help me with maximum power transfer

[zhi_yi]

hi there, please help me with this question..

it said that the maximum power transfer will reached if the internal resistance of the power supply (Rx) equal with the resistance of the load (RL) or Rx = RL. so, if we want to have the maximum power transfer of an amplifier circuit, is it we need to match the impedance of two circuit so the output impedance should be equal with the input impedance of the next circuit (Zout = Zin) ?

and then, from a power supply circuit, for example we make a simple adaptor using bridge rectifier, and then we filter the rectified signal with a capacitor, and then the output from the capasitor we connect it to the load, from that circuit, where does the Rx (internal resistance of a power supply) come from?

thank you all

 

[Borber]

Power supplies does not need any impedance matching. They are supposed to be as much as possible a voltage sources and thus their internal impedance should be as low as possible. If their internal impedance is high they will waste significant amount of power what is not wanted.
Another thing is matching load to power source with significant output impedance like at RF. RF sources like generators and amplifiers has output impedance usually 50Ω and load impedance must be matched to 50Ω for maximal power transfer.

 

[VVV]

Impedance matching is important especially when you are dealing with transmission lines, in order to minimize reflexions and also to really transmit the most power to the load. This is especially important in RF.
But in other types of amplifiers (such as audio, for instance) you do not need that.
Nor is it desirable, since the efficiency drops to 50% (keep in mind that you transfer maximum power to the load, but you dissipate just as much in the generator).

Power supplies are never impedance-matched. Their output impedance is to be as low as possible, to avoid voltage droop and to avoid power loss.
The output resistance of a power supply (how low it is) actually represents one of the criteria to describe how good the power supply is (the so-called load regulation).
Imagine you have a power supply and you add a resistor in series with it, effectively increasing its output resistance. Then the voltage that reaches the load is lower by an amount R*I.

 

[zhi_yi]

okay.. thank you very much.. what is the load regulation? and where does the internal resistance of the power supply come from? is it come from the capasitor and diode? i mean in the simple power supply with only bridge rectifier, and then the filter is only a capasitor..

thank you

 

[VVV]

In the simple rectifier+filter you are describing, the internal resistance is given by the transformer itslf, by the diode and by the cap.
The transformer's voltage drops because of its winding resistances.
The diode contributes only a small portion.
The capacitor contributes because the ripple increases as the load increase. Increased ripple will make the average of the output voltage drop.

 

[zhi_yi]

oo. thank you, and then, what is the winding resistance??is it like XL?

 

[VVV]

The winding resistance is really the DC resistance of the wire of the secondary, plus the reflected resistance of the primary to the secondary:
Rprirefl=Rpri/N^2 , where N is the turns ratio Np/Ns