# Calculating inductance values from transformer datasheet

I want to simulate this transformer using LTSpice:

This is a microphone transformer. To do a simulation in LTSpice I need to know the inductance values for the windings. The datasheet gives impedance values and a frequency range of 30-25kHz, which is strange from my understanding, since impedance can only be specified at a specific frequency. Is there some convention in the industry that would explain how to calculate the inductance values? Is it possible to calculate the inductance from these values?

• It takes a great transformer design to operate @ 2 freq decades so use Z(30Hz) to get -3dB for L Dec 9, 2019 at 20:44
• Thanks for the answer! I'm guessing it's gonna still be "eyeballing" it? Or is the lower end of the frequency response generally taken for inductance calculation in your experience? Dec 9, 2019 at 20:50
• Lower end for L , upper end from interwinding capacitance I get ~ for L = 3H @ 600 Ohms Dec 9, 2019 at 20:51
• maybe 300 Ohms so L ~ 1.6H maybe Dec 9, 2019 at 21:02
• @TonyStewartSunnyskyguyEE75 - the impedances given are those of the source or load. The reactive impedance of the transformer can be expected to be an order of magnitude greater. Dec 9, 2019 at 21:20

Low frequency cutoff is determined by the interaction of circuit resistance with primary winding inductance. High frequency cutoff is determined by the interaction of circuit resistance, leakage inductance, and winding capacitance.

The -3dB cutoff frequency of an LR filter is $$\ fc = \frac {R} {2 \pi L} \$$. Here resistance is the Thevenin equivalent of source and load resistances, which in a 600Ω circuit is 300Ω. So for a low frequency cutoff of 30Hz the transformer primary inductance should be 300Ω/(2π*30Hz) = ~1.6H. The ratio of primary to secondary impedance is 600Ω:25kohm; = 1:41.7, so the secondary inductance should be 1.6*41.7 = ~67H.

At the high frequency end the transformer's leakage inductance (caused by imperfect coupling) can have a large effect, as it forms an LC low pass filter with the winding capacitance. Without a specified leakage inductance all we can do is guess what the coupling coefficient might be, then choose a capacitance that produces the specified high frequency cutoff.

I put the specified 19.2Ω and 1140Ω winding resistances and calculated inductances into LTspice, chose a coupling factor of 0.998, and adjusted winding capacitances to get a high frequency cutoff at 25kHz. This was achieved with 340pF on the secondary winding, and (since winding capacitance is approximately inversely proportional to the number of turns) 2.2nF on the primary. The frequency response looked like this:-

For a more reliable simulation you would need to measure the leakage inductance and/or actual frequency response of the transformer. Your specs don't even tell us what 'frequency range' means. Is it the 3dB points, or something else?

Here's the measured frequency response of a line input transformer specified for +/-0.25dB from 10Hz to 60kHz:-

Sowter type 4383

• Yeah, the datasheet is really trying not to give out too much info. I'm guessing they mean -3db, unless there is an unspoken rule in the community that would suggest otherwise. Anyways thanks for putting time into the issue, it's much appreciated! Dec 10, 2019 at 8:16