Multisim Voltage Doubler Lab

Discussion in 'Homework Help' started by amano001, Sep 18, 2012.

  1. amano001

    Thread Starter Member

    Sep 7, 2012
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    Unfortunately My lab TA is terrible and does not know anything about the labs we are doing just the final answers. This forum seems to be pretty awesome so here are my list of questions please! I attached a picture of multisim and my lab. I am not getting the output that is expected of double voltage just a weird looking capacitor output. Maybe I am doing something wrong on the osciloscope. After todays lab I have been discouraged by how terrible some of the teachers are here.

    1. It seems to be hooked up correctly according to the lab except in multisim it requests a ground so I assume I put it in the correct place but not sure.

    2. I chose a ac voltage source instead of ac power as the component but not sure exactly if that was correct as ac power shows an RMS value while the ac voltage I chose gives me Vpeak.

    3. From What I have read about this doubler circuit it charges the C2 cap on negative cycles then uses the C1 to add in more DC voltage which gives you the double voltage? but why do they say it must be >10V

    4. The agilent oscilioscope has the two leads for X and Y but when we use it in the lab the BNC gets split into two leads each one going to ground and the other to measure. Does it matter with the simulator that there is no ground lead from X and Y do you just attach it how I have it attached?

    5. Why in a lot of my labs about diodes do they show a transformer then going to the diodes is there any specific point or can I just use the AC voltage source I have shown in my multisim every time. Seems like an extra component just to show that I can step down the voltage?

    6> not related to this lab but about loading effect. When I try to simulate the loading effect for the output of an opamp it doesnt seem to drop the voltage like I thought it would it just stays constant. I think I may be simulating something incorrectly. Its just a basic 741 inverting op amp with a gain of 20 with a load of 1k resistor on the output but when I take the load off its still the same voltage.

    Thanks everyone!
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Your oscilloscope display shows the AC component. To see the DC voltage, set that channel coupling to DC and set the scale to at least 50V/cm. since you are using 50V

    That display is normal when viewing with AC coupling.
     
    Last edited: Sep 19, 2012
  3. amano001

    Thread Starter Member

    Sep 7, 2012
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    Ah ok I also found that I can go to the measurements and set maximum and it outputs 20v from a 10v input. What exactly is that measuring if there really is not an ac component for that circuit?
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You are converting an AC voltage to twice the DC voltage at the output. Can I assume you've already covered Half-wave and full-wave dc supplies?

    In the lab, you are using a transformer to step-down the voltage to a safer level.

    As far as the loading question ... what load values are you using?
     
  5. amano001

    Thread Starter Member

    Sep 7, 2012
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    Yes, I know about rectifiers half wave and full wave.

    I meant in reference to your comment above about the osciloscope only showing the ac component. But the wave that is shown is the dc even without the setting you were speaking of? Do you mean that with the setting I am able to measure it more accurately. I am just confused because I am able to measure the height of that wave and see the doubling of the voltage without touching that setting.

    Yes the transformer question was more in question with one of the parts of the lab where you use the center tap that is grounded to create a full wave with two diodes.

    Thank you for your help!
     
  6. amano001

    Thread Starter Member

    Sep 7, 2012
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    Oh sorry the load values I am using on my inverting amp is 1k resistor. I was assuming I would see a small jump in voltage on the sim because the small current the op amp puts out for the resistor.
     
  7. JoeJester

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    Apr 26, 2005
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    When you use AC coupling on the oscilloscope, you are placing a capacitor in series to "block" the DC from view. This was probably done automatically as you have the scope set to Auto. The numbers above the trace should tell you something. I can't read them on your picture. I do see that channel 2 is 99.xx V which would be almost double of the 50 volts you've put into the doubler.

    The channel 2 signal you are looking at is the AC ripple on the DC. It has the same period as your input signal.

    Here are my simulator's circuit and oscope displays.
     
  8. amano001

    Thread Starter Member

    Sep 7, 2012
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    Ah Ok I finally figured out the DC coupling thing lol. Thank you for that. Why is it that when I am looking at an ac signal it does not matter if its AC or DC coupled it still shows the same signal?

    Also here is the picture of the transformer question I was referring to I was not sure why it needs to be center tapped for this to work. Without the center tap should it not just forward bias the bottom diode and go to load? I understand the centap makes the top +10v and the bottom -10v for example.

    Thanks again!
     
  9. JoeJester

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    Apr 26, 2005
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    DC coupled is Direct Coupling ... like a piece of wire.

    As far as your next question ... without the center tap grounded ... or ... connected to the load, you have no reference between the anode and cathode of the diodes. No current will flow through the resistor and either D1 or D2. D2 only conducts when the Anode is more positive than the cathode. At the time D1's anode is positive, D2 anode is negative. Without that centertap connection, there is no reference for the cathode.

    Post your 741 circuit you have a question about....
     
    Last edited: Sep 21, 2012
  10. amano001

    Thread Starter Member

    Sep 7, 2012
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    Here is the loading effect circuit I would think there would be a voltage drop now that the load is added but it stays the same on the final output.

    Thanks for all your help.
     
  11. amano001

    Thread Starter Member

    Sep 7, 2012
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    Here is the actual lab we had to do.

    From my understanding the 1st one is just a buffer which repeats the signal then the inverting amp has a gain of 220.

    Second one a load is added and should have a voltage drop because now the load is pulling current.

    Third the inverting amplifier is first and the buffer then repeats the signal.

    Fourth is the same as above just with a load added.For this one there should be a voltage drop on the buffer's repeated signal correct? The buffer just protects the original op amps signal from being affected?

    Thank You.
     
  12. JoeJester

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    Apr 26, 2005
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    What is the output impedance of the op amp? Your 1k load is probably way too large.
     
  13. amano001

    Thread Starter Member

    Sep 7, 2012
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    I cant seem to find it on the datasheet but I did read somewhere it said about 75 ohms. Its the 741 op amp by the way. When I do bring the load down near that 75 ohms it starts to drop dramatically why is that exactly.

    Thanks.
     
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