# AC - max power transfer

Discussion in 'Homework Help' started by StasKO, Jan 6, 2013.

Apr 28, 2012
48
0
hello everyone!

i think this problem gonna blow my head off.
its a lab experiment, and the subject is max power transfer theorem with an AC source and reactive components.

the circuit is in the pdf file.

what i did is calculate Zth and then taking its conjugate for Zload.

now im asked to simulate the power on the load as a function of frequency. as i understand it, peak power should be at the frequency i chose for the source, in this case - 10 kHz.

but i always get the simulation output not quite right. in it the max power is at a different frequency and i cant understand why.

thnx

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2. ### WBahn Moderator

Mar 31, 2012
17,748
4,796
1) Are you sure that you have calculated Zth and Z load correctly?

2) What kind of simulation are you performing? Are you sure that your measurements are being taken only in AC steady state and not being affected by initial transients?

Apr 28, 2012
48
0
1) yes. i double and triple checked it
2)im doing an ac sweep/noise simulation. i checked and there are no initial conditions for the reactive components.

Last edited: Jan 6, 2013
4. ### WBahn Moderator

Mar 31, 2012
17,748
4,796
Are you sure your calculation is returning real power and not apparent power?

Apr 28, 2012
48
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well all i did is calculate Zload using Zth. then i put the the corresponding components (a resistor and a capacitor) in capture. and simulated the voltage across the load times the currents through it. am i wrong this way? all i expected is to see peak voltage at the frequency that the thevenin and load impedances were calculated.

6. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
The simulation is showing power across R5

There isn't an R5 in your schematic (RLoad would be R4), it also shows the source as 5VDC.

Apr 28, 2012
48
0
yeah i changed the name of the load components just before i uploaded the pdf's.
i played a lot with the schematics so R5 is ok. it is Rload.

its 5VAC and 0VDC. i just deleted the letters VAC and left the 0VDC.

well after thinking about it i want to ask a question. because it seems that im misunderstanding something because Zth and Zload are correct.
my question is:

lets pretend that i have a circuit like the one i posted. i choose the components values (the R's, L's and C's) without knowing what the Zload should be for max power transfer. now i also choose the frequency. next, to find the values of the load, i calculate the thevenin equivalent impedance. this impedance will be dependent on the components values and the frequency i choose. now after i chose values i can claculate Zth and Zload for max power transfer. from the value of Zload i extract the R and C or L value. now in the original circuit i insert the newly found load components (which i remind you were found in a specific frequency that i randomly chose) and simulate the circuit using AC sweep and ploting the power on the load as a function of frequency. now here comes the confusion - should i always see the max power on the load (as a function of frequency) in the frequency which the load components were originally calculated or it doesnt have to be this way. my thoughts is that it doesnt have to be that way because at some frequencies, power could be higher than the power dissipated at max power transfer condition in the specific frequency that the load components were calculated. its just that at the other frequencies this power will not be the maximum power possible (since only one frequency meets the max power transfer condition).

am i right in my thinking? (and please tell me if i confused you more lol)
if im correct than its ok to see higher power on the load in frequencies other than the one that the load components were calculated.. right?

8. ### WBahn Moderator

Mar 31, 2012
17,748
4,796
Yes, you are correct. Setting Zload equal to Zth results in max power transfer compared to any other value of the load. But, as you change frequency, you are changing both Zload and Zth and there is no guarantee that they are both still equal. If Zth goes down and Zload stays about the same, it is entirely possible to get more power into the load at the new frequency than when they were matched. For the DC case with just resistors, this would be analgous to reducing the source resistance while keeping the load resistance the same.