# How a Secondary Resistor affects the Transformer Primary Side?

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## Main Question or Discussion Point

Let's assume a 2:1 transformer which has a 100V Source connected on the primary circuit and has no/negligible resistance, on the secondary circuit a 5 Ohms resistor is connected. Using the 'Impedance Transfer/Reflection' method, the primary circuit would act as if there was a 25 Ohms resistor connected to it, which means a current of 4 Amps would be flowing in the Primary.

My question is how the 5 Ohms resistor from the secondary side 'actually' affects/creates a 'resistance-like effect on the primary side? Aren't they separate circuits? Or does it has to do with the magnetic field that links one another?

A simple concise explanation would be very helpful!

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anorlunda
Mentor
Aren't they separate circuits? Or does it has to do with the magnetic field that links one another?
No they are not really separate. They are mutually coupled by the magnetic fields as you guessed.

No they are not really separate. They are mutually coupled by the magnetic fields as you guessed.
Thanks for the reply, just like what I thought (they are magnetically linked), but I am just having a hard time on how the 5 Ohms Resistor causes the Primary to act as if there was a 25 Ohms resistor, I'm guessing it has relation to Lenz's Law (Back EMF) from the secondary winding producing an opposing current in the Primary, but there still isn't any clarification and further explanation regarding the question above.

anorlunda
Mentor
The mutual coupling comes via the time changing currents in the coils. So if the secondary resistance was infinite, then no secondary current and there is no coupling.

So, i isn't that intuitive that changing the secondary resistance changes both secondary and primary currents?

The mutual coupling comes via the time changing currents in the coils. So if the secondary resistance was infinite, then no secondary current and there is no coupling.

So, i isn't that intuitive that changing the secondary resistance changes both secondary and primary currents?
Yes
The mutual coupling comes via the time changing currents in the coils. So if the secondary resistance was infinite, then no secondary current and there is no coupling.

So, i isn't that intuitive that changing the secondary resistance changes both secondary and primary currents?
Yes, it affects both of them, I just don't understand how the resistor from the secondary creates a resistance like effect in the primary (25 Ohms), the fact that the primary circuit doesn't have any resistor connected to it.

anorlunda
Mentor
Yes, it affects both of them, I just don't understand how the resistor from the secondary creates a resistance like effect in the primary (25 Ohms), the fact that the primary circuit doesn't have any resistor connected to it.
You sound confused. If it affects both of them, how would you describe the effect? Would the effect not look analogous to a resistance in the primary circuit?

rude man
Homework Helper
Gold Member
View attachment 239280

Let's assume a 2:1 transformer which has a 100V Source connected on the primary circuit and has no/negligible resistance, on the secondary circuit a 5 Ohms resistor is connected. Using the 'Impedance Transfer/Reflection' method, the primary circuit would act as if there was a 25 Ohms resistor connected to it, which means a current of 4 Amps would be flowing in the Primary.

My question is how the 5 Ohms resistor from the secondary side 'actually' affects/creates a 'resistance-like effect on the primary side? Aren't they separate circuits? Or does it has to do with the magnetic field that links one another?
The secondary resistance reflected into the primary is 20 ohms, not 25 ohms.

The transformer, containing no resistance in either primary or secondary windings, itself dissipates no power. So in order to conserve energy, the power supplied to the load must equal the power furnished by the primary source.

So V22/R2 = V12/R1. So the primary source MUST "see" R1 = (V1/V2)2R2 = 22R2 = 20Ω.

Whethere or not you understand principles of magnetic coupling, energy conservation forces you to realize that the primary source "sees" 20Ω.