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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 '14 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.

Why do you think it would matter which order they are plugged in? – Kyle Kanos Mar 3 '14 at 15:23
Yeah, this isn't a physics question as is. If you were to edit it to explain why you think it would matter, as @Kyle said, if that reason is something to do with physics, then it might be on topic for us. – David Z Mar 3 '14 at 15:25
This question appears to be off-topic because it is about consumer electronic devices. – David Z Mar 3 '14 at 15:25
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 '14 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 '14 at 21:38
up vote 9 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 shouldn't make any difference. Assuming you have a regulated supply, which is virtually always the case with a cell phone, then if you plug the charger in first, followed by the cell phone, then of course the charger will have a regulated 5v at its output. This probably is how must people charge their phones, plugging the charger in first.

But if you plug the phone into the charger, and then plug the charger into the wall, there will not be any harmful voltage spikes, because 1) the charger is going to ramp up from 0 to 5v, and not start at some higher voltage and drop to 5v; and 2) the time it takes to go from 0 to 5v for most switching regulators is usually significantly less than a millisecond.

If you have an unregulated supply (not for a cell phone, but some other electronic device), then the "no-load" voltage of the power supply may be significantly greater then the rated voltage; for example 9v for a 5v supply. Here it may make a slight difference what order you plug them in.

If you plug the power supply in first, it is going to be at (say) 9v, until you plug in the electronic device, and then its load will bring the supply down to somewhere around its rated 5v. Note in this case, you will always be starting at a higher voltage than the rated voltage since the power supply has already plateaued at the no-load voltage.

If you plug the device into the power supply first, and then plug the power supply into the wall, then there may or may not be a temporary spike above the rated voltage depending on the load imposed by the electronic device. Since you are starting from 0 instead of the no-load voltage and ramping up, the device will certainly be subject to an over-voltage condition for a shorter period of time.

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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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What people commonly call chargers are really just transformers followed by a rectifier and possibly a regulator. If you plug the "charger" (the charger is actually the circuit that controls the battery charge from that supply) without the cellphone, it's basically an open-ended transformer: the only current drawn is the one from the iron losses and the creation of the field within the transformer, which is small. Plugging your phone in at that stage is just like switching a switch on. As for the other way around, the transformer will be ramping up because of the inductor. The first rule of design is "never trust the user". If it really mattered there would be warnings, but I agree that understanding is always better.

P.S: USB chargers are assumed. As tcrosley said it might actually matter for different chargers

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Actually most cellphone chargers are fairly sophisticated. The state of the art is probably the one cubic inch charger that Apple supplies with its iPhone. Here's a schematic and here's a link to the datasheet for the switcher IC. Check out the block diagram on page 3. Instead of feeding directly into a transformer, the line voltage is converted to high voltage DC and then switched at 70,000 times a second, which results in an itty bitty transformer. – tcrosley Mar 4 '14 at 1:03
Indeed commercial systems do go a long way from the basics, since it has to be optimised... Thanks, very informative (especially the schematic, I didn't know it was available...). – Mister Mystère Mar 4 '14 at 9:37
There are sites like this one that do "teardowns" of products as soon as they hit the shelves, and reverse engineer the schematics along with the bill of materials. – tcrosley Mar 4 '14 at 11:41

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