My second laptop power cable began to get flaky .. intermittently failing to
power/charge the laptop/battery.
I decided to get a replacement for it in the shape of an 'Expro Universal
Laptop AC Adapter'
My previous power brick is rated at 18.5V for 3.5A .. but this universal
power brick only does voltages in whole numbers: 12V, 15V, 16V 18V, 19V @
4.5A and 20V, 24V @ 3.75 Amps.
.. it says: "covers between volatages also" on the above linked
Which voltage should I choose, 18V or 19V?
What might be the problem for the Laptop since I can't choose 18.5V?
It seems to be working fine on 18V at the moment in initial testing.
I am not familiar with this brick. Almost all of them put out voltages
greater than indicated. Most items don'e care. Only if you need
filtered output would that even be a possibility. When charging
batteries you need excess voltage anyway.
If you can, look at the output with a scope.It is likely to be merely
rectified ac. Only during the peaks will current flow to charge the
battery. I would just use the 18V setting.
If you burn your stuff up, I said nothing. This advice is worth less
than you paid for it.
Based purely on user experience (try and find a spec), laptops
*are* sensitive to the voltage delivered by the adapter. One
person had a failure after 30 days of using a "universal" adapter,
so some care must be used. If you feel more heat, coming from the
laptop power management area, that would be a sign you're feeding
it too much voltage. Or, if too little voltage, you might find the
battery never has a full charge on it.
If all systems are functional at 18V, and you're getting a full charge
on the battery, no further experiments are necessary. The allowed range
is down around +/- 1V or so, so you should do your level best, to match
the original spec if possible. The difference between 18.5 and 18.0 is
As one other poster indicated, verifying the voltage is a good idea,
since the new adapter will have a tolerance on the stated numbers,
and a setting of 18.0, may not be delivering 18.0 after all.
If you want to verify the voltage, that'll be harder to arrange. An
auto mechanic, would probably use sewing needles clamped to multimeter
probes, as a way to pierce the insulation on the wires and take a
reading. An engineer would likely prefer to use an adapter cable,
with perhaps a male and female barrel connector, so you'd get a
voltage sample safely. If the meter probes short together, while
you're taking your voltage reading, that could damage something.
(The adapter probably has overcurrent protection, but you never know.)
As far as I know, the laptop adapters are switching power supplies,
as a transformer/bridge rectifier/cap solution would be too heavy for
portable usage. Switching power supplies are much lighter by
comparison. A switching supply will deliver pure DC, and a
garden variety multimeter set to DC volts, is all you need
to check the output. There are two possible measurements
you can make - open circuit voltage from the adapter, or
check the voltage while the adapter is in usage.
The adapter regulates to the voltage "it can see" - if the adapter
had remote sensing (two sense wires that snake down to the load end),
it can compensate for the voltage drop in the cable under load. (ATX
power supplies use that method, but only typically on the 3.3V rail.)
If the adapter only checks the voltage as created inside the adapter,
then the adapter will have poor load regulation, and you'd see some
variation (no load vs full load), down at the connector end. The
voltage drop in the cable, gets subtracted from the regulated
voltage inside the adapter.
If you don't want to even think about the multimeter thing, no problem,
just continue to use the 18.0 setting.