# Why don't transformers have capacitors?

I recently learnt about how resonance in an LC (inductor capacitor) circuit could increase the efficiency of the circuit, for application in something like Resonant Inductive Power Transfer.

Excuse my naivety, but I couldn't help but wonder then why don't transformers also have a capacitor in them to achieve resonance and increase efficiency? Is it due to requiring too large a capacitance or because it wouldn't be worthwhile?

• If it works you should patent it, not ask the question here :) – my2cts Nov 20 '18 at 12:38
• An ideal magnetic transformer has essentially infinite bandwidth but despite its name an ideal magnetic transformer has no internal inductors and operates as an ideal impedance scaler, that is for $Z_{load}$ the apparent input impedance is $kZ_{load}$ for any load impedance. If you include a capacitor to improve the impedance match by resonating the inductance then the operating bandwidth of your match narrows down. And of course it is done everywhere in RF circuits to improve the match to antennas and to amplifiers. en.wikipedia.org/wiki/Transformer#/media/File:TREQCCT.jpg – hyportnex Nov 20 '18 at 14:25