Linear variable voltage psu for laptop and desktop motherboard repair

[varunme]

can we use Linear variable voltage psu for laptop and desktop motherboard repair,
like the linear variable voltage regulator, lm317 or anyother option to build better one ?

[mariushm]

Re: Linear variable voltage psu for laptop and desktop motherboard repair

In theory you could make a linear power supply, but in practice it's not cheaper.. so no.

LM317 and other basic, cheap linear regulators can only provide up to 1-1.5A of power. Even the less power hungry laptops require 4-5A of power.

More expensive linear regulators could do 4-10A (and you could connect several to work together for higher current) but then you still have the problem of heat dissipation: the difference between input voltage and output voltage is converted to heat by linear regulators.
So for example, if your laptop needs a 19v 4A power supply and you have a 18v AC RMS transformer, the input voltage to linear regulators will basically be 18v x 1.414 - 2v = 23.5v so you'll waste (23.5v - 19v) x 4A = 18 watts as heat. The heatsink would be bigger than the laptop if you don't use a fan.

[varunme]

Re: Linear variable voltage psu for laptop and desktop motherboard repair

A parallel transistor like 3095, togather with lm317 can handle big currents isnt it ?
I plan to supply dc voltage to lm317 using a pc smps

can i go that way ?

[Khron]

Re: Linear variable voltage psu for laptop and desktop motherboard repair

I'm wondering why would you need a *variable* voltage PSU in the first place? Supply voltages for computers are pretty standard - 3.3v, 5v, 12v (for desktops), and 16-20v coming out of laptop chargers.

A PC SMPS can take care of the first three, and the latter could be taken care of by, say, a second modded ATX supply (mods to raise the 12v output to the aforementioned 16-20v), or something like a car-adapter (which is basically a boost-converter), that can produce the 16-20v from the first ATX's 12v output

[eccerr0r]

Re: Linear variable voltage psu for laptop and desktop motherboard repair

I think varunme wants to have a variable supply because repairing multiple machines would then not require a stock of different power supplies - one for every single type of machine run into. There is no reason why a linear supply couldn't be used except they will generate a heck of a lot of heat.

That being said, some laptops have custom power bricks with the ID function, and may or may not charge properly without the ID bit if you don't somehow override it. This may force people to get the proper brick, even they are all still the right voltage...