I've recently been trying to figure out how to use transformers, coils, and inductors in general. My question here is whether transformers produce a load or not. For example, if I apply current to the primary coil, is it a short circuit, and how to they do it with mains power? My guess is that a load on the secondary coil produces a load on the primary coil. Is this true? Please correct me if I'm wrong.

  • \$\begingroup\$ if I apply current - AC or DC current? \$\endgroup\$ Jan 11, 2017 at 3:49
  • \$\begingroup\$ en.wikipedia.org/wiki/Transformer \$\endgroup\$ Jan 11, 2017 at 3:59
  • \$\begingroup\$ I was testing it using DC, but I'm pretty it requires AC to work properly \$\endgroup\$
    – Scrapper
    Jan 11, 2017 at 4:14
  • \$\begingroup\$ The primary must have enough inductance so that only a small current flows in the primary coil at the transformer's specified frequency. Then, yeah, the load on the secondary also loads the primary. \$\endgroup\$
    – user57037
    Jan 11, 2017 at 6:59
  • \$\begingroup\$ Motors are loads. They're inductors. I guess transformers would produce a load of some sort. Otherwise connecting a transformer to AC would be a short circuit. \$\endgroup\$
    – Bradman175
    Jan 11, 2017 at 11:28

2 Answers 2


It depends what you mean...

They produce a reactive load, which is to say electricity flows through them, but they don't consume energy.

There's also losses which are usually quite small and are a real load and consume energy to make a small amount of heat, and that humming sound, etc.

If you connect a load on the output side of the transfomer, it appears electrically as-if there was a transformed version of the load at the input side.

  • \$\begingroup\$ My main question was whether a short circuit is produced by any one of the coils. My tests with an old transformer have shown little to no resistance. \$\endgroup\$
    – Scrapper
    Jan 11, 2017 at 4:01
  • 3
    \$\begingroup\$ They have little resistance at DC. At 50/60Hz AC, the "resistance" is quite high because of the coil and the iron core. Please read about inductors first before you try to understand the transformer. \$\endgroup\$
    – Janka
    Jan 11, 2017 at 4:09

As mentioned earlier, learning about inductors helps before learning about transformers. Reading about Lenz law will be helpful in understanding the operation of inductors and transformers.

Transformers, when neglecting losses, do not create a load all by themselves. It transfers a load from the secondary coil to the primary coil, assuming that the voltage source is on the primary. If there is no load on the secondary (as in there is no current taken from the secondary, which means the secondary is open-circuited), then no load is taken from the source to the primary too. Under this condition, only a small 'magnetising current', which is always present flows (ideally this should be zero).

When you apply an ac voltage to the primary coil, it does not cause a short. Instead an nearly equal and opposing voltage is present in that coil, so that very little current actually flows. This opposing voltage is due the magnetic flux generated within the coil, and follows Lenz' Law. No short actually happens here, as long as the secondary coil is kept open.

For a dc voltage however applied to the primary alone, in the beginning there is little current, but in the steady state as dc voltage is a constant one, the flux changes little and the opposing voltage is non-existent. Here due to the small resistance as you measured, a near-short is very much possible.


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