Laptop power supplies, does current matter?

Using a lower current rated brick (the 5 A on the 8 A laptop) would result in one of the following:

• Melted power supply or cord, as the laptop starts drawing too much current
• Working laptop, with little to no charging of the battery (or charging the battery, but no working laptop) as 5 A is enough for one, but not the other
• Perfectly working laptop, as even though the brick is rated at 8 A, your laptop only draws 5 A (or the 5 A is perfectly capable of pumping 8 A)

Using a higher current rated brick (the 8 A on the 5 A laptop) should be fine - the laptop will only draw a theoretical maximum of 5 A, so that's the maximum that will be put out by the power supply.

This, of course, assumes that the polarity is correct - otherwise you'll likely just go poof (or unlikely catch fire). Sometimes there's a diagram, sometimes you have to check, sometimes you just cross your fingers and pray (the latter is not recommended for expensive toys like laptops).

Basic rule of thumb for power supplies: The voltage has to be right; the amperage just has to be high enough. Your laptop will draw differing amounts of power depending on what it is doing.

Remember that the power supply has to meet the maximum possible power consumption of the laptop. e.g. burning a DVD whilst simultaneously running CPU at max, connecting to a wireless network, and thrashing the hard disc. In "normal" usage, you'll be drawing much less than 8A.

So I would suggest:

1. Don't rip any DVDs while using the lower-rated power supply.
2. If you're worried, get a Kill-a-Watt (or equivalent for your country), plug your laptop in using the right adapter, and measure the power usage during various tasks.

Basically the current rating is a maximum value. This means that a 15.6 V/8 A can replace a 15.6 V/5 A powersupply. What's important in a power supply for a laptop is that the voltage output is the same and the current are the same or above the original power supply.

You'll want to make sure the connector is really the same (same polarity, same size, it does not wiggle or anything). Ideally they would be from the same manufacturer.

I purchased my HP 6730B business laptop used. It was in a very nice condition, had Windows 7 Professional and ran great until the charging unit started beeping. I could shake it at first and tap on it and it would stop beeping and work again. This got more frequent within days.

Then the tapping and the shaking stopped working, so U placed it in the freezer for 20 minutes. This also worked for another week or so. Then the beeping became louder and neither the freezer trick nor the tapping worked. So I blasted it with a hammer a few quick times and it squeeled like a stuck pig and the beeping got louder and is now steady (whereas before it was intermittent).

In any case, I got one from a friend that is lower in volts by .5. It's 18.5, not 19. Also it has less amps (3.5, not 4.7). The wattage is different: 65 watts, not 90 watts.

There hasn't been a noticeable difference, except that when the old charger had completely charged the battery the icon would read "battery fully charged", "plugged in, not charging" — now it only does this sometimes and I noticed my cooling fan is on more often, although it has gotten considerably warmer where I live, so I'm thinking this is a temporary fix.

I'm going with the original manufacturer specified unit ASAP, and although lower watts/amps/voltage works, original gear is always better.

Not quite so simple as presented above: there's a critical difference between regulated and unregulated power supplies. Unregulated ones only hit their target output voltage at their rated load, and will generate a higher than desired voltage at lower loads. A regulated power supply will always generate the expected output voltage.

The most direct way to work out whether your laptop PSUs are regulated or not is if you can get a volt meter and check the voltage directly from the plug: if it's the rated 15.6V, then they're regulated power supplies and the above comments apply: so long as the power rating is high enough, you can substitute. If you find that the output voltage is a few volts higher than you expected, then the supply is unregulated, and will drive lower-power equipment at a higher-than-specified voltage, which may cause it damage. For example, I have an unregulated 9V "wall wart" power supply for a 'phone, which from its plug (the one that goes into the 'phone) appears to generate 12V.

That said, much equipment supplied with an unregulated PSU will have internal regulation circuitry, so you may be ok anyway.

(Unregulated power supplies have fewer components, and are cheaper as a result.)