Need Help with Full wave rectifier in Pspice

Discussion in 'Homework Help' started by MarsG, Sep 14, 2015.

  1. MarsG

    Thread Starter New Member

    Sep 12, 2015
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    My homework assignment is to design a Full Wave Bridge Rectifier (Attached the circuit I built in pspice and the full description of the assignment below.) The rectifier is to have a filter Capacitor across the Load.
    Given:
    Input = 120v-rms, 60Hz
    Load = 155 Ohms
    Voltage Across individual diodes = .7V (1.4 together)

    The Avg Outpt Voltage is to be 15v with a ripple of +- 1V peak

    Calculations
    120V-rms = 170v pk (Vrms= Vpk/sqrt2)

    with the voltage drop from the diodes (1.4v)
    The transformer has a ratio of 170:17 and a inductance ratio of (1/10)^2

    The ripple frequency is double the source
    60Hz----> 120Hz
    Iload= 15V/155Ohms

    Vripple = Iload/fC

    The transient analysis does not seem right, can someone help me figure out the problem (wiring or maybe the wrong capacitor value).
    I attached a screenshot below.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The filter capacitor must supply the total load current for ~8ms until the next current pulse arrives from the FWB.

    To achieve a given amount of ripple, use the following method.

    I*t = q = C*ΔV

    Rearrange and solve for C

    C=I*t/ΔV

    ΔV (ripple) = 1V, so C= I*0.008/1 = 8*I mF = 8*(15/155)mF = 0.77mF = 770uF (Much bigger than you calculated...)
     
    Russmax likes this.
  3. MarsG

    Thread Starter New Member

    Sep 12, 2015
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    Thank you, I hellso.png had originally put 800uF but thought it was wrong and took another approach, My marker was also in the wrong place.
     
  4. shteii01

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    Feb 19, 2010
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    I thought they said ripple is +-1 volt, would that make delta V = 2 volts?
     
  5. MikeML

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    The problem spec is somewhat ambiguous, but if so, that would cut the required cap in half...
     
  6. MarsG

    Thread Starter New Member

    Sep 12, 2015
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    Oh, I misunderstood it, I thought it was +- 1 of 15V lol.
    ti.png
     
  7. Russmax

    Member

    Sep 3, 2015
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    I'm pretty sure the ripple spec means he cannot go below 14V. Or above 16V.
    Regards
     
  8. shteii01

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    Your average is 15 volts. (14+16)/2=15 volts, that is your average. So the highest value you can have is 16 volts. The lowest value that allowed to have is 14 volts. The delta V is 2 volts, 1 volt above 15, 1 volt below 15. You must stay in this "band" between 14 and 16 volts.
     
  9. MarsG

    Thread Starter New Member

    Sep 12, 2015
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    Should I use a lower input peak voltage at 160V to get a 15V at the output cause right now it seems im slightly above the 16V peak at output
     
  10. shteii01

    AAC Fanatic!

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    I think the ripple is given to you to figure out the smoothing capacitor. Just use MikeML formula to get the capacitor.
     
  11. MarsG

    Thread Starter New Member

    Sep 12, 2015
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    Is the sine wave supposed to drop under 0, cause the peak voltage from peak to peak is actually 17
     
  12. shteii01

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    Forgive me for asking this, but: Why do you give a damn?
    You are given an input.
    You are asked to produce output.
    You are given some details about what should be placed between input and output. Everything else is up to you. If the output of the transformer is something... Is that part of your assignment? Are you required to produce transformer output that have particular behavior? Because if you are... I can not find this part of your assignment. Are you holding out on us?
     
  13. MarsG

    Thread Starter New Member

    Sep 12, 2015
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    Not really, I just don't want to be marked off for anything so im including everything.



    Is their any way to read the values on the simulation from the markers in order to get the current values.
    Current.png
     
  14. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Ok. About that sign wave. Check a few things:
    1. Are you sure the signal has settled? I noticed in your assignment that they note that you want to wait for signal to enter steady state. So maybe if you wait, that signal will rise up and be, like you said, from 0 to 17 volts.
    2. Check if the simulated components have some offset.
    Is your input signal 120 VAC RMS centered on zero? Or is it from 0 to 120? Stuff like that.

    You should be able to get current readings by placing current meter in series. So if you want current thought R1, put current meter in series with R1. If you want current through D2, put current meter in series with D2. And so on.

    Finally. Remember that your diodes drop 0.7 volts each, 1.4 volts total. I think you noted that yourself. So you actually want output of the transformer to be 16+1.4=17.4 volts. I would be happier if I did not actually hit 16 or 14. Just stay inside the band, that is why they gave you that +-1 volt band to contain the ripple.
     
  15. MarsG

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    Sep 12, 2015
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    How would I be able to center it at 0 in pspice? by using an offset?
     
  16. shteii01

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    Hm... I don't use PSpice. For general use I got old copy of MultiSim Textbook Edition that I got with a textbook back in 2000-2002.
     
  17. MarsG

    Thread Starter New Member

    Sep 12, 2015
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    Oh alright. since the output is supposed to be 15V at the load I just added the voltage drop from the diodes (1.4V) at the transformer secondary to be 16.4 and it came down closer to 15V. 169.7 : 16.4 ratio 120V-rms*sqrt(2)=169.7 peak
    cdazc.png
     
  18. shteii01

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    Feb 19, 2010
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    Yes, but now your load voltage drops below 14 volts and that is bad. Your ripple must be between 14 and 16 volts, that is requirement of your assignment.
     
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