# Transformer impedance question

#### ddickey

Joined Sep 28, 2011
55
Primary has 100 turns.
Secondary #1 has 50 turns and a 10 ohm impedance.
Secondary #2 has 25 turns and an impedance of 25 ohm.
What is the primary impedance. I know the answer but don't know how it was derived. Any help would be appreciated.

#### bountyhunter

Joined Sep 7, 2009
2,512
I think you calculate the primary impedance from each secondary (using the turns ratio) and then parallel the two values on the primary side. I can't remember if the impedance reflects back using the turns ratio or the turns ratio squared? I suspect the latter.

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#### jegues

Joined Sep 13, 2010
733
Use impedance matching to match the impedance on secondary #2 to secondary #1. Combine the two impedances and match that impedance to the primary winding.

Impedance matching is related to the square of the two turns ratio through which you are matching.

#### ddickey

Joined Sep 28, 2011
55
The formula that my textbook taught number of windings primary/by number of secondary, square the answer and x the secondary impedance so my answer was coming up 62 ohms. Bounty Hunter, your answer is correct.