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How to understand Laptop Adapter power ratings?

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benali72

Member
Joined
Sep 23, 2007
Messages
26
Hi,

I have a bunch of old laptops and power cord adapters. Some match up, but some don't.

Could I ask for help in understanding how adapter power ratings work? (I searched the web but couldn't find good explanations).

I see Volts and Amps rated listed on each adapter and on the back of each laptop.

Amps-- my understanding is that the laptop can use an adapter with Amps equal to or above its own rating (an adapter with lower Amps rating won't work, but one with more is ok. The laptop just takes the amps it needs, as long as they're available).

Volts-- my understanding is you want to match the laptop and adapter voltage ratings exactly. Or you can use an adapter that has maybe a volt less than the laptop (but the adapter should never have more volts than the laptop or it could cause problems or even permanent damage).

Watts-- I got no clue on this! Most web explanations seem to ignore Watts, and the few that had something, I wasn't able to relate it to what I found about Amps and Volts (just some complicated mathematical relationship). Is there anything I should know about Watts that would help me match up laptops and suitable adapters?

Thank you for your help.
 
From my limited knowledge, you seem to have understood everything correctly. The effect (in watts) is calculated from the voltage and current (in ampere). Basically, I think as long as the voltage matches, the A and W will "follow" eachother, so whenever one is equal or larger than you need, the other is as well.

Depending on how the power connector looks like, you may want to check the polarity of DC power supplies. Plugging in one with the opposite polarity can (will?) cause some damage. As far as I understand, there is no polarity with AC power supplies since it is alternating anyway.
 
You've pretty much answered your own questions. The only thing I might add is, beware of dual-voltage units as well. Some machines, like my GW2K HandBook 486, require a higher voltage to boot, then step it down for running/charging.

--T
 
As carlsson said, Watts are just Volts times Amps, so you can usually ignore Watts, as long as you know both the Volts and Amps for both pieces.

And, correct, Voltage is the important one. Make sure it matches perfectly, INCLUDING POLARITY. Polarity is usually signified by a little logo that looks vaguely like this:
+ -(•- -
The arc/circle (some have the arc going almost all the way around the dot) means the outside of the plug, the dot in the middle means the center of the plug. In my diagram's case, this means that the outer part of the plug is positive, the inner part is negative. Make sure that you have the same polarity for both the adapter and the laptop.

And some people claim that having TOO high an Amperage can damage the laptop, but as long as it's not a few multiples over, you'll probably be fine. (It *SHOULD* work by the laptop only drawing as much amperage as it needs, but if the adapter is TOO high Amperage, it is possible that it will try to force too much to the laptop.)

And, as Terry says, make sure the adapter lists ONLY the exact voltage (or voltages) the laptop says it needs. I used to have a laptop that needed two voltages, so its adapter had four connections, two for one voltage, two for the other. But as long it has a fairly standard circle-and-dot plug, you're probably fine.
 
Thanks... plus i found a "universal adapter"

Thanks... plus i found a "universal adapter"

Thanks, guys. I had totally missed the part about "polarity," so I appreciate your help.

Along the way I discovered a product that tries to be a "universal laptop adapter" called the Antec NP-100. It looks like it doesn't cover every situation but it sure covers many. There's a good review of it here --- http://www.dvhardware.net/review116_antec_np100.html

I figure if I have to buy an adapter I'll go for the universal one if I can... then I can probably use it for more than one system.
 
I go along with the crowd, voltage should match, amps should at least be equal. If those parameters are met, then the available amps is a non-issue. Consider your car stereo...it's connected to a 12 volt battery that can deliver 100 amps or more.

But on the subject of voltage, I have a Compaq Presario 1610 laptop that "needs" 16 volts. The AC adapter puts out 18 volts when not connected to the computer. I regularly run the laptop on a 12 volt AC adapter with no ill effects whatsoever. In fact, the computer doesn't get nearly as warm running on the 12V as it does when running on the 16V adapter.

Kent
 
DimensionDude said:
I go along with the crowd, voltage should match, amps should at least be equal. If those parameters are met, then the available amps is a non-issue. Consider your car stereo...it's connected to a 12 volt battery that can deliver 100 amps or more.

But on the subject of voltage, I have a Compaq Presario 1610 laptop that "needs" 16 volts. The AC adapter puts out 18 volts when not connected to the computer. I regularly run the laptop on a 12 volt AC adapter with no ill effects whatsoever. In fact, the computer doesn't get nearly as warm running on the 12V as it does when running on the 16V adapter.

Kent
Click to expand...

The 'extra' voltage is split off internally, with part of it directed to charging the battery, and the rest to keep the laptop running. The charging circuit accounts for the extra heat (especially if it's trying to charge a dead or shorted cell). If you check the battery pack, the voltage is probably around 10 - 12 volts, which is all that it really needs to run.

I also use an old universal laptop adapter from RS, which outputs 15 - 24 volts, and most everything in between, by changing a resistor pack. Unfortunately, RS no longer sells anything like it, but they still carry a large variety of tips for it.

--T
 
This discussion is really useful. I've got a couple of machines without powerpacks and I've been wondering about these issues myself.

Good info!
 
benali72 said:
Thanks, guys. I had totally missed the part about "polarity," so I appreciate your help.

