Expro universal power adapter doesn't do 18.5V

Hi all,
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.
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.. 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.
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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.
Reply to
Salmon Egg
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 likely fine.
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.
Reply to
123Jim Inscribed thus:
The 18 volt setting will be fine. Most bricks are only a nominal voltage +-5%. The only laptops that will care are the ones that look for the ident from the power brick.
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The only problem os MY FACE IS SMELLY....
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Thanks go to those who replied on topic I used a multi-meter (and some soldering wire I happened to have handy) ..... it tells me the adapter is producing 18.51V ... so that'll do nicely!
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