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LT Spice - Bridge Rectifier

  1. Dec 10, 2008 #1
    1. The problem statement, all variables and given/known data
    I am to build a linear power supply in four stages. However, at stage two, i have to add a bridge rectifier to the ouput of my transformer. My lecturer at university says i should get a graph that is a sine wave, with the negative values reflected into the positive x axis (Volatage). I was wondering were am i going wrong?

    2. The attempt at a solution
    http://img399.imageshack.us/img399/1367/ltspice1gm7.jpg [Broken]

    Any help would be greatly appreciated.

    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Dec 10, 2008 #2

    The Electrician

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    Gold Member

    You need to plot the difference between rectifidp and rectifidn. Or, alternatively, plot the voltage across your load resistor, R5.
  4. Dec 10, 2008 #3
    Yeah, had a suspicion it might have been that, but it took me a while to work out how to enter an expression. Only started using the program a few days ago, so i'm still getting used to it.

    http://img218.imageshack.us/img218/138/ltspice2rc0.jpg [Broken]

    Many thanks for your help.
    Last edited by a moderator: May 3, 2017
  5. Dec 10, 2008 #4
    I've got another problem. For a full wave bridge, with a capacitive filter, we are told the following relationship between Vdc [V(Regout)-V(Rectifidn)] and Vac [V(Secondary)]:

    Vdc = 1.41 x Vac

    However, i'm obtaining the following results:

    http://img386.imageshack.us/img386/5310/lt4vr1.jpg [Broken]

    I'm just wondering if i have the wrong formula, or taking incorrect readings?

    Last edited by a moderator: May 3, 2017
  6. Dec 10, 2008 #5


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    Homework Helper

    A couple of things to keep in mind here:

    That formula uses the rms voltage, not the amplitude, for Vac.

    Also, it does not account for the 2 diode drops in the rectifier voltage.
  7. Dec 10, 2008 #6
    Ah, forgot about rms. Thanks.

    That equation was given a bridge rectifier (4 diodes) with a capacitor and load resistor, but exluding the regulator. So i assume to calculate Vac, if i want Vdc of 5, i simply calculate 5+the voltage acrross the regultor, all divided by 1.41.
  8. Dec 11, 2008 #7


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    Homework Helper

    You can check that on your Spice circuit, "transformer1 stage 5".

    If you plot

    V(rectifidp) - V(rectifidn) ​
    V(secondary) ​

    you can see if the regulator input voltage equals the amplitude of the transformer secondary.
  9. Dec 11, 2008 #8
    Yeah, thanks for your help.
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