# 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.

#### ddickey

Joined Sep 28, 2011
55
Got it, thanks guys.

#### jegues

Joined Sep 13, 2010
733
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.
You should understand that the formula you mentioned above is only valid if your are matching impedances to the primary winding.