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I'm currently investigating the livelihood of a laptop adapter (as seen in an other question).

It's possible that it is faulty in that there are measurements of correct voltage but with wrong current. What's the expected behavior of a laptop? It currently doesn't even start [not even a led].

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up vote 1 down vote accepted

To put it in the simplest way, the amount of current used by laptop is a function of its internal resistance (and that's a big huge simplification) and input voltage. On old and very cheap power supplies, if they can't provide needed current, they will overload and overheat. Today, some power supplies will give as much current as they can, which may or may not be enough to run the laptop, or will detect overload and shut down.

If the current which the adapter provides is higher than needed by the laptop, everything should work fine.

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It might be lower. What will happen then? – j riv Jan 2 '11 at 12:05
Well, as I said, the power supply could be overloaded and be shutting down or it could provide too little current for laptop to even turn on. Form what I've heard, most newer power supplies will limit output current and be able to provide some current even if they can't give enough to run laptop at full power. Still, you should at least be able to get LEDs to turn on. What are characteristics of the adapter you suspect is broken and the one which shipped with laptop? – AndrejaKo Jan 2 '11 at 12:10
Simply put, a power supply with the right voltage does not force more current into a device; the device takes what it needs. You don't worry a moment about connecting a 5W phone charger into a socket which can supply 2KW to your electric kettle. Just as long the mains voltage is right. – Lenne Jul 21 at 8:04

PSUs (in the UK at least) are required to have some form of overcurrent protection in case of an increased load due to a fault. Forms of protection include thermal protection, current limiting, output shutdown and the basic fuse.

Thermal protection, a simple temperature cut-off that resets itself.

Current limiting means just that, as the load increases the current is prevented from rising above the max allowed and in extreme conditions the PSU will trip or the output voltage will fall to zero.

Shutdown means the output will switch off until the load is removed and sometimes requires a PSU reset.

If this post refers to your laptop fault then that clicking you can hear is possibly either the on-board regulator or the PSU thermal trip resetting

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Just a thought, can you take the battery out of the laptop?

I have had a few occasions where the battery is faulty and prevented the laptop from turning on. Removing the battery allowed it to be powered from the PSU.

Worth a try.

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