1. Junior Member
Join Date
Sep 2007
Posts
26

## How to understand Laptop Adapter power ratings?

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?

2. 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.

3. 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

4. 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.

5. Junior Member
Join Date
Sep 2007
Posts
26

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

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.

6. 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

7. Originally Posted by DimensionDude
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
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

8. 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!

9. Originally Posted by benali72

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.
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
Last edited by Terry Yager; October 31st, 2007 at 05:13 PM.

10. 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.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•