Question about Voltage and AC Adapters

November 16, 2007 4:53 PM

Am I ruining my laptop with this AC adapter?

have a question about AC adapters and voltage.

For work, I use a Toshiba Satellite U305-S7467 laptop. It's a great machine, but I forgot my power plug at home this morning. I've been meaning to buy a second unit for a while now (one to have at home, one to have at work), so this more or less forced me to act today.

I went to a Radio Shack a few blocks away from where I work and ended up buying an iGo AC power adapter. Here's my question: am I hurting the laptop by using this adapter?

On the laptop itself, the specs read: DC 19V -- 3.95 A. I checked on the Toshiba site and the specs said that it included a 75 W adapter.

Here's the unit I bought this morning:
http://www.igo.com/product.asp?sku=3726890

On the back of this iGo unit, it reads: INPUT 100-230 V -- 2A, OUTPUT: 15-24 V, 65 W max.

The guys at the store said that most laptops don't draw 75 watts continuously, so it shouldn't be a problem. But I'm wondering if I'm doing damage at all to the laptop by running it on a power adapter with a lower voltage. The problem is I just don't know enough about how they work to make a decision. Normally, I would just buy a replacement dierctly from Toshiba, but I was kind of in dire straits this morning.

What do you think?

posted by zooropa to computers & internet (11 comments total)

I think it's unlikely to be a problem. I'm pretty sure the laptop power jacks connect to another regulator inside the machine that deals with charging the batteries properly and so on.
posted by hattifattener at 4:58 PM on November 16, 2007

(If there is a problem, I think it's more likely to be from the fact that the iGo adapter appears to be only loosely voltage-regulated, putting out "15-24 V" when your laptop expects a steady 19. So when you're not drawing much power, the voltage from the adapter might drift up to 24V and possibly damage something. Dunno.)
posted by hattifattener at 5:12 PM on November 16, 2007

I wouldn't worry about it. Just because the transformer is rated for 75watts, doesn't mean the Toshiba is drawing 75watts. Typically the draw would be approx 20% less than the max on the transformer for optimal transformer life. So 80% of 75watts being 60watts, you're well under the 70watt limit of the igo unit.
posted by wile e at 5:12 PM on November 16, 2007

For practical purposes, if it is working fine right now, don't worry about it.

Ideally you would want to monitor the temperature of the on board voltage regulator over a few hours of use.. You might be able to feel for unusual hot spots, and it would be worth doing, but it isn't really likely to warn you of a problem, and unfortunately anything more thorough isn't feasible.

Pay attention to the temperature of the power adapter you bought. It might be overloaded, which could lead to overheating and failure of the adapter.



Am I ruining my laptop

Don't think of it as a process.. In general, if you get it wrong, electronics fail spontaneously and catastrophically. There is a heating up factor, because it takes between several minutes and several hours to reach peak operating temperature, so spontaneous failure can happen quite a while after the connection is made. It can even happen much later, due to a hot day, or a thick coating of dust..
For the most part though, the idea of doing damage over time just doesn't apply to electronics.


Also, a very detailed response here.
posted by Chuckles at 6:20 PM on November 16, 2007

OUTPUT: 15-24 V

It is very odd that a switching power supply based adapter would have so much output variability. I suppose it must be related to the input voltage range, but I didn't think that was typical..
posted by Chuckles at 6:30 PM on November 16, 2007

Chuckles, it's a universal powersupply for a range of laptops. I think they have multiple outputs, ranging from 15-24 V. Which is tapped depends on the adapter.
posted by Good Brain at 6:54 PM on November 16, 2007

Switched based on the plug you stick in the end of the cable? I don't know how they get that to work...

Ah, I see. The tips have extra wires so that they can set the power supply output level. Neat idea, but not exactly a universal solution. Lots of 5.5mm OD, 2.1mm ID devices with all kinds of voltage ratings.. And, kind of pricey..

Whatever works :P
posted by Chuckles at 7:10 PM on November 16, 2007

The comment in the other thread suggests that it is bad to use an adapter with lower amps.
posted by Mr. Gunn at 7:48 PM on November 16, 2007

Well, the datasheet claims it can do 90watts, but the marketing says 70. The amazon page says it automatically chooses voltage, which scares me. Considering you've done a charge already I would not worry about it.
posted by damn dirty ape at 7:52 PM on November 16, 2007

Doing exactly that (only with a DC adapter) damaged the battery in my old laptop. Of course, it was much more on the ragged edge of being enough power, to the point that I had to turn down the backlight brightness for it to output enough to charge the laptop.

In the worst case, you'll be accelerating the demise of your battery. Not an ideal outcome, to be sure, but far better than accelerating the demise of your laptop!

If it's not oscillating between charging and not, it'll probably be fine.
posted by wierdo at 10:14 PM on November 16, 2007

Don't think of it as a process.. In general, if you get it wrong, electronics fail spontaneously and catastrophically.

Well, you can damage batteries over time by doing weird things to 'em.
posted by delmoi at 1:03 PM on November 18, 2007