From: Paul Keinanen on Sat, 05 May 2007 18:35:57
+0300

On 5 May 2007 04:32:02 -0700, Tim Shoppa wrote:
William E. Sabin wrote:

"Computer Grade Capacitor" might possibly refer to the low value of internal
series resistance and inductance which are important parameters in switching
regulator power supplies.

Well, maybe "low inductance" by 1960's standards. A computer power
supply would have to filter 60Hz, or maybe 400Hz ripple then.

Apart from aviation, the last time the last time I heard someone using
400 Hz for powering computers was in the early 1960's with a 50/400 Hz
motor/generator in the basement, feeding the anode supply of some tube
computers :-), in order to reduce the transformer and filter capacity
size.

Large electrolytics labeled "computer grade" were quite common in low-
voltage supplies required for digital electronics from the early 1960s
and
into the next three decades. Once the TTL family of digital ICs "took
off" all digital electronics seemed to require 5 VDC supplies. :-)

Since the early TTLs were - relatively speaking - power-hungry, the
supply current had to be much higher than for old tube architecture
equipment. With that was the requirement for filtering the raw DC
from (nearly-always) full-wave rectification resulting in 120 Hz
ripple.
The end result was a market need for higher-Farad electrolytics.
Low-ESR in those large-capacity electrolytics was, in my opinion,
less of a problem for switchers than it was to cut down on the
internal
heat dissipated by the ripple current in that ESR.

Yes,
there were switching supplies back in the 60's/70's too, but then
switching frequencies were much much lower (think about it: they used
2N3055's or their predecessors, transistors that take a millenium to
turn off!) than typical switching supplies today.

Outside TV EHT generators, primary switching power supplies were rare
in those days, simply because bipolars could not handle the rectified
mains voltage in those days, not at least with any significant
current, so linear power supplies was the norm in those days.

I agree with that, Paul. The switch-mode supplies did not increase in
either market quantity or acceptance in OEM equipment until the
late 1970s. One frequently-overlooked example is the small supply
(a switcher) used in the Apple ][ personal computer. Well over a
quarter million made and sold to power a small computer using (then)
legacy TTL devices. While some CP/M-based PCs existed using
switching supplies, other PC makers saw the advantage and started
using switchers also, even the "IBM" that became the norm for PCs
after 1980. While most switching supplies were still radiating RFI
into
the late 1990s, that did not trouble PCs operation. After multi-
millions
of PCs have been made with switchers for power supplies in over two
decades, it would seem (to many) that "switchers have always been
used." :-)

Clearly for their purposes, surplus 2N3055's and old big-can "computer
grade capacitors" can be put to very good use in many ham power
supplies. Make sure you get some LM723's too, a very versatile part
that modern parts (each good in their own narrow niche) have not
completely replaced yet!

My guess is that amateur radio is the last application in which linear
power supplies are used, since it is much easier to make a low noise
linear power supply than a switcher.

Much SIMPLER. :-) Today a typical under-1-Ampere supply consists
of an AC transformer, bridge rectifier, one filter capacitor, and one
three-
terminal regulator. Positive or negative polarity, no real change in
components. Guaranteed voltage regulation and the legacy three-
terminal regulator ("7805" for +5 VDC output) has over-current shut-
down. Spec sheets on three-terminal regulators include maximum
internal noise generation (very low) which few bother to notice. :-)

Excess power dissipation in a linear regulator? Actually not much.
A typical +5 VDC supply for 0.5 A uses a typical 8.5 V average raw
DC input and a three-terminal regulator drops about 3.5 V average
for a 1.75 W average loss converted to heat. The TO-220 cases of
three-terminal regulators are built to handle that (and a bit more) if
mounted to a "heat sink" of copper-one-side PCB. Three-terminal
regulators are entering their fourth decade of existance and couldn't
be more convenient to use for a defined, regulated output voltage.

73, Len AF6AY