Variable Power Supply Multi Sim

Discussion in 'Homework Help' started by amano001, Nov 4, 2012.

  1. amano001

    Thread Starter Member

    Sep 7, 2012
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    0
    Hello everyone,

    I have built this power supply and works wonderfully but when I try to multisim it, I cannot figure out why it does not want to work correctly.
    I am not sure if its maybe the parts or something that I am using wrong but touching the variable resistor does not effect the voltage out and when I try to add a multimeter at the output it says invalid error comes up. The primary to secondary ratio I used was 10 which from my understanding should give me 12 volts on the secondary side but somehow im getting 30v?

    Thanks!
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,238
    Your oscilloscope is hiding part of the circuit so I can't tell what is happening.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Looks like its a LM317 regulator circuit, but your scope is connected to the AC PRIMARY input and not the DC output!!
     
  4. amano001

    Thread Starter Member

    Sep 7, 2012
    35
    0
    Hey guys thanks for taking a look. The oscilloscope is the end of the circuit there is nothing behind it and yes it is a LM317. I will take a look at the input and output where I connected.

    Thanks again.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    ?????????????

    :p:p:p:p

    Look again

    Here's what you posted:
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    I can see that one connection is connected to the Mains input of the transformer.
    The scope is hiding the second connection of the scope.
    There is U2 behind the scope wich is the LM317.

    Bertus
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Why is the mains voltage 120V peak? The mains voltage in North America is 120V RMS which is 169.7V peak.
     
  8. amano001

    Thread Starter Member

    Sep 7, 2012
    35
    0
    lol sorry the pic that I took and the one that got posted were completely different! Here is a better one and with the voltage input I was just trying to make sure the transformer was stepping down the voltage correctly so I went with 120 peak because it was just easier to see for me the step down. The ratio I used for the transformer was primary to secondary turns = 10. Then I tried to add just a 10 to 1 audio transformer but still nothing I keep getting random errors sometimes and it does not want to run.

    I also wanted to add that the I took out the variable resistor just for testing purposes and put a 1000 ohm one in for R2.

    The formula I used for the LM 317 was v = 1.25(1+R2/R1) which with my setup should output 7volts. Which seems to be the correct voltage on the output but the input is still not working correctly. The input into the LM317 is fluctuating up and down a lot on the oscilloscope but on the multimeter it holds steady at 15volts. Is it maybe something im reading wrong or hooked up wrong when doing my readings? I have the oscilloscope is DC coupled.

    Thanks again !
     
    Last edited: Nov 6, 2012
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You posted two new schematics.
    The second one has a tiny 1uF main filter capacitor so of course the input voltage to the regulator has lots of ripple.

    I don't know why your SIM software thinks the input to the regulator is 15V.
    Oh, it is Multisim messing up again.

    Why aren't you using a power transformer? Its input and output voltages will be labelled.
     
  10. amano001

    Thread Starter Member

    Sep 7, 2012
    35
    0
    Hey I am so sorry I was trying to mess with a lot of the components and forgot to switch them back. I changed the capacitor back to 1000uF and put in a power transformer 10 to 1. I am now getting 22v on the input which still does not make sense to me.

    Here is the new picture.

    Thanks.
     
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    The 10:1 transformer reduces the 120V peak on the primary to 12V peak on the secondary. The bridge rectifier charges the main filter capacitor to +10.6V since there is no load.
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    The 22 volts you see is probably because you have 12 volts between the center-tap and each of the other sides.
     
  13. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    use just the voltage source and the transformer.

    Measure the voltage between the center tap and either side and measure the two ends (no connection to the center tap).

    You should see 24VAC peak on the end to end measurement and 12VAC peak on the center tap to either end.

    120V peak is not the same as the 120Vrms that comes out of an outlet as explained to you earlier.
     
  14. amano001

    Thread Starter Member

    Sep 7, 2012
    35
    0
    Hello again,

    So I think I figured out the problem was a multisim issue or a part I picked but when I finally switched out the bridge to another model it worked. Here is a picture of all the readings one before the bridge one after and one at the final.

    The only thing I dont quite grasp fully is the RMS and Peak, I do understand the difference but I am trying to make sure I understand the readings on the multimeters mostly.

    So out of the transformer makes sense its 12v RMS which is 16v Peak approx. Then it goes through the bridge (this is where I get confused) does it become then 32v peak 22 RMS because of where I took the reading it now after the bridge which combines both the positive swing and negative swing of the AC signal? and is the 32 volts peak or RMS? and is the 7volts on the output RMS or Peak? I get confused since I associate RMS with an AC signal and once its rectified I feel like it becomes a peak voltage for some reason.


    I just wanted to add that with the 2 diode rectifier is only giving me 16 volts after the 2 diode bridge which I would assume would give me the same read after the 4 diode bridge of 32 volts, Why is that exactly. Does this not mean that I am getting less voltage from the 2 diode bridge over the 4 diode bridge?

    Please clarify and you guys rock btw thanks so much for your time!
     
    Last edited: Nov 8, 2012
  15. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    In the first picture, you have set xmm3 to DC, but the output of the transformer will be AC.
    This is why there will be a faulty reading on that meter.

    Bertus
     
  16. amano001

    Thread Starter Member

    Sep 7, 2012
    35
    0
    Hey I just wanted to ask a quick question about this rectifier circuit. When You have the top and bottom connected of the transformer and say its 12 volts measured from the top to ground and then also 12 volts measured from the bottom to ground essentially you have 24 VAC across the transformer correct? Then does that mean that 24volts is also going across the load after it is rectified? I am getting confused with some of the pictures I see of rectification that the negative swing is just being inverted to a positive swing there by only giving 12 volts across the load for both positive and negative swings. But what I have seen in some formulas like 2vmax/pi seems to give me the other version that actually the load sees 24 volts = 12 volts from the positive swing and 12 volts from the negative swing that is being rectified to positive.

    Thanks.
     
  17. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    Have a look at this page of the eBook:
    Rectifier circuits
    And a capacitor after the rectifier will act as a kind of peak detector:
    Peak detector

    Bertus
     
  18. amano001

    Thread Starter Member

    Sep 7, 2012
    35
    0
    Here is my final question lol I swear!

    Why is it my math does not match up at all with what multisim is saying

    2vpeak/pi should give me 2*(24rms*sqrt2)/3.14 = 21.6 approx but when I change the capacitor right after the rectifier it keeps increasing my voltage? The capacitor from my understanding should just be smoothing out the 21.9 volts not adding to its amplitude. Is multisim screwing up or something?

    here is two pictures below one with .1 uf cap and one with 1000 uf cap both (24.75 and 32 respectively) are way off from the 21.6 I should be getting.
     
  19. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    Multisim is almost always wrong.
    Use an accurate more modern SIM program like LTspiceIV.
     
  20. amano001

    Thread Starter Member

    Sep 7, 2012
    35
    0
    Here was my attempt at LTspice lol I can't seem to get it working properly, not sure what is wrong. I am using the .op point to see the voltages on different wires DC after the rectifier but getting no voltage. I put together the transformer like in the tutorial and made a spice directive to connect them. The inductance ratio is 100 to 1 which should be 10 to 1 voltage ratio from my understanding.

    The regulator is 1.5A positive adjustable and just some regular diodes from the bar. Not sure about the sine wave I created at all I went grabbed a voltage source turned on Sine made Vamp 120v to try and get 12 v on the out and 60hz.

    Thanks!
     
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