Re: Apple IIgs 200w Power Supply

["Luke Forat" <iceman@nospamdigidude.homeip.net>]

    Henry,

    the "switching" terminology comes from the fact that you don't keep the
transistor ON all the time with a variable Base BIAS such as in a linear
regulator but your switch it off and on using Pulse Width Modulation (this
keeps the transistor much cooler and is more efficient). Varying it's duty
cycle regulates the output. Typically, an optocoupler or another small
transformer is used for the feedback to regulate it (the HV part is isolated
from the low voltage output for safety). The whole power supply is lighter
because your transformer core is much smaller due to the higher operating
frequency (1-2Khz or even more for modern PSUs).

    If you place your ear near your Apple when you turn it off (or on, but
it is more recognizable when switching off), you will hear a brief "squeak":
it is the switching PSU (my 1980 Apple//e and all the older Apple IIs we had
in school manifested this). The squeak is the brief "ramp up" (or down if
you are switching it off) and is emitted by the transformer's core, which
vibrates reacting to the lower frequency while it "ramps" up.

    Luke


"Henry" <apl2research@comcast.net> wrote in message
rq6dnZeQ0JAS00feRVn-jQ@comcast.com">news:rq6dnZeQ0JAS00feRVn-jQ@comcast.com...
> Are they switchers?  I didn't look at the transistors or the transformer
too
> closely.  Must be some really old switching technology because as far as I
> know you need two transformers to make a switcher and the Apple power
> supply's only had one.  I just assumed they were linear.
>
> Thanks for the correction.  I'll have to look further in to their design
one
> of these days.
>
>
> Henry S. Courbis
> www.GSE-Reactive.com
> Apple II Series Legacy Hardware - Come take a look at what we have to
offer!