Electronic – How do AC adapters achieve low current


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The image above shows how a typical AC to DC adapter works, high voltage (230V) at low current is fed into the primary coil and then gets stepped down to a lower voltage with higher current, afterwards it gets rectified to DC and finally a capacitor turns this DC to pure DC.

The power from the primary coil has to be the same as the power at the secondary coil (assuming no power loss). Let's for instance use an example, a phone charger, most phone chargers normally output 5V@1A DC which means 5W. Since the voltage from the wall is 230V this means that for there to be 5W there has to be 5W/230V = 0.0217A which means that the resistance at the primary coil is 230V/0.0217A = 10599\$\Omega\$

This is the part that I don't get, how can an inductor (coil), something with almost no resistance, have so much resistance. At first I thought that they would put a resistor before the coil but then I figured out that all the power would be used up by the resistor and wasted as heat instead of reaching the coil.

So what is really going on? Do they make these coils with materials that have really high resistance or am I missing something? Probably the latter is true.

Any help is appreciated.

Best Answer

The following approach may also help a little

The ohmic resistance of an ideal coil is 0 ohm in DC, but in AC the coil resistance is called inductive reactance and its amplitude can be calculate by the following formula:

\$X_L=2 \pi f L\$


  • \$X_L\$ is the Inductive Reactance of the coil (in your case the primary of the transformer) in Ohms
  • \$\pi\$ =3.14
  • f= frequency of the AC signal (probably 50 Hz in you case)
  • L=inductance of the coil measured in Henries

An inductor or coil is a device that, when subjected to an increasing electrical current flow, generates a back voltage that opposes this current. Inductance quantifies how much energy an inductor can store

Please note that heat losses in a non ideal transformer are made by the wire resistance and by parasitic currents the transformer core and not by XL.