bridge rectifier issue

Discussion in 'The Projects Forum' started by idmond, Nov 1, 2010.

  1. idmond

    Thread Starter Member

    Oct 5, 2010
    18
    1
    Hi all
    our professor told us to design a project for full wave rectifier with bridge circuit.
    well, i used 220v -to- 16v center tapped transformer and connected only one half of the terminals(the middle and right wires) to the bridge circuit (4 diodes). then, i measured the voltage on the terminals of the transformer and after the bridge and everything was fine (about 15.6v AC after the transformer and the same voltage DC after the bridge) , but when i applied a smoothing capacitor (47000uf/50v) after the bridge to reduce the ripple voltage and measured the voltage after the capacitor, it became 23v dc :confused: with no AC!!!

    How 15.6v dc suddenly become 23v dc after the capacitor! i just can't get it.

    any ideas on what i might have missed or forgot to do??!!
     
    Last edited: Nov 1, 2010
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The 16v rating is an RMS rating; or Root-Means-Squared. Basically, it's an average of one-half of the wave's voltage.

    When you rectify and filter it, the sine waves' peak voltage (less the Vf, or forward voltage, of the diodes in the bridge) will be accumulated across the filter capacitor. If there is no load on the filter capacitor, the Vf of the diodes will be negligible. The DC voltage will be 1.4142 * the RMS AC voltage, less the bridge diodes' Vf, or V(RMS) / .707107 - Vf * 2
     
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  3. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
  4. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
    In the rectifiers I've looked at, ∆V (of a diode pair) is ~1v at 0.2mA of current and still reads at ~0.75v with no load other than the DVM. To my thinking this is not negligible.
     
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