Under Voltages

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I know various manufacturers have outputs from their powerbricks ranging
from 14 volts IBM to 18 volts Compaq.

What exactly are the potential damages caused by running at less than
the rated voltage (amperage remaining the same).

For example the two units I mentioned before what would happen if I had
only 12 volts available?.

I know batteries do drop in voltage output as they discharge so does the
circuitry allow for a voltage drop and then cut out at say 7 volts or

Thanks for any help.

Re: Under Voltages

John LaBella typed on Tue, 04 Mar 2008 14:54:42 GMT:
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Hi John... 7 volts is too far off. If you try you could risk damage to
either one. Plus the less voltage, the more the the laptop will draw to
make up for the loss of power. Plus even if you have the voltage right
and enough amperage, you also have to have the polarity correct as well.
As you can fry either the adapter or most likely the laptop as well.

If you don't listen to me and the polarity is correct, I would assume
what would happen is that the power light on the laptop will flash,
indicating that the adapter is faulty. In this case, the voltage is too


Re: Under Voltages

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There are several things that might happen.

1. First, the amperage will not be the same.  The current *rating* of the
bricks may be the same, but the actual current drawn depends on voltage
and load.  If your laptop is designed to draw 70 W from a 20 V supply,
that's 3.5 amps.  If it works when connected to a 14 V supply, it will
be drawing 5 amps for the same power.  The heat dissipated in many
components is proportional to the square of current, so some components
in the laptop power supply might be dissipating twice the heat they were
designed for.

2. The laptop might operate from the brick, but might fail to charge the

3. If the voltage is substantially too low, the laptop probably won't
operate from the power brick at all.

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Impossible to say for sure without knowledge of the design.

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Yes.  But the laptop may not have the same voltage limits for the
external power supply as it does for the battery.  Generally, the power
brick voltage will *not* drop unless it's defective or overloaded, so
no need to design the laptop to operate on low voltage from the input.
Also, the external voltage generally has to be a few volts above the
battery peak voltage in order to fully charge the battery.


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