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

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

1. ### ninjaman Thread Starter Active Member

May 18, 2013
309
1
hello all

I have a problem:

V thev= 22
R thev= 1500

Rload = ? (thought im guessing 1500)

I got these values
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,
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
496
64
Multisim? Use Calculus. A "maximum" ring any bells?

3. ### WBahn Moderator

Mar 31, 2012
24,454
7,653
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
496
64
Because the OP said "Rload = ? (thought im guessing 1500)"

5. ### WBahn Moderator

Mar 31, 2012
24,454
7,653
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
496
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
24,454
7,653
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 Active Member

May 18, 2013
309
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
4,071
1,774
Show your screen shot. That might help us.

10. ### ninjaman Thread Starter Active Member

May 18, 2013
309
1
hello JoeJester

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
4,071
1,774
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
24,454
7,653
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
5,370
2,029
It looks identically connected in both diagrams?

14. ### ninjaman Thread Starter Active Member

May 18, 2013
309
1

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
4,071
1,774
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.

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16. ### WBahn Moderator

Mar 31, 2012
24,454
7,653
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 Active Member

May 18, 2013
309
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