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serpico4321

Hello All,


I am currently hitting a barrier with regards to the basics of this question. It is for a HND so I may have put this in entirely the wrong forum. Apologies if so.


A 50HZ supply is connected to various given impedance's connected in series and parallel. Calculate the value of the load impedance and the maximum power transfer.


Maximum power transfer I understand is basically the Thevenin/Norton equivalent of Rth. But to acquire Rth surely you nee two values of ohms law?


Though THERE IS NO EMF indicated in the question. Is it possible to simply use the frequency given 50Hz and the impedances to calculate the maximum power?


Any help in the right direction would be vastly appreciated
 
Can you not ask your lecturer to expand on this or is it long distance learning, in which case you would still have tutor support?

Sorry I can't help further, i don't understand what you are on about at all! Way above my level.

Good luck.
 
Hi,
You have over simplfied it a bit. The classic case is a radio transmitter which has a resistance at it's output of 50 ohms, to make maximum use of the power the load also has to be 50 ohms. If the load is greater than 50 ohms then you are not at the maximum power transfer level. If the resistance of the load in this case is less than 50 ohms then excess power will be dissiapted in the source and not transfered to the load. Do a paper calculation for the power in both resistances for a load of 49 ohm, 50 ohm and 51 ohms.
 
Hi,
You have over simplfied it a bit. The classic case is a radio transmitter which has a resistance at it's output of 50 ohms, to make maximum use of the power the load also has to be 50 ohms. If the load is greater than 50 ohms then you are not at the maximum power transfer level. If the resistance of the load in this case is less than 50 ohms then excess power will be dissiapted in the source and not transfered to the load. Do a paper calculation for the power in both resistances for a load of 49 ohm, 50 ohm and 51 ohms.

Where does the 50 ohms come from no mention of it in the OP? There was a mention of Thevenin/Norton. Many many years since I did that!
 
Where does the 50 ohms come from no mention of it in the OP? There was a mention of Thevenin/Norton. Many many years since I did that!
The 50 ohm is a standard impedance at RF frequencies, you will find most signal generators have an impedance of 50 ohms to match 50 ohm coax. I was using 50 ohms as an example.
 
Thanks for the replies!
I'm looking to get the maximum power transfer for the load impedance.
AC Circuit Theory Maximum Power Transfer AC Theory - EletriciansForums.net

To calculate the power surely requires either current or voltage to which I am only given the 50Hz supply and the calculated impedance. I've attached a drawing (yeah I know!)


Chr!s,

'There are two methods, the trial and error to plot from 0 - 3 ohms, or the first derivative method'


I've not heard of the trial and error plot, is there any chance (please!) you could elaborate on this. Using either of these applications you can find the maximum power from only Frequency & Impedance?

Thanks
 
Thanks for the replies!
I'm looking to get the maximum power transfer for the load impedance.
View attachment 17445

To calculate the power surely requires either current or voltage to which I am only given the 50Hz supply and the calculated impedance. I've attached a drawing (yeah I know!)


Chr!s,

'There are two methods, the trial and error to plot from 0 - 3 ohms, or the first derivative method'


I've not heard of the trial and error plot, is there any chance (please!) you could elaborate on this. Using either of these applications you can find the maximum power from only Frequency & Impedance?

Thanks

Post the complete question in full.

To be able to calculate the Maximum power transfer you need to know the EMF, im not so sure that is what is being asked of you.

Id of though you need to just find the Thevanin equiverlent, so match the Source to the Load between A and B terminals.
 
Chr!s, thanks.

The question asks: 'determine the value of the load impedance that will dissipate the maximum power and the value of this power.'

As there is no EMF stated and no current given, just frequency and impedance's then i'm sure there is not enough values to compute the maximum power transfer?
 
You've got all the info you need.
Convert all the resistors and the inductor into one impedance.

Do you know the answer?
 
Thanks, Archy. Got that bit, using the modulus of impedance then combining resistors in parallel and in series, came up with 332.53 ohms.

Though to calculate the power transferred is P=I^2xR so would I not need another value? The supply says 50Hz not 50V
 
Agh...I'm doing it again, read the question!

All you can say then is...

P[SUB]MAX[/SUB] = V[SUP]2[/SUP] / Z[SUB]Th[/SUB] ,when Z[SUB]L[/SUB] = Z[SUB]Th[/SUB]


I get a different value by the way, but it's been a long time since I did this stuff.
Knew I shouldn't have got involved! :)
 
Last edited by a moderator:
Chr!s, thanks.

The question asks: 'determine the value of the load impedance that will dissipate the maximum power and the value of this power.'

As there is no EMF stated and no current given, just frequency and impedance's then i'm sure there is not enough values to compute the maximum power transfer?

Rs = Rl for maximum power transfer

So, calculate the thevanin equivalent to find the load.
 
Thanks, Archy. Got that bit, using the modulus of impedance then combining resistors in parallel and in series, came up with 332.53 ohms.

Though to calculate the power transferred is P=I^2xR so would I not need another value? The supply says 50Hz not 50V

It's just asking for a load resistance which will achieve mpt
 
Archy, Thanks for giving it a go, once i'm done with it believe me, I don't want to look at it for a long time! what value did you get out of interest?

Chr!s, The thevenin equivalent is no problem, but i'm assuming the question is asking for a value of the power transferred to the load in watts?

Do you think it is just asking for the load impedance only which will have to be equal to the circuits internal impedance for maximum power transfer? (as you say Rs=Rl). I hope so!
 
Archy, Thanks for giving it a go, once i'm done with it believe me, I don't want to look at it for a long time! what value did you get out of interest?

Chr!s, The thevenin equivalent is no problem, but i'm assuming the question is asking for a value of the power transferred to the load in watts?

Do you think it is just asking for the load impedance only which will have to be equal to the circuits internal impedance for maximum power transfer? (as you say Rs=Rl). I hope so!

Yes load impedance only, it won't matter what the emf is, you have matched the two impedances so Rs=Rl
 
Nice one, It's starting to get clearer, alas, there's still some fog!

Determine the value of the load impedance that will dissipate the maximum power (Thevenin impedance equivalent of the circuit) DONE

And the value of this power? (Do you reckon its just the way its worded?)
 
Nice one, It's starting to get clearer, alas, there's still some fog!

Determine the value of the load impedance that will dissipate the maximum power (Thevenin impedance equivalent of the circuit) DONE

And the value of this power? (Do you reckon its just the way its worded?)

Well if Rs = Rl then the power dissipated in the load will be 50% of the total power dissipated in the circuit.
 

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