I'll provide a standard Mains->USB adapter as an example.

We know that the output of the USB will remain the same no matter what voltage between the allowed range is applied at the input.

If we're using this adapter in the UK, then we'll be applying 230v across the input. Are we wasting any power at this voltage compared to using the device at a lower voltage, say 110V?

  • 4
    \$\begingroup\$ In ATX power supplies, the efficiency rating is higher for 230V (see the 80 Plus requirements (at least for some ranges)). But a Mains -> USB adapter will have a different design, so it might be the other way round. \$\endgroup\$
    – Arsenal
    Commented Sep 15, 2016 at 13:36
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    \$\begingroup\$ @Arsenal that's more or less a given for anything but a low power design (where parasitic losses dominate) because the higher voltages on the AC side mean lower I^2*R losses. \$\endgroup\$ Commented Sep 15, 2016 at 20:41
  • \$\begingroup\$ AFAIK the opposite is true: The efficiency is higher in 230V mode. \$\endgroup\$
    – Al Kepp
    Commented Sep 16, 2016 at 12:46

4 Answers 4


You talk about transformers and power adapters as if they're the same but they're not.

An (oldfashioned) transformer based power adapter will be large and heavy and usually only suited for either 110 V or 240 V AC. Not both unless there is a setting for it which selects a different tap on the transformer.

Modern power adapters are quite different, much smaller and lighter in weight (for the same power rating) and can handle a wide input voltage range like 80 V to 240 V AC. These adapters are isolated switched mode power adapters and contain a very small transformer operating at a high frequency.

Your USB adapter will undoubtedly be of the second (switching) type. Does it fit in your pocket easily ? Then it's a switcher.

It depends on the design of the switched mode power adapter if it is more or less efficient at 110 V or 240 V AC. The design could be optimized for 110 V and thus be less efficient at 240 V. Or the other way around. There is no general truth here.

Maybe you think that the excess voltage when using 240 V instead of 110 V would be "burned off". Well it isn't, switched converters handle this in a more efficient way so that only a small amount of power is lost with any input voltage and any output current.

The way that such switching converters van handle such a wide input voltage range is a result of the way these converters work. The electrical energy from the input is converted into magnetic energy in the transformer and then back to electrical energy again. A transistor at the input side of the transformer switches the input power (at 100 kHz or so) and thereby determines the amount of power going into the transformer. So no more power than what is needed (at the output) is fed into the transformer ! This is a very efficient solution to control the power and thus voltage at the output.

  • \$\begingroup\$ Thank you for your answer, you've nearly hit the nail on the head with your last paragraph. Regarding the switched mode supply, is this switching on/off at a very high frequency in order to supply the 5V we need at the USB level? If so, how does the supply know what frequency corresponds to the voltage we're applying at the input level? \$\endgroup\$
    – James Hyde
    Commented Sep 16, 2016 at 10:26
  • \$\begingroup\$ The switching itself has no relation to the 5 V ! The switching is just to make an AC signal which can be transformed to a lower voltage. The frequency you're talking about is irrelevant, it has no relation to input or output voltage ! The mains frequency (50 Hz or 60 Hz) does not matter either as the mains voltage is rectified making it a DC voltage. Then that is switched at a "random" high frequency (100 kHz or so) and then transformed to a low voltage and rectified again and it appears at the output. \$\endgroup\$ Commented Sep 16, 2016 at 10:48
  • \$\begingroup\$ These switching adapters are rated for 50 to 60 Hz but most will work at DC (0 Hz !) up to 1 kHz or so. As long as the input voltage can be made into a DC voltage bij the input rectifiers, the supply will just work. I suggest you also read this related question, answers and the links: electronics.stackexchange.com/questions/204145/… \$\endgroup\$ Commented Sep 16, 2016 at 10:49
  • \$\begingroup\$ Let me add a small explanation about how these converters handle that wide input voltage range. \$\endgroup\$ Commented Sep 16, 2016 at 10:55

The switching currents are lower at higher voltages (as they must be for the same output power), so conduction losses will be less at higher input voltage.

Supplies designed for only 120VAC input often have an input doubler to produce a 300V rail, however 600V is a bit too high for comfort so that's rarely, if ever, done for supplies that can handle 240VAC input (including wide range input supplies).

The lowest efficiency of a switching supply (for a given output power that is within normal operating range- say more than 10% of rated output power) is typically at the minimum input voltage.

For a given input voltage, the efficiency usually peaks for a load current somewhere in the normal operating range (it is a convex curve with a maxima).

To make this clear, however, this is a second order effect, the efficiency might be (for a certain load) 80% with 240V in and 75% with 120V in. There should not be an enormous difference, assuming the supply is well designed (all bets are off with counterfeit junk).


The Mains to USB adaptor almost certainly is a switch mode power supply and probably contains a circuit similar to this

Flyback PSU

If it were a simple transformer it would be heavy and either get really (dangerously) hot when used with 250Vac or would have a switch to wire two primaries in parallel when used with 110V mains and in series when used with 230V mains.

This type of circuit is called a Flyback power supply and works by the switching device turning on and current builds up in the primary. When the switch turns off the transformer flies back and current can flow into the secondary.

The transformer and opto-isolator provide isolation for safety reasons and the transformer is switched at relatively high frequency (high tens or low hundreds of kHz) to minimise the size, and cost, of the transformer needed.

This type of circuit is usually more efficient with high mains than low, meaning it will waste less power at high mains. That said its highly efficient at any mains. The control IC need not be a TOPSwitch: many manufacturers make ICs for this.

  • \$\begingroup\$ Thank you, however you've not mentioned anything about possible power wastage at different voltages. As the answer stands, it doesn't answer my question. \$\endgroup\$
    – James Hyde
    Commented Sep 16, 2016 at 10:21
  • \$\begingroup\$ @lightswitchr I've extended the answer to emphasize the point but my original answer did state that these designs are generally more efficient (waste less power) with high mains. \$\endgroup\$ Commented Sep 16, 2016 at 10:51
  • \$\begingroup\$ The answer to that is that it is the design of this circuit which takes care of the different input voltages. It is a switching converter and those converters are by their design capable of doing this. \$\endgroup\$ Commented Sep 16, 2016 at 10:51

It depends on type of your transformer. Usually they are made for standard voltage like 230 V and their loss of power for that voltage might be around 1% or even less. If you change input voltage it might drop, but only a few % not like half for 110V. If half the power would be just wasted it would mean it's converted to heat.

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    \$\begingroup\$ And that seems what? You forgot to end Your answer... \$\endgroup\$ Commented Sep 15, 2016 at 14:07
  • \$\begingroup\$ OP almost certainly means a switched mode power supply - not a transformer. Welcome to EE.SE. \$\endgroup\$
    – Transistor
    Commented Sep 15, 2016 at 18:32
  • 1
    \$\begingroup\$ Don't flag answers for low-quality unless its length (which this this meets the length req) or content (there is no bad links or any other malfeasant information). Its better to let the auth respond or edit the post yourself, if its bad use the voting system. \$\endgroup\$
    – Voltage Spike
    Commented Sep 15, 2016 at 20:31

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