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Posted by Dave Martindale on March 11, 2008, 2:16 am
Please log in for more thread options >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).
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
battry.
3. If the voltage is substantially too low, the laptop probably won't
operate from the power brick at all.
>For example the two units I mentioned before what would happen if I had
>only 12 volts available?.
Impossible to say for sure without knowledge of the design.
>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
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.
Dave
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