OT: Need PC MB help!

From: jpero_at_cgocable.net <(jpero_at_cgocable.net)>
Date: Fri Dec 24 10:16:47 1999

> Date: Fri, 24 Dec 1999 18:44:55 +0000 (GMT)
> Reply-to: classiccmp_at_u.washington.edu
> From: ard_at_p850ug1.demon.co.uk (Tony Duell)
> To: "Discussion re-collecting of classic computers" <classiccmp_at_u.washington.edu>
> Subject: Re: OT: Need PC MB help!


Hi Tony,

> Towers lists a D45H2 as a PNP power transistor, Vce=30V, Ic=10A. Of
> course that's the one you could measure the parameters of, since it's
> still good. I can't fid any HB numbers in Towers -- are you sure it's a
> transistor and not a 3-terminal regulator, for example. Maybe you could
> trace out a little of the circuit around this transistor to see just how
> it's used.

That's what it is. Transistor(s) acting as "regulator" controlled by
microampere regulator like Astec '431 in a TO-92 (ie found on
pcchips boards), or usually by (usually national semi.) 2951 IC.

That D45H2 explains the 10A rated. P5 cpus draws about 3A or less
for sub 100mhz, 100 < 150 is about 3A to 4A, 166 and 200 is about
4.25A and 4.6A. Don't forget: chipset, cache draws on this 3.3V
current as well. But! Linear regulators are inefficient at
channeling from 5V to 3.3V, very often mounted on too *small*
heatsinks or in poor or stragent airflow. That is why they run hot
enough to make one's fingers jerk out with a owch!

I have swapped or robbed other boards for good power transistors in
that design about 10-15 times. That is very common occurance.
They just simply blow for no reason. If a chip is really bad it
will heat up like crazy but not that sudden death in linear
regulator except if it's really aged and on the edge and adding that
cache stick pushed it over. And failure mode is shorted and
externally usually no sign of failure like holes or smells.

This is why m-board makers have since converted to switching inductor
regulator design for reliablity and more current capacity around
'95-96. Very little heat, most of heat if any is from coils.
Failure very rare in my experience regarding those switcher regulator
even in cheapo boards. (!!)

PSUs in laptops used same idea and often mosfet and like doesn't have
any heatsinks at all or just very modest one. Look into notebooks,
pii/piii/athlon/ss7 boards for that examples.

Wizard

> -tony
 
Received on Fri Dec 24 1999 - 10:16:47 GMT