I have created a transformer using a small cylinder (a tube) made of ferrite. The primary coil has \$N_{1}=5\$ turns of coil wire, while the secondary \$N_{2}=50\$ turns. According to a recent question of mine, a real transformer has an equivalent circuit like the following: Where:

  • \$L_{P}\$ is the primary leakage inductance
  • \$R_{P}\$ is the primary copper loss
  • \$R_{C}\$ is the core losses due to eddy currents and hysteresis
  • \$L_{M}\$ is the magnetization inductance
  • \$L_{S}\$ is the secondary leakage inductance
  • \$R_{S}\$ is the secondary copper loss

(taken from here)

My question is how can I determine each quantity in the above circuit, using \$N_{1}, N_{2}\$? If any additional information is needed please let me know (plus how can I calculate them, if it is not obvious). I don't seek to create an 100% precise model, just a circuit that works correctly (for example with an AC voltage source connected to the primary coil and a capacitor connected to the secondary coil the circuit must work like a band-pass filter).


1 Answer 1


It’s really tricky to theorize on leakage inductance given only that you have a non-looping ferrite rod. If the rod was in fact a full magnetic core you can make the assumption that all the flux produced took the path through the core. But, then, the problem would be that there can be no leakage inductance i.e. the two coils would be perfectly coupled and, when perfectly coupled, there is no leakage inductance to form a tuned circuit with the secondary capacitor.

I would suggest that you measure the inductance of the secondary with the primary shorted in order to ascertain the effective leakage inductance that would form the tuned resonant circuit.

  • \$\begingroup\$ Thank you. What about \$L_{M}, R_{C}\$? \$\endgroup\$
    – Bram Fran
    Dec 30, 2019 at 11:27
  • \$\begingroup\$ Lm can be measured by leaving the secondary open circuit. Although it will be in series with Lp it will usually swamp it but, if you are just after the effective inductance that can be used to calculate the tuned frequency, the method in my answer is the most direct way. Resistance vales can be estimated from the winding length of each turn and using the diameter and resistivity of the wire material however, if operating at frequencies where skin effect is significant, you need to take it into account. \$\endgroup\$
    – Andy aka
    Dec 30, 2019 at 11:48

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