Simple Capacitor and Resistor Circuit

Discussion in 'Homework Help' started by exidez, Sep 12, 2008.

  1. exidez

    Thread Starter Member

    Aug 22, 2008
    26
    0
    I did a lab last week to investigate the affect a capacitor has on a circuit.
    The circuit was simple. 15V(pk-pk) supply in series with a 1kΩ resistor and a 0.18 μF Capacitor. The frequency was at 500 and 2000 Hz. I want to calculate the RMS voltage across the circuit & and then just the capacitor

    I calculate the total impedance to be
    500Hz:
    Z = 1000 - j1768.39 => 2031.55 Ω < -60.57°

    I(RMS) = (7.5/sqrt(2) < 0°) / 2031.55 Ω < -60.57°
    =2.61mA < 60.57°

    then Ohms law V= IR
    2.61mA * 1000Ω = 2.61 V (RMS)

    2000Hz
    Z = 1000 - j442.10 => 1093.37 Ω < -23.85°

    I(RMS) = (7.5/sqrt(2) < 0°) / 1093.37 Ω < -23.85°
    =4.85mA < 23.85°

    Ohms law V= IR
    4.85mA * 1000Ω = 4.85 V (RMS)


    These calculated voltages are different to what the DSO gave me when i tested it
    500Hz
    Circuit
    pk-pk: 14.8V
    freq: 500 Hz
    Period: 2000ms
    RMS: 5.05V

    Capacitor
    pk-pk: 12.8V
    freq: 500 Hz
    Period: 2000ms
    RMS: 4.39V

    2000Hz
    Circuit
    pk-pk: 14.4V
    freq: 2000 Hz
    Period: 500μs
    RMS: 4.9V

    Capacitor
    pk-pk: 5.68V
    freq: 2000 Hz
    Period: 500ms
    RMS: 1.98V


    Have i done something wrong????
    I have simulated this circuit on the computer and the values seem to be right, so am i calculating this wrong???
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Most likely what you are seeing is the fact that the real components you used have a tolerance associated with them.

    hgmjr
     
  3. exidez

    Thread Starter Member

    Aug 22, 2008
    26
    0
    that would explain the the difference in small voltages but when i calculated the RMS Voltage at 2kHz my RMS Voltage was higher than the 500Hz
    Where the DSO gave me a lower RMS voltage for the 2kHz and a higher voltage for the 500Hz
    im sure tolerance is not to blame for that.... ?
     
  4. exidez

    Thread Starter Member

    Aug 22, 2008
    26
    0
    dounf out what i was doing wrong. With ohms law i was multiplying by the resistance and not the reactance! duh

    Anyway, here is my next questions. Why is is that when i was inputting 5.3V RMS, the circuit only displayed 5.05V RMS for 500Hz and 4.9V for 2000Hz ?
    i never changed the voltage at all. There was this .25V RMS drop somewhere from thevery beginning and i dont know where it went. Is this normal?
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Do you mean that the open circuit generator voltage was 5.3V, and it dropped when you loaded it? Signal generators have non-zero output resistance, often 50 ohms. Check the specs on your generator.
     
  6. exidez

    Thread Starter Member

    Aug 22, 2008
    26
    0
    thats probably it. Thanks
     
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