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Discussion in 'General Electronics Chat' started by anathema, Mar 22, 2004.

  1. anathema

    Thread Starter Member

    Feb 29, 2004
    17
    0
    Hi all, I'm just a student and have only few experiences with real circuits. I just open an equipment and look that there is a 0.01 capacitor and 10ohm resistor in parallel between the chassis ground (connected to the ground of the wall) and the circuit ground. The connection is made after the transformer, after the rectifier section.
    Maybe sound stupid to many but, what do they do (R and C)? What happened if they were connected before the rectifier section? or if both grounds were shorted?

    Anathema
     
  2. anathema

    Thread Starter Member

    Feb 29, 2004
    17
    0
    sorry for the two messages, I wasn't sure if the first was posted, an error appear
     
  3. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    143
    The R C configuration after the rectifier sounds like a smoothing circuit. After the transformer the AC will be rectified into positive AC pulses, however this is relatively useless for a practical application. The capacitor charges on the rising rectified AC waveform edge, and discharges on the falling edge, this has the effect of smoothing the rectified waveform out. The capacitor rating is dependant on the time between AC waveform peaks dt and is aimed to keep the voltage drop off dV to a minimum. Ideally dV/dt should be equal to 0.

    If the R C network was placed before the rectifier, the capactor would charge and discharge progressively with each positive and negative AC waveforms (i.e. the pre-rectified waveforms). Ultimately the effect at the rectifier output would be minimal.
     
  4. Optikon

    New Member

    Mar 18, 2004
    8
    0
    Apparently, they seem to not be part of the rectifier section. Without more details, my guess is that they are there to shunt common-mode current back to earth ground (chassis). For all practical purposes, the 10 Ohm connects the circuit ground to chassis ground (why this is performed has to do with the design details)
     
  5. anathema

    Thread Starter Member

    Feb 29, 2004
    17
    0
    I think you are right Optikon ;) . I've just read a paper from Jensen (www.iso-max.com) showing a "star grounding" configuration between earth ground (third prong), chassis and circuit ground, like the one of the power supply I looked. They say this configuration avoids ground noise coupling.
    The connection must shunt hum and common-mode current noises to earth ground, but wouldn't be better if the grounds were shorted? It's there some kind of rule to put impedance between the grounds in the star ground config.?

    Anathema :)
     
  6. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    :) hi,

    i think what he has there is a capacitance multiplier supply. the cap and resistor are there to reduce higher order harmonics. it has nothing to do with rectification. :)

    anathema, as to your other post about mic pre-amp the coil value cannot be guessed since we have not seen the schem. although we would probably have a calculated estimate. what kind of schem do you have that it does not state the value of the coil in "microhenries"

    as a suggestion do a google search for mic pre-amp that uses a single/double transistors. you would have an easier time making it. :)
     
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