max power transfer, multisim (yes i said multisim!) problem

Discussion in 'Homework Help' started by ninjaman, Oct 30, 2014.

  1. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello all

    I have a problem:

    V thev= 22
    R thev= 1500

    Rload = ? (thought im guessing 1500)

    I got these values
    Rthev Rload total
    e 10.9995 10.9995 22
    I 7.333mA 7.333mA 7.333mA
    r 1500 1500 3000
    p 80.65mW 80.65mW 161.318mW

    I tried this on multisim and found that I was getting 80.65mW but when I used a 1500 ohm load I got zero W, nothing at all. if I changed the load to 1505 I got 80.66mW, 1499 I got 80.66mW. but nothing with 1500. I have to do a screen print to prove my maths but cant get the power to show???
    any help would be great

    also, my method
    I removed source,
    added two resistors,
    voltage over total resistance equals current,
    current times Rthev equals voltage drop across Rthev
    current times Rload equals voltage drop across Rload
    then current squared times Rthev for power, and
    then current squared times Rload for power

    thanks

    simon
     
  2. amilton542

    Active Member

    Nov 13, 2010
    494
    64
    Multisim? Use Calculus. A "maximum" ring any bells?
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    Why are you guessing about what load impedance will result in maximum power transfer. The whole point of learning about this is to KNOW that.

    I'm confused. You provide a table of the results you got and it includes 80.65 mW for a 1500 Ω load, but you then say that you got 0 W for a 1500 Ω load. Which is it.
     
    Last edited: Oct 30, 2014
  4. amilton542

    Active Member

    Nov 13, 2010
    494
    64
    Because the OP said "Rload = ? (thought im guessing 1500)"
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    Maybe I'm misreading things, but I interpreted this as being the question asked what is Rload need to be equal to to achieve max power transfer and the OP is guessing that it is 1500 Ω.
     
  6. amilton542

    Active Member

    Nov 13, 2010
    494
    64
    Oh no, I've done it again. That's twice this week. Forget what I said, I've misread your post.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    No problemo. Once more and you win the trifecta! (Unless, of course, I come from behind and beat you, always possible.)
     
  8. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1

    hello WBahn

    sorry I didn't write it correctly. I used a 1500 ohm resistor as the load and got nothing on the power meter, then I changed the resistance by 5 ohms to 1505 ohms. this gave me a value. so I think its something that multisim didn't like. this is the problem im having. I have to do a print screen showing the value as proof.
    the question was does anyone know what I can do to get the power meter working with 1500 ohm exactly.

    thanks


    simon
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    Show your screen shot. That might help us.
     
  10. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello JoeJester

    upload_2014-10-31_16-37-57.png upload_2014-10-31_16-38-30.png
    in the one on the right I changed the load resistance to 1501 ohms and can measure the power output, but I get nothing with 1500 ohm resistor.
    this is the problem im having.

    any help!!!
    thanks
    simon
     
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    It would be more helpful if we could see how you connected the meter with the 1500 ohm load.
     
  12. WBahn

    Moderator

    Mar 31, 2012
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    Can you display what it is seeing for the voltage and current values separately?

    I agree that something hinky appears to be going on.

    What happens if you fix the load resistor at 1500 Ω and make slight changes up and down to the other components? What if you cut the values of all the components in half or double them?
     
  13. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    2,802
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    It looks identically connected in both diagrams? :confused:
     
  14. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    upload_2014-10-31_18-4-22.png upload_2014-10-31_18-5-26.png
    upload_2014-10-31_18-7-35.png upload_2014-10-31_18-8-8.png
    here with half value resistor twice the current.
    I don't think multisim likes it when I do this.
     
  15. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I know it looks like the connection is fine .... and it should work, unless ... can you move R2 and make sure that line connected to the top of R2 moves with your relocation. That is the ONLY way I can see you would get such an indication IF the connected line doesn't move.

    I don't use multisim, but, if you ran a dc analysis stepping the resistance of R2 from o to 3k ohms you would see something like the chart below. The same could be seen if you did the same thing in excel spreadsheet.
     
  16. WBahn

    Moderator

    Mar 31, 2012
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    What happens if you connect the wattmeter to the other resistor (when they are both evenly matched)?

    Have you tried opening up a brand new schematic and doing it from scratch to see if you get the same weird results?
     
  17. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello WBahn

    I have tried it a few ways and still get the 0 Watts. I did however try the wattmeter and a separate volt meter across the load and that worked. which is strange. I think I will do that for my result. as long as I can show the correct value.

    thanks for your help fellas!

    simon
     
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