Running a 3b1 continuously

[Bill Mayhew]

My system, impulse, is a 3b1 with 2 megabytes of RAM and a
Miniscribe 6085 (67 meg) disk drive.  With the monitor screen
switched off, impulse draws about 65 watts from a 120 vac, 60 Hz
line.  With the monitor turned on, the system draws about 80 watts.
With winchester seeking, power consumption would make brief
excursions around 100 watts.  Switching the machine on causes a
transient load of about 150 watts for about 10 seconds while the
winchester drive spins up.

I measured the figures above with a real honest to goodness
wattmeter, rather than calculating values.

I don't like the idea of leaving the CRT running (even if the
screen is blanked, the anode high-voltage supply is still running)
while I am not at home and/or asleep.  I installed a toggle switch
in the 12 DC power lead going to the CRT.  I got nervous about
leaving CRTs turned on while unattended from the Televideo 950
terminals that we used to have.  In more than one instance, the
flyback transformers in the Televideo terminals failed in such a
way that they caught on fire.  The horizontal drive arrangement in
the TVI 950 is quite similar to that in many monochrome CRTs.  The
culprit was an electrolytic coupling capacitor that attached the
flyback to the collector of the horizontal output transistor.  The
capacitor would eventually short, causing a large DC current to
flow in the flyback transformer and set it on fire due to the
resulting heat build-up; the fault current was not large enough to
blow the fuse in the power supply.  There were also some horror
stories about Zenith TVs that allegedly spontaneously caught fire
after being switched on for long periods several years ago.

To answer the question being asked, a modest stand-by type of
back-up power supply is sufficient for the Unix PC.  The power unit
in the Unix PC seems to have several hundred miliseconds of
ride-through capacity.  A stand-by type of back-up power supply is
one that might take one or two missing cycles of AC power before it
switches over to battery.  I bought a used Questa 200 watt UPS
several years ago for my Unix PC for $100, and it seems to do the
job just fine.  The Questa unit has an 8 amp-hour internal sealed
wet cell battery, which is enough to run the Unix PC for about 20
minutes.  There are input terminals on the back of the Questa
supply to allow something like a car battery to be attached, thus
enabling a Unix PC to run for hours.  The Qesta supply is only
rated for maximum load for 20 minutes, and must be derated to 2/3
load for continuous duty; this is still more than enough for a Unix
PC.  I like the diminutive size of the Questa power pack; it is
about 1/2 the size of a shoe box.

I think if I were buying a stand-by supply today, that I would go
with one of the ones made by Minute Man; they look well made, and
their % load and % charge bar-graph LED indicators are helpful.

Bill
wtm at impulse.UUCP