On 15/01/12 10:52, Aley Keprt wrote:
Colin, thank you very much!!!
Also I always wondered why there is a zener diode (I never saw this kind
of computer power supply) instead of IO 7812.
It's a cheap and nasty way of getting 12v.
And technically, why there is that R1 resistor? Is its purpose to limit
maximum current on 12V line (i.e. something like a cheap reversible fuse)?
The voltage coming of the transformer is nominally somewhere like 18v.
To get it down to 12v all the current is put through the 33ohm resistor
which has the 12 zener tied between the other side and ground.
The 12v supply (drawn from memory) is:
(excuse the poor ascii graphics.)
----18v------\/\/\/\/\--------------12v------
| 33ohm |
| ¬
=== /\ 12v zener
| 4700uF --
| |
-----0v--------------------------------------
So under no load there is a drop of 6V across the 33ohm resistor which
gives a current of 0.18A which is just over 1W to be dissipated as heat.
This also means the zener has to lose over 2W of heat as well. As the
load from sam increases, by running two disk drives for example, more of
the current goes through sam and takes the load off the zener but the
resistor still has to burn off the excess voltage.
The problem with this type of regulation is the resistor has to have a
small enough resistance to let enough current through under max load
conditions and heavy enough to dissipate the heat.
If you're not using all of the 0.18A available, e.g. you only have one
floppy drive, then you can increase the 33ohm resistor which will still
drop the excess volts but draw a smaller current, and so be cooler.
These days, of course, a 7812 would more than likely be cheaper than a
big resistor :-)
Nev
