Why 19v adapter on laptop when they run on 12v

[selldoor]

Couldnt decide between Power supply and troubleshooting a laptop but a general sort of a question that I was asked and thought 'I should know the answer to that but dont.' Have since tried to find an answer on line but nothing concrete.

As title - most laptops have a 19volt adapter but run off 12volts.
Other than needing a higher voltage to charge the battery is there some good reason for that.
This also ties in with the old chestnut - if my car is 12 volts and the laptop runs off a 12 volt battery why do I have to get a 12> 240v(120v) inverter then use an adapter to get it down to 19v to run the laptop - should it be possible to feed 12 volts -direct into a laptop to power it.

Simple answers only please.

[rogfanther]

In the car : it runs in 12V, but depending on motor speed, etc, the generator varies the voltage, so you could have peaks of like 19V going to your electronics. One could also use a DC-DC converter with a wider voltage input ( say, 10-30V ) for in-car equipment.

In the laptop : I don´t know

[larrymoencurly]

I think the power adapter voltage is related to the battery voltage, and many laptops offer both a standard 4-cell lithium battery (12V) and an optional high capacity 6-cell battery (18V). So I guess using 19V makes the circuitry inside the laptop that regulates the charging a bit simpler.

[tom66]

Because the laptop needs to charge the battery and run from it at the same time. A buck converter is used to charge the battery; but this needs a higher voltage than the battery supply. And once you go a few volts above, you may as well go 5-10V above, because the efficiency can be improved with higher voltages (less cable voltage drop.)

You should never run a laptop directly off a car battery, because:-
- During cranking the battery drops to as low as 6V for several seconds. Who knows what the laptop will do. Maybe it will cut out, or it could lock up and crash, or get into an unknown state.
- During running the battery varies from 12.5V - 14.4V.
- Spikes of from -30V to +70V for 400ms are present due to load dumps and transients in a typical automotive system. YES, the supply does go negative sometimes for long transients which will easily destroy any typical semiconductor power converter. I'm currently designing a product which must plug into the OBD-II socket on a car - which has absolutely NO filtering.

Another reason for the 19V may be because it allows the 12.5V - 14.4V from a car to be boosted to 19V, using a single topology (boost), reducing the cost.

[ben7]

Perhaps the reason is similar to this?: Why do modern motherboards use 12v for the CPU power and not 5v?

[budm]

TOM covers just right. I agree with Tom's statement.

[severach]

So that every voltage regulator is buck. A buck converter requires the input to be a couple volts more than the expected output. Having to add boost circuitry to a few buck converters would raise cost and part count. Like water they like voltage to always be flowing down hill.

Laptops have a system voltage that supplies the high current regulators including the charger. The system voltage is the battery or the power adapter, whichever is higher. The system at battery voltage is enough to run all the 5v and lower voltages. The charger is disabled. When the AC adapter is connected the system voltage jumps and the low voltages stay the same by adjusting PWM. The system at AC adapter voltage is high enough to allow the battery to be charged through its buck converter.

Why some laptops are so voltage sensitive is not clear. The buck converters don't much care what the input voltage is. Any voltage from 15v to 25v would work.

[tom66]

It's quite likely that they have an UVP and OVP circuit to detect abnormal conditions. My 20V ThinkPad will not charge from a 19V Dell adapter.

[b700029]

Originally Posted by severach

So that every voltage regulator is buck. A buck converter requires the input to be a couple volts more than the expected output. Having to add boost circuitry to a few buck converters would raise cost and part count. Like water they like voltage to always be flowing down hill.

Laptops have a system voltage that supplies the high current regulators including the charger. The system voltage is the battery or the power adapter, whichever is higher. The system at battery voltage is enough to run all the 5v and lower voltages. The charger is disabled. When the AC adapter is connected the system voltage jumps and the low voltages stay the same by adjusting PWM. The system at AC adapter voltage is high enough to allow the battery to be charged through its buck converter.

Why some laptops are so voltage sensitive is not clear. The buck converters don't much care what the input voltage is. Any voltage from 15v to 25v would work.

The main system bus is usually in the 14-19V range for most laptops. There are some exceptions for those that have lower power requirements (Eee for example, 9.5V but should be OK with quite a bit higher - check schematic to be sure).

Originally Posted by tom66

It's quite likely that they have an UVP and OVP circuit to detect abnormal conditions. My 20V ThinkPad will not charge from a 19V Dell adapter.

They all have comparators that will detect if the system bus voltage is higher than a certain threshold to enable charging and other things, they might have ones for overvoltage too but I'm not too sure on that (again, depends on the exact model, check schematic to be sure.) Also some Thinkpads have adapter sense too, it's analog and uses a resistor on the center pin unlike Dell's EEPROM.