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Laptop power supplies, does current matter?

I have the opposite problem outlined in Laptop power supplies, does current matter?

My AC adapter (19 V, 3.95 A, 75 W, 3-pin) Part No. PA3468E-1AC3 for Toshiba Laptop L40 18Z is now useless. I am currently risking using an IBM Thinkpad adaptor (my old laptop) after checking the specification. The centre pin polarity is the same, I cannot currently find any references to Wattage output for the IBM adaptor - but the only difference I can find is in current and voltage.

The defunct Toshiba adapter is rated 19 V and 3.95 A. The IBM adapter is rated 16 V and 4.5 A.

I've used the IBM for several hours with no ill effect - risky I know! The lower voltage means that the battery isn't charging - but my concern is the higher current - 0.55 more.

Could this damage the sensitive internal electrics in the longer term - or is it unlikely to make a difference. If it works, which it does - could I confidently continue to use this until I've sourced a proper replacement?


3 Answers 3


Here's the deal. The voltage SHOULD match (power adapters provide a steady voltage over a range of currents), whereas the current MUST meet or exceed.

If the voltage is too low, you'll get either nothing, or odd / failure behavior when the power draw on the system gets high. If the voltage is too high, you'll risk damaging the equipment you are powering.

If you have too low a current, you will either burn out the supply, trip a breaker or fuse in the supply, or the equipment will not work or fail unpredictably at high load.

If you have too high a current, the system simply won't draw as much power as the supply can provide. The supply might be a little less efficient, but no harm is done.


The current rating is the amount that's available. It's not getting forced on the device that it's connected to. For comparison, you can plug your toaster into the same outlet as you'd plug your vacuum cleaner into, but the current they draw (and can handle) is quite a bit different. (This is for comparison to the output of the adapter, ignoring the AC input side.)

I'd be more worried about the undervoltage. It may put a strain on the voltage regulator in your laptop and may also put a strain on your battery (which you might consider removing while you're using the mismatched power supply).

  • 3
    TL;DR: you need as much or more amperage as your device specifies. You need matching voltage, within a given margin. If your voltage is off, Bad Things <tm> happen.
    – Daniel B.
    Commented Aug 5, 2011 at 16:03

If the battery is not charging it should not do any damage. You might save a bit more by dimming the LCD.

Besides: 16V x 4.5A = 72W power.