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Phone & Ipod Chargers


Do phone chargers and Ipod chargers draw power when not connected to their device? The answer is: it depends on the type of charger.

There are two basic types of technologies used in power supplies (a charger is basically a power supply). It is useful to have a general understanding of the two types of power supplies. So, without getting into the technical issues, the two technologies are:

1) Transformer Power Supplies

Transformers have been traditionally used in power supply designs for many years. A transformer transforms high AC voltage to low AC voltage. Then some simple circuitry changes the AC to DC.

2) Switch Mode Power Supplies (SMPS)

Switch mode power supplies are a newer technology. They convert the AC to DC, then switch the DC on and off very quickly at different rates to produce the required voltage.


The major differences are outlined in the table below.

Transformer Power SupplySwitch Mode Power Supply
Produce heat even when not used, so are less efficientProduce little or no heat when not used
Have a heavy transformer in themDo not have a heavy transformer
Have a narrow or fixed input voltage (like 230-240 volts)Can have a wide input voltage (like 100-250 volts)
Draws some power even with no loadDraws negligible power with no load

So it is simple to tell which type of power supply your phone or Ipod uses for its charger. If it is warm when not connected, has very limited input voltage range (read the label) and feels a bit heavy for its size, then it is probably a transformer power supply.

If it doesn’t get warm when not connected, has a wide input voltage, and feels like there is nothing in it, then it is probably a switch mode power supply.

So, does your charger draw power when not plugged into its device? The above table gives some hints to this answer. In practical tests, I cannot measure the power drawn by my switch mode power supply phone charger when not plugged into my phone, as it is too low for my power meter to read. It did draw about 4 watts when connected to the phone and charging. It reduced to 2 watts once the phone was charged.

On the other hand, when I measured the power drawn from a transformer power supply, it measured 3 watts with nothing connected to it.

I want to thank Mark from Canberra, Australia for asking this question. I hope this helps you understand and use your technology better.

Further reading

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Please Note: Due to other commitments I'm currently unable to spare the time required to answer further questions in the way I'd like. Hopefully this is a temporary situation. I'm sorry for this inconvenience.