Along the way I discovered a product that tries to be a "universal laptop adapter" called the Antec NP-100. It looks like it doesn't cover every situation but it sure covers many. There's a good review of it here --- http://www.dvhardware.net/review116_antec_np100.html

I figure if I have to buy an adapter I'll go for the universal one if I can... then I can probably use it for more than one system.
Click to expand...

Seems like a fair enough price. You probably won't regret buying one, especially if you're dealing with multiple laptops.

I didn't notice anything about the amperage output tho. Perhaps it would be wise to inquire before committing to purchase it. Using an underpowered power supply can be just as dangerous to your hardware as over-voltage. The consequences become readily apparent, when those puffs of MajikSmoke(tm) come wafting out of your power supply, as the laptop tries to to draw more power than your transformer is rated for, which leads to overheating and eventual meltdown.

--T
 
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Well, it says that the DC output is 100W, so, at 24V, you could draw a maximum of 4.16A, @21V, 4.76A, etc, etc.

Probably not quite that simple, but, a good guideline.
 
A bit related: based on the output ratings on an adapter, can I estimate how much power it draws on the input line? As an example, I got an Atari 1050 with a 110V power supply. The supply is marked as outputting 9VAC 31VA. Now I'm thinking to get a step-down converter from 230 to 110V, but am unsure how heavy one I need, given that I will own very few 110V devices and generally not so power hungry. The converters are available in 45W, 75W, 100W, 300W, which I presume is the maximum load on the 110V side.
 
carlsson said:
A bit related: based on the output ratings on an adapter, can I estimate how much power it draws on the input line? As an example, I got an Atari 1050 with a 110V power supply. The supply is marked as outputting 9VAC 31VA. Now I'm thinking to get a step-down converter from 230 to 110V, but am unsure how heavy one I need, given that I will own very few 110V devices and generally not so power hungry. The converters are available in 45W, 75W, 100W, 300W, which I presume is the maximum load on the 110V side.
Click to expand...


I'd think that the 31VA is near enough to 35W, but I'd be a bit concerned about pushing the 45W converter to its limit. Depending on the price differences I'd go with the 75W or 100W just for that extra bit of safety margin.

Kent
 
Ok, here's what I did...

Ok, here's what I did...

Ok, based on everyone's advice, here's one thing I did ...

I have a Compaq Presario 1685 laptop (it's a AMD K-6 CPU @ 380 mhz, with 64M memory and a 4 gig drive).

But I didn't have any adapter. Not very useful !

Then, looking around in the basement, I found I had an "orphan" IBM adapter. (I didn't even know what it was, it was just knotted up inside a pile of printer and monitor cables).

The markings on each item are--

-----item-------- --Volts-- --Amps--
Presario Computer.........19.........3.16
Presario Battery............14.8......3.0
IBM Adapter.................16.........4.5

Based on this information and the advice eveyone gave, I figured it very likely the IBM adapter would work fine... and it did!

Even leaving it on for long periods, no problem or over-heating.

Then I installed Damn Small Linux (DSL 3.0) in addition to the Windows 98 already on the machine, and I have a nice fun little box. THanks, everyone.
 
Correct Adapter/charger

Correct Adapter/charger

MY PACKARD BELL EASY ONE SILVER 2120 LAPTOP ADAPTER (INPUT 100 - 240V 1.5A 50-60Hz), (OUTPUT 19V 3.16A LPS) STOPPED WORKING. I HAVE BOUGHT A UNIVERSAL ADAPTER (INPUT 120 - 240V 60/50Hz 1380mA, (OUTPUT 19V 3.5A. IT HAS VARIOUS VOLTAGE SETTINGS, I HAVE OBVIOUSLY SELECTED 19V. MY QUESTION IS, IS IT OK TO USE DESPITE THE DIFFERENCES AS STATED ABOVE?

ANY IDEAS FOLKS?
MANY THANKS.:confused:
 
Should be, as long as you have the voltage set for whatever the country you are in generates on the input side and the polarity on the output plug is correct for the computer you are about to plug it into.
 
Hi
Most adapters show wattage as the AC input side.
One can't just assume what goes in comes out the
cable. Most switchers are in the 85 to 95% efficiency.
This means there is some loss.
Switchers are funny things with light loads. Many will
over shoot the voltage when too lightly load. Still this
is usually a large factor. To see this type of problem,
one might need to be different by factors of 5 to 10
or greater.
Many cheap adapter have such poor regulation that
they depend on the load to hold the voltage down.
Dwight
 
Dwight Elvey said:
Many cheap adapter have such poor regulation that they depend on the load to hold the voltage down.
Dwight
Click to expand...

Hmm..this might explain something. I have two genuine ZX-Spectrum power supply units. One is from New Zealand, the other is from Europe. Both have input 220-240V.

However, with a voltmeter, one has 12v output the other 15v! The voltage is suppose to be 9V?? Yet they both seem to run the Spectrum perfectly satisfactorily? (I tend to use the one that showed 12v).
 
Is that 12V unloaded? As regarding power supplies, the ones in the UK were built for 240V and could get overheated if run at a lower voltage. The Swedish Sinclair importer already from the beginning rejected UK power supplies and instead bought German ones which were built for 220V and of a different build quality.

I don't know about the ZX Spectrum internally, how strong 78XX chips it has. Recently I read about the Acorn Electron, which natively has an 18V AC power supply. Someone claimed it runs fine on as little as 5V DC (!), which seems remarkable if it is accurate. Perhaps it has some external expansions that would draw more power if they're attached, or otherwise a lot of power must be converted into heat inside the computer.
 
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