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I'm currently investigating the livelihood of a laptop adaptor and I noticed, unlike another adaptor it drops voltage to almost zero after I also attempt to measure its current [while it did show voltage at first, before measuring current]. The other adaptor normally shows voltage and then normally shows some current and then if it turns to measure voltage, it shows it normally again.

I assume the adaptor is faulty, however, I want to make sure if laptop adaptors turn off themselves if they aren't used normally by a laptop.

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up vote 2 down vote accepted

If the adapter is fancy enough, it could have some sort of protection from unexpected currents. Usually, this is implemented as a short circuit protection, so if you're incorrectly measuring current, you could be tripping it.

What load did you use when measuring the current on the adapter? Also, how many watts/amperes can the "faulty" adapter provide and at which voltage?


Here are some simulations showing how to properly measure current and voltage Voltage measurement

Here we see power supply V1 at the left connected to the load R1 on the right. Meter XMM1 is connected in parallel, with its positive lead to supply's positive side and its negative lead to supply's negative lead. Ground is there because of the way program runs its calculations. We get 19 V.

Current measurement

Here we see how to properly measure current. Meter is connected in series with load, so current goes through it and then to the load. We have to actually cut open a piece of the wire and connect meter there! This isn't very convenient for most power laptop power supplies. The current is around 4.5 A, which is the maximum value of my power supply.

Good way to cause electrical fire

Here we see the incorrect way to measure current. If we remember Ohm's law, current is voltage divided by resistance. Since ammeters are designed to be connected serially into the circuit, their resistance should be zero. That means that current through ideal ammeter should be infinite! In this simulation, we have a nearly ideal ammeter and power supply, so we have 19 GA! Of course, realistic power supplies can't get anywhere near that (power plants make current in kA ranges) and realistic meters have much higher resistance. So in real world, this would catch on fire or blow a fuse.

Here's a nice video showing what could be happening with components of power supply improperly connected:

Here's a nice video showing how to correctly use a multimeter:

To cut the story short, since you mentioned in the other question that you can't get even LED to light up, you probably ruined the power supply and there's a chance that meter's fuse could have blown too.

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There was no load at all, directly to the output, before connecting it to laptop. It's 19V-9.5A [~180W]. – j riv Jan 2 '11 at 12:03
@leladax That's the surest and fastest way to burn (as in set on fire) your meter and your power supply! I'll update my answer soon with correct way to measure current. – AndrejaKo Jan 2 '11 at 12:11
@leladax I updated my answer. – AndrejaKo Jan 2 '11 at 12:40
@AdrejaKo: +1 for a better answer with pics than mine! – Andy Jan 2 '11 at 12:43
The LED didn't show up before I measured anything. – j riv Jan 2 '11 at 13:54

I have never heard of such a feature...

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How are you measuring current?

You need to put the probes of your multimeter/ammeter in series with a line you wish to measure current in.

If you are simply placing the probes across the supply lines in the same fashion as you measure voltage, then you are shorting out the PSU and it is almost certainly current limiting itself. This is a Good Thing, otherwise it would quite probably fry itself.

If, when it is disconnected from the laptop, you measure voltage of about 19V DC on the output, then the PSU is probably OK.

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