Am I using the wrong power adapter?
March 1, 2006 11:33 PM   Subscribe

used old IBM power adapter on new IBM laptop. The voltage was the same, amperage was lower, but now the machine won't boot with a major Windows 2000 registry error. Am I frying my machine?

After my old-skool ibm laptop died, I just bought a newish used laptop. I wasn't supplied with a power adapter for the new one, but I was assured I could use my old one. my laptop says it takes 16V 4.5A. The power adapter output is rated at 16V 3.36A. I figured these would be close enough.

When I plugged the adapter in, the computer fluctuated back and forth quickly from charger to battery power, like the plug was loose. The machine booted fine several times, then I tried to leave it off and charge the battery as it was fairly dead. After 30 minutes charging, I turned it back on, and now Windows 2000 tries to boot and gives me a fatal registry error. I can't even get into Safe Mode. I'm looking at a complete reinstall, assuming there's not something wrong with the HD.

Is the amperage affecting the system somehow? I have no experience with the difference between voltage and amperage.
posted by Parannoyed to Computers & Internet (8 answers total)
If the amperage is lower, then that means it can supply less total power then the old one. Rather then frying your computer, you could be starving it of juce.

Basically V = IR, where V is voltage, I is current (measured in amperage) and R is resistance. V is a constant, R is determined by what the computer is doing and I is dependant on the other two. The more work the computer is doing the less resistance (because when it's turned off, no electricity goes through R is infinite and I is zero).

So if the computer tries to do too much, the current won't get high enough and something will happen, like the total voltage dropping or who knows what. And the machine will lose power.

That's the high school physics version, anyway.
posted by delmoi at 11:49 PM on March 1, 2006

Response by poster: helpful! thankyou.

Now, I would assume that if the power stops being adequate the computer would simply go into battery mode.

I can't see how that could cause damage to my memory. Am I wrong? Anyone?
posted by Parannoyed at 12:00 AM on March 2, 2006

Best answer: I don't know Ohm's law, but I support and fix laptops. The shop I'm in is lacking power supplies for a number of their laptops, and we have to play swap'em all the time.

With not enough amps you're probably damaging your laptop and/or battery.

With matching voltage and no more then an amp or two more you can usually power a laptop for quite a while, but chances are pretty good you'll eventually run in to problems.

And you can power a laptop for a while with not enough amps, but chances are even better that you'll run into problems, and sooner.

Symptoms of mismatched power supplies include mis-charging batteries, battery not charging at all or beyond a certain point, power bricks overheating, inaccurate battery meters or charging indicators, unexpected shutdowns, reboots and more.

Effects and consequences can be power supply failure, battery failure, charging and system power circuit failure, and pretty much everything else, including the cpu, memory, mainboard and more. Including the (remote) possibility of batteries exploding or power supplies catching fire.

A damaged or failing desktop/tower PC power supply unit providing improper amps or volts will often damage desktop hardware the same way.
posted by loquacious at 12:12 AM on March 2, 2006

Response by poster: Wow. Okay, good to know. I could find nothing like this on the Net. Do you think problems are likely to show up within a few minutes of the wrong adapter being plugged in, or is it more likely to take a while?

posted by Parannoyed at 12:34 AM on March 2, 2006

I would think the lower amperage is the reason that you are constantly switching back and forth between charger and battery power. The adapter is not providing enough juice to both keep the battery charged AND run the laptop. I don't know if the constant back and forth context switch in Windows would necessarily cause huge problems, but I would think that switching power profiles constantly like that certainly isn't a GOOD thing.
posted by antifuse at 3:21 AM on March 2, 2006

Best answer: antifuse's explanation of the mode switching problem you experienced while the laptop was in use sounds very sensible. I wouldn't have expected that to lead to problems charging the battery while the laptop was off though. Also, despite the explanation that follows, I don't see any reason why you would see a registry error connected to the improper power adapter. Also, despite what I am about to say, I probably would have done all the things you did (confirmed here) - it should have worked...

To connect delmoi's and loquatious' answers...

Common cheap battery charging circuits can be extremely sensitive to every detail of the power supply they operate from; however, I would have thought charging circuits in laptops (or laptop batteries) were very sophisticated. loquatious' experience makes that seem much less likely.

It is certainly possible to overheat/burn-up electronics by supplying too little power. This can happen due to low voltage, low current, or both.
These are just the first examples I could think of...
low voltage: Think of transistors as voltage controlled resistors. The higher the voltage, the lower the resistance of the resistor. Power dissipated = I^2xR. So, if for some reason the current wanted to stay the same but the voltage was lower, you would see more power dissipated, which could overheat the part.
low current: Some parts, or circuits, might rely on a maximum current limit to turn off. If you give that device too little current it may stay on improperly and then overheat.
combined: It is very common for too little voltage in one area to lead directly to either too little or too much current in another area, and vice versa, and every other possible permutation you can imagine.
All this being said, well designed charging circuits (well designed electronics in general) should easily account for the current provided (to some extent the voltage too), and adjust accordingly (either work, or fail to operate without damage).

Related questions:
You should probably check out Why so many AC adapters?

People with apple laptops don't seem to have any problem swapping adapters: Can I use an iBook's power adaptor for my PowerBook? yellowcandy 's problem might have been related, but probably not: Got to get myself connected... If you think your problem is strange, check this out: Why does my laptop run 10 degrees hotter when plugged into a ground fault circuit interrupter receptacle?

Finally, here are some questions that discuss battery charging, if you are interested: Can you mix rechargeable batteries with different mAh ratings?, How do I prolong the life of my electric toothbrush's battery?, simultaneous homebrew batteries charging/use?
posted by Chuckles at 6:02 AM on March 2, 2006 [2 favorites]

Response by poster: Gosh, you guys are the bestest. Long live the Hive Mind!
posted by Parannoyed at 7:55 AM on March 2, 2006

A new laptop power supply thread - The sound of silence.

Also, improved 'low voltage' description:
low voltage: Think of transistors as voltage controlled resistors. The higher the voltage, the lower the resistance of the resistor. Power dissipated = I^2xR. Now, consider some transistor in the middle of a complex circuit. If the voltage controlling it droops, it's resistance goes up, but the current flowing through it doesn't necessarily drop accordingly. You would see more power dissipated, which could overheat the part.
posted by Chuckles at 9:45 AM on May 19, 2006

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