Apple Orchard 1980 Fall v1n2 reduced

FALL 1980
THE APPLE ORCHARD
PAGE 67
DON'T OVERLOAD YOUR APPLE 11
by Ken Silverman
Copyright © 1980 all rights reserved
As you shop for that next attachment
for your Apple to turn it into
an APPLEMATIC, that will liquefy,
chop, and dice your programs -
BEWARE you don't overload the
Apple power supply.
There is such a proliferation of
plug-in e(luipment now made for
the Apple . 11 that it has become
difficult to make a decision as how
to use the eight slots to best
advantage.
If, for instance; you were in the
market for a. serial interface card
and found that two of the ones you
looked at would handle the job at
about the same price, which one
would you purchase All operating
criteria being equal, you might see
which one uses the least amount or
current. This information is not
normally given in the manufacturers
operating manuals, but if you
took the time to write them, they
would most likely give you the
information.
This area of current drain can be
very important from the point of
view of overloading your supply.
The Apple Reference Manual
(A2L0001A), on page 92, specifies
the limits of the switching power
supply.
Full load power output:
+5v: 2.5 amp
-5v: 250ma
+12v: 1.5 amp*
-12v: 250ma
*This +12v can supply 2.5 amp
intermittent load if not run for
more than 20 minutes and is followed
by 10 minutes at normal
load.
The power supply has a built in
protection circuit and if you short it
or have no load on it, the built in
oscillator will stop and cut all
output. It will try and restart the
oscillations about every half second
and when the impairment is removed
it will start up again. This
also happens when an overload
condition is present on the supply.
The reference manual states that
this cycle can continue indefinitely
without damage to the power
supply. In some cases the oscillator
might not start again which could
be caused by a faulty supply, or the
fuse FU1 might be blown (located
inside the supply). If this happens,
it must be fixed by an authorized
repair center.
The majority of peripheral cards
use the +5v and +12v outputs, and it
is in this area you should configure
your system so that the limits are
not exceeded.
As a starting point, for the +5v,
the manual specifies 2.5 amp is
available. This figure is the amount
of current the supply can deliver to
the Apple and any additional plugin
cards. First you have to determine
what the motherboard of the
Apple is using before any 1/0
devices are plugged in. This information
is on page 104 of the
manual, which states that 1.5 amp is
consumed by the motherboard
(with 48K). This now leaves only 1.0
amp for your 1/0 devices. I did
check several Apples and found the
average to measure 1.46 amp, but in
a few cases, using low power
RAMS, the system used only 1.2
amp for only a savings of 300 ma. I
would advise using the 1.5 amp
figure unless you can get a qualified
technician to measure the motherboard
for you. (The current meter
goes in series with pin 3 for +5v
measurement - see drawing on
page 104 of the reference manual).
The +12v supplies 400 ma to the
motherboard which leaves 1.1 amp
in the normal mode and up to 2.1
amp for an intermittent load.
Using CHART A and adding up
the current drain of each card you
should be able to keep within the
limits specified. Some cards do a
power down when not in use or not
addressed and this should be taken
into account when adding the
drains. The following method
should be used when adding up the
power:
1. Add those currents in which
there is no difference between
ON & OFF.
2. Some cards are turned on for
system use - like the Applesoft
Firmware card. These also
should be added as if always in
the ON condition.
3. Take the current for the rest of
your system and add the OFF
drain.
4. Total the above.
5. Add the DIFF column to the
total for any one item from
instruction 3 which this should
be the maximum drain at any
one time.
A typical system might consist of
an Applesoft firmware card, Disk
controller, Apple High Speed Serial
Card (for a printer) and maybe a
D.C. Hayes modem. Referring to
CHART A for the +5v column and
using the instructions:
CARD +Sv OFF ON DIFF
Applesoft Card 381ma 411ma 33ma
Disk Controller 180ma 275ma 95ma
Apple Serial 38ma 166ma 128ma
Modem 184ma 194ma 10ma
In this example the firmware and
the modem would be ON at the
same time with either the disk or
serial. This could give a maximum
drain at any one time of 951 ma
(with serial card in use). This same
procedure should be used for the
+12v supply.
· I attempted to test as many cards
as possible and those that were
made available to me by friends
and local stores. If you have a card
that is not listed, please write me
and I will try to get the measurement.
I discovered while working on
this article that some systems, even
though they were within current
limits, had a tendency to crash. This
crash, or the system going out to
lunch, is not a power supply probblem.
The Apple could have a
thermal (heat) problem inside the
Apple case. With just a few periph-