I've taken out a beafy transformer with 8 leads from PCB of a multimedia device of some sort. (Possibly a TV).

2 leads are obviously the primary coil (thicker leads and wires) the rest are almost same awg wire (and are connected to much thinner copper wires)

Is there a way to measure the turn ratios of these secondary coils vs the primary one accurately, without unwinding the coils?

I could use this transformer on some other projects if the TTRs are known.

Research and readings:

  • Measuring the resistance of coils separately. [I don't think my DMM is accurate enough for such low ohm values. Plus, due to the unknown differences in copper gauges of different coils, I don't think resistance readings being useful.]
  • Measuring inductance of coils. [Don't have LCR meter.]
  • Applying alternating voltage (though it's not accurate, as it doesn't take into account the losses, and is frequency-dependant, any instructions on how to do it safely is welcomed)
  • 4
    \$\begingroup\$ "2 leads are obviously the primary coil (thicker leads and wires) ..." Um, no. If it's a step-down voltage transformer the currents will be higher on the secondary so they'll need the thicker wires. Be very careful - you seem to be missing some education on this topic. \$\endgroup\$
    – Transistor
    Oct 30, 2021 at 18:05
  • 1
    \$\begingroup\$ The assumption that thick coil is primary and thinner wires are two secondaries would make sense if it is a step-up transformer for driving fluorescent backlight tubes. For utilizing a random transformer in your projects, you need a lot more parameters than turns ratios. Most likely it is a transformer for a switch mode power supply of some sorts, so it might have been most useful when used with the circuit it came with. Most likely it is not a 50 or 60 Hz transformer. \$\endgroup\$
    – Justme
    Oct 30, 2021 at 19:38
  • \$\begingroup\$ If you mean thicker insulation on thinner wire, those two leads might be the primary, but otherwise, dangerous assumption. And if the insulation colours are brown (L) and blue (N) for gear sold in Europe, it's a pretty safe assumption. \$\endgroup\$
    – user16324
    Oct 31, 2021 at 13:43

1 Answer 1


With a function generator and a 2-channel oscilloscope.

Feed, say, 1 kHz sine-wave to one winding with the function generator, while monitoring both that winding and the other winding with the scope. Vary the frequency, say between 50 Hz and 200 kHz, looking for a range of frequencies that gives high output (but not a specific frequency that gives you a spike of high amplitude: that's a resonant frequency, and it doesn't give you the correct ratio).

Be careful not to generate a voltage that is too high for the input of the scope. If so, reduce the input level from the function generator.

Take the ratio of the output voltage over the input voltage, and that will be close to the turns ratio. The frequency of the generator will give you an idea whether that transformer is a line voltage transformer (50/60 Hz), an audio transformer (~ 1 kHz), or a switchmode / pulse transformer (~ 100 kHz).

  • \$\begingroup\$ "Vary the frequency ... looking for the frequency that gives the highest output", what do you mean by highest output? \$\endgroup\$ Oct 30, 2021 at 19:19
  • 2
    \$\begingroup\$ The amplitude of the signal in the scope with change with frequency. Find the frequency that results in the largest value amplitude. \$\endgroup\$ Oct 30, 2021 at 19:30
  • 2
    \$\begingroup\$ No, don't vary the frequency looking for resonance. That's stupid talk if trying to find the turns ratio. \$\endgroup\$
    – Andy aka
    Oct 30, 2021 at 20:33
  • \$\begingroup\$ OK, so, what frequency then? Given that we don't know if it's a line, audio, pulse, or switchmode transformer. \$\endgroup\$ Oct 30, 2021 at 21:32
  • 1
    \$\begingroup\$ Use a frequency where the response is flat. If you are having trouble interpreting, take some measurements and update your question. \$\endgroup\$
    – Mattman944
    Oct 30, 2021 at 23:27

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