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I asked this question on Physics StackExchange but was advised to ask it here.

When I charge me cell phone or notebook or whatever device what should I plug in first? Should I at first plug in usb charger to the AC wall outlet and only then to the cell phone or vice versa? What are the precise physical arguments for this? And are there any differencies at all? I think it is safer to plug in wall outlet first because of peak charge. Can someone give me a technical explanation? Thanks in advance.

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closed as off-topic by Leon Heller, Nick Alexeev, Matt Young, Adam Davis, Joe Hass Mar 3 at 22:45

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Leon Heller, Nick Alexeev, Matt Young, Adam Davis, Joe Hass
If this question can be reworded to fit the rules in the help center, please edit the question.

If it makes a difference which you plug in first, then you shouldn't be using the device in the first place. –  Scott Seidman Mar 3 at 16:06
The correct answer is, "Use the charger according to your manufacturer's instructions." Generally, however, it doesn't make a difference which order you do it in. If you must follow a specific order, plug the charger into the AC power first, then plug the device to be charged into the charger. Why? Because I said so. That's about as good advice as you can get from anyone without specifying exact part numbers, and other specific information about the environment they are being used in. –  Adam Davis Mar 3 at 21:38

3 Answers 3

up vote 8 down vote accepted

Such chargers1 are (at least, supposed to be) basically just constant-voltage supplies, i.e. whenever connected to AC power they should offer the specified voltage, no matter if a device is already connected. For a modern switching power supply, there's quite a lot of circuitry included that makes sure this goal is met pretty reliably, so indeed you don't need to worry about anything here.

But older power supplies are often built much simpler: occasionally, these would consist only of a transformer and a roughly smoothened rectifier.


simulate this circuit – Schematic created using CircuitLab

When connected to a device while not on AC, nothing much happens at all (a simple diode can make sure no current is "draining back" from the batteries), so we shouldn't worry about that.

However, the other case, connected to AC but not to any device, is not so uncritical. The thing about transformers is, they only work properly (in the sense of, providing a fixed ratio between the voltages on both sides) when there's a current in both windings. However, when running idle, the voltage in the secondary coil exceeds the nominal voltage. That's especially a concern because the smoothening capacitor will then charge up to that voltage, and if you connect a device at that instant it may suffer damage from the excess voltage.

1 Note that a properly designed actual battery charger is not a constant-voltage supply, but the part that controls this is nowadays build into the device rather than the power supply.

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With any modern "electronic" charger it should make no difference - it should be safe "by design" in either case. Any reputable brand of charger will not create an output surge on startup - which is the main thing to be concerned about.

Starting a supply while loaded will place larger stresses on it than otherwise, but modern chargers must anticipate and accommodate this without damage.

The circuit example supplied by leftaroundabout may have a preferred mode of us BUT no modern charger in any way resembles this circuit. If you looked for a cheap enough poorly built and non designed enough charger you may have problems, but the equivalent is true in any situation.

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It is not just the matter of circuitry, but mechanically joining the conductors, too. When you try to connect your charger to outlet, or usb to its port, it is not always a smooth connection, but you might connect and disconnect them few times before the connection becomes (mechanically) stable.

So, I would recommend connecting the usb port first, and then connect the AC, so that circuitry in the charger reduces the surge of connecting the AC.

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