On Fri, 2005-09-09 at 08:17 +0100, Ted.Harding@nessie.mcc.ac.uk wrote:

On some laptops, the external mains supply passes through an external transformer and it is the reduced voltage which is plugged in to the laptop.

Pedant mode on, but technically it is not just a transformer. A transformer of the size required to power a laptop would be rather large, expensive and cumbersome. All modern computers use something called a Switch Mode power supply to do the voltage conversion. The principles within are very different (and not really worth going into here) but SMPSUs are far less cumbersome (and more efficient) than a traditional regulated transformer supply would be.

On others, the mains lead is plugged directly into the laptop, and the voltage reduction is done internally.

I remember early Tosh Satellites being this way...the old battle grey ones. You don't see it now because power requirements have tended to trend upwards and there simply isn't the space (or internal cooling capacity) to deal with an internal PSU.

In the former case, heat generated in the reduction is dissipated externally, while in the latter it has to escape from the interior of the laptop.

This is true and most likely part of the reason why the PSU is now always an external brick.

All laptops still have some degree of internal voltage conversion. It's only the bulk of it that is done externally. So generally the brick will convert mains AC down to 12-19V DC which is then converted down to various other voltages (and converted up for the backlight) internally.

So, other things being equal, one would expect the latter kind to run hotter than the former.

Slightly yes...but you would hope that designers had taken that into account. That said a well designed PSU shouldn't need to dissipate as much heat as say the CPU.

Remember that you cannot create or loose energy, only convert it.

So a PSU with 90% efficiency can only turn the other 10% into heat because it is not able to provide energy output by another means (i.e kinetic) so a 100 watt 90% efficient PSU will need to dissipate 10 watts of heat.

However in energy terms the only (tangible) output of a CPU is heat. So a CPU that consumes 40 Watts of energy has to dissipate 40 Watts of heat. (physics buffs on list will be quick to point out that this is not 100% true...but it is near enough)

This is why (except in rare cases) you don't see fans on the PSU brick.

This certainly seems to correspond to my experience. I have had 4 (ancient) laptops, 3 still working. One of these has the mains plugged in directly, the others all have an external transformer.

I have never had heating concerns with the ones which have external transformers, while the "internal" one does run pretty warm, and periodically exhibits screen-flashing/breakup (especially when running "graphics heavy", as in X with lots of re-draw).

I've never experienced that on the old Tosh'es but I'd say that it sounds like a problem with the Video RAM...Memory chips can fail in interesting ways..one of them is heat related failure where bits fail beyond a certain die temperature. When you are pushing the Graphics or Processor things may heat up enough to cause the underlying error condition.

In short I doubt that the fault is due to excessive heat, but simply induced when the system temperature gets close to it's operational limits.

(There is an internal fan, which cuts in occasionally, but it seems to be a sleepy bugger and doesn't often wake up ... )

it is more likely that the fan is there to cool the CPU more than anything else...it's been so long since I have been inside one of those Tosh'es I can't remember the exact layout

Any comments welcome, and will be read with interest!

By pressing the lid close button you are probably causing the graphics hardware to reset thus the "bad bits" will be gone for a while.

Hope that helps

Wayne