Why do laptops need 19V whilst desktops need 12V? If no parts use 19V in a laptop, then what is/are the voltage(s) for the components that are typically 12V in a desktop? (e.g. fans, disks, DVD drives, floppy drives, GPU, CPU) What is the main voltage of the motherboard if it isn't 19V? Also, what form factor are laptop internal PSUs (like ATX in a desktop)?
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1In my experience, most modern laptops use external PSUs. Which might be because of the heat. Also, I have a Thinkpad fan lying around here, and it is labeled 5V.– berndbauschCommented Jul 29, 2021 at 0:45
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If there's no form factor PSU, then there must be some way its power supply is controlled by software.– user1477539Commented Jul 29, 2021 at 0:48
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SATA standard requires 3.3V, 5V and 12V.– berndbauschCommented Jul 29, 2021 at 0:52
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If there's no form factor PSU, then there must be some way its power supply is controlled by software. .... Yes. There is power supply circuity on the motherboard after it gets the main 19V.– anonCommented Jul 29, 2021 at 1:06
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"Why do laptops need 19V whilst desktops need 12V?" -- You're comparing apples to oranges. A desktop uses AC voltage, not "12V". The internal PSU of a desktop has a 12VDC rail. The laptop simply avoids using components that require 12VDC.– sawdustCommented Jul 29, 2021 at 3:52
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2 Answers
Laptops typically do not have power supply units (PSU), like desktops. Since laptops are designed to be as light and thin as possible, putting a PSU in the laptop is not feasible. There are some electrical regulation components in the laptop, to make sure the power coming in the correct voltage and amperage. However, laptops PSUs are the "brick" in the power cable. That is what converts the alternating current (AC) from the wall's power socket to to direct current (DC), which the laptop uses. Laptops do not conform to form factors, like desktops. There is no AT, ATX, mini, micro, etc. Again, since laptops are designed to be as light and thin as possible, the design of every laptop is unique.
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No form factors? Then what part is responsible for turning off the laptop after shutdown? Commented Jul 31, 2021 at 18:52
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@user1477539 I do not understand what you mean. Form factors have nothing to do with power or shutting down. When modern computers shutdown, they send an APCI signal to the motherboard when complete, which turn off power to computer. However, they are not really off. They are still drawing a small amount of power to detect the power button being pressed and/or other power on signals like Wake-on-Lan.– KeltariCommented Aug 1, 2021 at 2:09
A lithium ion battery has a full charge voltage of 4.2 volts.
Please see Li-Ion cell voltage
Li-ion battery also has a voltage and capacity rating. The nominal voltage rating for all lithium cells will be 3.6V, so you need higher voltage specification you have to combine two or more cells in series to attain it. By default all the lithium ion cells will have a nominal voltage of only ~3.6V. This voltage can be allowed to go down upto 3.2V when fully discharged and go as high as 4.2V when fully charged.
So if you have 4 cells, you get to the higher voltage that a laptop uses. Desktops have no operating batteries. That, and regulation, is the primary reason.
With respect to your question on a separate PSU in a laptop, there is simply no room in a modern laptop. My laptop is 1/2 inch thick including screen. No room for an Ethernet port let alone a PSU. All Laptops are different. So for PSU, very largely, no such thing.
See also this good article from Stack Exchange.
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Another question I had: "what PSU form factor do laptops use?" Commented Jul 29, 2021 at 0:47
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The power supply in a laptop is part of the motherboard (with some charging parts in the battery). There is not a separate PSU box like a desktop. Many laptops today are 3/4 inch or less in thickness including the screen,– anonCommented Jul 29, 2021 at 0:50