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Voltage tolerance for a laptop

juliasdream
Grafter
Posts: 260
Registered: ‎09-06-2007

Voltage tolerance for a laptop

Does anyone know how much excess voltage a laptop can stand?
The charger is supposed to put out 18.5V and I just tested it  with a multimeter and it's reading 19.33V
I am wondering if thats a bad sign and if that's the likely cause that has killed the battery(Thankfully not the laptop)
5 REPLIES
wacky17
Grafter
Posts: 134
Registered: ‎25-06-2008

Re: Voltage tolerance for a laptop

It isn't just the voltage you have to look at when using a adapter on a laptop, take note of the amps too.
alanb
Grafter
Posts: 459
Registered: ‎24-05-2007

Re: Voltage tolerance for a laptop

I assume you are measuring the voltage while the PSU is disconnected from the computer, in order to get access to the PSU output plug. In this situation, a slightly high voltage reading is normal.
If you measure the PSU voltage while no current is being drawn it will normally be a few volts higher than the rated output. You'll only get an accurate voltage reading when the PSU is plugged into the computer and it is drawing some current.
Community Veteran
Posts: 14,420
Thanks: 721
Fixes: 12
Registered: ‎01-08-2007

Re: Voltage tolerance for a laptop

I wouldn't worry about it. Alan has explained it pretty well (nice one fella I'd often wondered about it myself but never realised Ohms affected PSUs with no load). I've had a few adapters with higher voltages than the device and never had any harm done to any of them.
I need a new signature... i'm bored of the old one!
alanb
Grafter
Posts: 459
Registered: ‎24-05-2007

Re: Voltage tolerance for a laptop

It is also worth bearing in mind that the active components in a laptop all operate at lower voltages (typically: 15 volts, 12 volts, 5 volts or 3 volts) and that the 18.5 volts from the adaptor has to be converted to these voltages inside the laptop. (Though some laptops that use flourescent backlighting for the LCD display have to generate several thousand volts to drive the backlight too.)
The 18.5 volts rating is nominal, it does not need to be exact, but probably should not be lower than this value. It will have been chosen by the hardware designers because it is the lowest possible value that they thought could be reliably converted to the required voltages and also charge the batteries without wasting excess energy which would be converted to heat. 19 volts also seems to be a common rating for laptop power adaptors.
alanb
Grafter
Posts: 459
Registered: ‎24-05-2007

Re: Voltage tolerance for a laptop

I think the upward drift of the voltage on power converters when they are not under load is largely due to the design of the smoothing circuitry on the output stage of the PSU okrzynska.
The components in the smoothing circuit act a bit like a rechargeable battery when the circuit is not loaded. This is most noticeable on old-style transformer-based power converters which used large capacitors for smoothing that can cause the voltage to appear maybe 30% higher than it should be. Modern switch-mode power supplies only require a lightweight smoothing circuit, so the voltage increase under no-load conditions should be much smaller, and may not even exist in a well designed switch-mode circuit.