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My laptop uses a charger outputting 6.4A with 19V (120W). This charger was a replacement for the old one, the specs were almost completely matching (19.5V and 6.15A on the old charger), everything works perfectly.

There is, however, another laptop, which doesn't see the charger when working and barely charges with the 20V and 2A of the AC adapter, which is 40W. It sounded like a clear indication of not enough Amps, so I checked that the voltage and connector were (almost) matching and tried it with my adapter (3 times more powerful), and it worked perfectly.

I have heard before that it is completely safe to bring more Amps, because amperage is pulled, not pushed, so I believed the experiment to be safe, and was going to buy the same charger for the laptop's owner -- but she has also asked her brother, and her brother told that it is not safe to change amperage by more than 2A, some googling revealed the same statement about 1.5A. So I would need to find out the original amperage and choose a charger carefully.

So: Is it completely safe to use a charger with considerably bigger amperage if other specs, including polarity and connector shape, are matching? Is there any limit to this?

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    Thank you for clearly indicating your question. Yes, it is completely safe. See superuser.com/a/600426/873319 and superuser.com/a/247315/873319 – dsstorefile1 Apr 24 '18 at 22:37
  • @dsstorefile Those 2 questions (and a couple of others) indicate that it is safe for some exact amperages. However, I wish to know if there is any limit. E.g. would it be OK to use a 20A charger with a laptop that just requires 1A? :) – Baskakov_Dmitriy Apr 24 '18 at 22:41
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    There are some power supplies that push amperage. These are generally for scientific purposes or electronic labs. Your not likely to run into any of those. I routinely charge electronics with adapters with greater amperage than the spec. – Dan Sp. Apr 24 '18 at 23:00
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    Her brother is flat-out wrong, as is anybody or anything else that makes a similar claim. Consider that if you plug a "100 watt" light bulb into an AC socket, that AC socket can provide anywhere from 1500 to well over 2000 watts (depending partly on where you live). Does the light bulb flare up and burn out? No. Looking at it the other way - You could use that same 6-amp power supply to run a single LED (pulls about 20 milliamps) with no problems. – Jamie Hanrahan Apr 24 '18 at 23:56
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    With respect of chargers "barely working" - some laptops have circuitry in them to detect a non-original charger and only charge at a slow rate. This could explain some of what you were seeing as well. – davidgo Apr 25 '18 at 4:00
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You are correct, it is completely safe to use a charger with a higher amp rating. Her brother does not understand basic electronics.

The electronics formula is quite simple: Voltage=Current * Resistance. Therefore Current=Voltage/Resistance.

The voltage is fixed, and the resistance varies depending on the exact paths the power takes to feed the components of the computer - thus the current drawn varies DEPENDING ON THE PARTS IN THE COMPUTER - not the power supply.

There are - as you appear to be aware - an instance where this is not true - where the available current is less then what is desired - in which case the voltage would drop (assuming there is no protection circuitry). If - hypothetically - the manufacturer were to design the computer to expect a lower voltage because of the current drop by drawing too much current, then your brother would be correct - however this design choice would be insane, because it means building a system that is dangerous and designed to fail in a dangerous way, and there is no advantage to it (even if the manufacturer wants the system to eventually fail, there are safer ways to accomplish this - without exposing them to the same unquantifiable liability)

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    Correct except for the fourth paragraph. When the current drawn by a device exceeds the safety ratings for the AC adapter you're using, the AC adapter's output voltage doesn't drop; instead, the current flows through it anyway. (And yes, there could be some current-limiting circuitry, or a fuse, or a circuit breaker, but good design requires that we consider these reliable but instead provide sufficient power.) This extra current could cause failure including fires, melting, or other damage. – Mathieu K. Apr 25 '18 at 0:25
  • A common metaphor for this relationship is water flow. The flow rate (amperage) is determined by the pressure (voltage) and the diameter of the pipe (resistance). (A resistor being basically a narrow segment of pipe placed between two bigger segments.) – ssokolow Apr 25 '18 at 3:46
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    Yes it's safe, but the analogy of the laptop as a resistor only works perfectly for actual constant voltage, and then the key point is that the power supply is trying to maintain its rated voltage, not its current limit. The laptop isn't a resistive load; giving it slightly more voltage will probably reduce its current draw, assuming its internal switching power supply has constant efficiency for charging the battery and powering its +12V and other supply rails. It probably uses a buck converter to step down the input DC voltage. – Peter Cordes Apr 25 '18 at 4:08
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The charger does not put out 6.4A at 19 Volts. The current is not a characteristic of the charger. It is capable of putting out 6.4A if the load requires it. Big difference. Depending on the load the charger may never put out 6.4A. No charger or power supply can control both the voltage and current output unless it could somehow control the load. Ohm's law specifies what current will flow through a specific load with a specified voltage.

The charger could be rated at exactly what the laptop requires, or it could be 10A, 100A, or even 1000A. The laptop would charge exactly the same way and could in fact not tell the difference.

A properly designed charger would for it's own protection limit it's current output if a failure in the laptop were to cause an excessive load. Otherwise a catastrophic failure and fire may result. The laptop should itself limit the current, even if only with a fuse, and not rely on the charger to do this.

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