OT : determining the impedance ratio

Let’s say I have an unknown output transformer that I want to know “the impedance” …

A transformer, through electromagnetic induction, “transfers” alternating electric energy from the primary to the secondary (AC current)

A load Z2 connected at the secondary is seen at the primary as an impedance Z1. This change in the impedance is the impedance ratio of the transformer.

If N1 is the number of turns in the primary and N2 the number of turns in the secondary, the ratio voltage presented to the primary / induced voltage in the secondary is as follows:

output power transformer voltage ratioIdeally, all the power is transmitted from the primary to the secondary :

power output transformer power impedance ratio

So impedance ratio is the square of the turns ratio :
Impedance ratio transformer

Following this schematic, if I measure V1 and V2, I can deduce the ratio :

OT schematic measure impedance ratio voltage(R1 = 100R is to measure the voltage accross R1 and deduce current, not to burn my PT !)

My PT is 230V at primary, 36V RMS at the secondary.

At first, I test a known OT (5kΩ at the primary for 16Ω at the secondary) :

output transformer ratio impedance V1 known transformerPT (left) and OT (right) – V1 : 36V

output transformer ratio impedance V2 known transformerV2 : 2V

So : output transformer ratio impedance V1 / V2 known transformerAnd then a 16Ω loudspeaker at the secondary will be seen as a 16 x 324 = 5184Ω at the primary. That’s correct for an OT given for 5kΩ !

Now, let’s test my unknown OT (from a Vox Student) :

output transformer ratio impedance V1  unknown transformerV1 is still 36V ;-)

output transformer ratio impedance V2 unknown transformerV2 = 1.2V

That gives :

output transformer ratio impedance V1 V2 unknown transformerSo a 16Ω loudspeaker at the secondary will be seen as a 16 x 875 = 14kΩ at the primary.

Thanks to ProjetG5 team for great ideas, as always !

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