opamp than 741
     
    Last edited: Nov 22, 2015
  3. Esteban

    Esteban

    15
    0
    May 24, 2015
    Thanks GPG for replying

    Sorry what do you mean with max current? The max current that can afford the CT?

    The rectifier does not need filtering, the μC is programmed in order to capture and hold the maximum value read on the adc channel.

    What opamp do you recommend?
     
  4. GPG

    GPG

    452
    66
    Sep 18, 2015
    Max current that you wish to measure and the range of ADC .

    LM358 is internally compensated dual for example.Multiple sources,variety of packages
     
    Last edited: Nov 22, 2015
    Esteban likes this.
  5. Esteban

    Esteban

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    May 24, 2015
    Thanks!

    From the single phase motor I'm using, the maximum current that I'm expecting to measure is 30 A at primary winding, which will be 600mA at secondary winding.

    I'm also using a pic18f4550, it has a resolution of 10 bits and can be adjust between 0V and 5V.

    Should I use any specific burden resistor value? I'm not expecting more than 1.5 A at secondary winding.
     
    Last edited: Nov 22, 2015
  6. GPG

    GPG

    452
    66
    Sep 18, 2015
    Hence the Q abt range. You can set the resistor value to a convenient value eg power dissipation may be a factor so for 1W dissipation R = 1/1.5A² =.44Ω
    Say .39Ω (E12 value) this will give .585V RMS across the resistor = .827pk
    If the output of the rectifier is to be ~5V the reduce the input resistor to ~ (.827/5) x 10K = 1.6K nearest E12 1.8K Got to err on the lower side for headroom =~4.5V pk
     
  7. GPG

    GPG

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    66
    Sep 18, 2015
    I see you shifted the goalposts! You can repeat the calcs if you wish
    If you make it more sensitive put a clamp on the adc input to protect it against overvoltage edit Put one on it anyway
    And the resistor should be connected ala
    https://www.google.co.nz/search?q=k...-8&oe=utf-8&gws_rd=cr&ei=CINRVqSnHOLXmgWWnboo
     
    Last edited: Nov 22, 2015
  8. Esteban

    Esteban

    15
    0
    May 24, 2015
    Thanks GPG for replying!

    This rectifier does not invert the input.

    I do not need to amplify the signal on the rectifier, for a 1Ω burden resistor: the μC takes the maximum voltage sampled in 25ms and holds the maximum voltage which will be Vpeak = 1Ω*Ipeak (any other resistance value can easily work at code), since I'm not expecting a max current greater than 1.5A, 1.5A * 1.4142 = 2.1213 amps peak, then μC Vref+ could be adjust to 2.2 V,the problem is that if I use a burden resistor greater than 2.35Ω I'll get a voltage with more than 5 Vpeak and the controller cannot measure voltages greater than it's bigger Vdd :/!

    So I've to limit the burden resistor to 2.35Ω or less... you mentioned power dissipation is there any other factor I should consider?

    Sorry I didn't understand the last quote :(
     
  9. GPG

    GPG

    452
    66
    Sep 18, 2015
    N
    The sign is a tilde not negative which I have used to designate "about"
    https://en.wikipedia.org/wiki/Tilde
    Its the main one. In the example I used 1W. A 2.2Ω will dissipate 3.3W
    The rectifier will produce an amplified waveform by changing one resistor enabling a lower burden resistor
    The rectifier should be connected directly to the burden resistors leads
     
    Esteban likes this.
  10. Esteban

    Esteban

    15
    0
    May 24, 2015
    Hi!, I finally used the circuit with an AC motor and it's perfectly working yeeiii :D!

    But I've a problem with the rectifier dual supply :(

    I have used a very basic ±12V power supply and also a pc power supply for the opamps, using any of them the rectifier output started giving crap signal. After noticing it, I tried the school laboratory BK Precision DC power supply and the rectifier is perfectly working.

    Is there any DC power supply circuit you recommend? I've to put all the project together on a single board so I need a dc supplier maybe 6 volts, I' will change 741 for a single power supply opamp an LM358 if I get one.
     
  11. GPG

    GPG

    452
    66
    Sep 18, 2015
    1) Bypasses on amplifier supplies 10μF in parallel with .1μF ceramic on + and - near the supply pins.
    2) have you compensated the 741s? (LM358 is internally compensated and one will replace both 741s)
    3) You need a dual supply to run the rectifier
     
    Last edited: Nov 24, 2015
  12. Esteban

    Esteban

    15
    0
    May 24, 2015
    Thanks man! I've thought bypasses but I've never compensated op amps, by compesated you mean frequency compensation right? I gonna search about it.
     
  13. GPG

    GPG

    452
    66
    Sep 18, 2015
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