I bought a 115 to 15V toroidal transformer.The turns ratio of this should be (115/15)=7.66 ideally.

Now the problem I'm facing is that upon wiring it I'm getting a calculated turns ratio of 6.97, which doesn't match the datasheet.

I plugged it into my wall outlet and measured 122 V with my voltmeter coming into the primary side. On the secondary of the transformer I am getting an output voltage of 17.5 V.

This give me: (122/17.5)=6.97 turns ratio.

Why is this? The toroidal transformer also had this wound plastic which encircled the coils and I cut that off. Could that be the reason why this is happening or is it just how things in the real world work?


2 Answers 2


Transformers are usually rated (volts in / volts out) based upon full load current. You are measuring during no load.

So you measured the no load (unloaded) voltage is 17.5 v = 122/6.97 v.

The Un-loaded voltage is a more accurate measurement of the turns ratio.

Fully loaded, you will get lower voltage than 17.5 volts, which results in your transformer's rating of 15 volts output.

Fully loaded means that there will be a voltage drop on the internal resistance of the transformer,

  • \$\begingroup\$ ah so simple! makes sense since there is no voltage drop under no load conditions! \$\endgroup\$ Jul 3, 2014 at 2:59
  • \$\begingroup\$ @user2464665 . . I love the sound of the bell ringing over ones head when understanding is made \$\endgroup\$
    – Marla
    Jul 3, 2014 at 3:01
  • \$\begingroup\$ and hopefully you also saw that the unloaded voltage gives a good measure of turns ratio, where loaded voltage does not. \$\endgroup\$
    – Marla
    Jul 3, 2014 at 3:06
  • \$\begingroup\$ i will have to also test it while it is loaded and find out. Hopefully my transformer wont be funky since I removed the plastic coating off the coils \$\endgroup\$ Jul 3, 2014 at 3:19

You are measuring the unloaded voltage of the transformer, at a higher than nominal input voltage.

As you load the transformer its output voltage will drop (mainly due to power dissipated in the resistance of its windings) until at full load it should deliver the rated voltage.

The percentage difference between the two voltages (no load and full load) is called the regulation of the transformer.

At the nominal 115V, you would see 17.5V * 115/122 = 16.5V.

This is exactly 10% higher than the rated voltage, so your transformer is designed for 10% regulation (quite a common value) and if you bought it from a good source, you should see 10% regulation quoted in its specification or datasheet, which is how this sort of surprise is prevented in future.

EDIT: the "wound plastic" encircling the coils is (was!) its insulation : if you have cut that off the transformer is not safe any longer and while it probably still works I cannot recommend its use, as even the slightest damage to the wires under that plastic can expose you to dangerous voltages or risk starting a fire caused by a shorted turn on one of the windings.


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