Question

If a transformer steps down a 120-V source to 6.3 V and is connected to an 8 ohm loadWhat is the value of the impedence reflected back to the source?

Answer 1

How do I figure out the value of impedence reflected back to the source?

Answer 2

The secondary current is given by
\[I _{s}=\frac{6.3}{8}\]
The turns ratio n is
\[n=\frac{120}{6.3}\]
The primary current is given by
\[I _{p}=\frac{I _{s}}{n}=\frac{6.3}{8}\times \frac{6.3}{120}\]
The impedance reflected back to the source is
\[Z _{p}=\frac{V _{p}}{I _{p}}=\frac{120\times120\times8}{6.3^{2}}=8\times(\frac{12}{6.3})^{2}\]
So we have the result that impedance reflected back to the source is
\[Z _{p}=\frac{R _{s}}{n ^{2}}\]

Answer 3

\[Z _{p}=8\times(\frac{120}{6.3})^{2}\]

Answer 4

so 2902 ohms?

Answer 5

Yes. I get 2902.5 ohms.

Answer 6

yeah i see how you got that thanks

Answer 7

You're welcome :)

Answer 8

So we have the result that impedance reflected back to the source is
\[Z _{p}=R _{l} \times n ^{2}\]
where
\[R _{l}\ is\ secondary\ load\ resistance\]
and
\[n ^{2}\ is\ the\ primary/secondary\ turns\ ratio\ squared\]