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[TenTec] Re: Alternate Power Switching

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Subject: [TenTec] Re: Alternate Power Switching
From: geraldj@ames.net (Dr. Gerald N. Johnson, P.E.)
Date: Thu, 12 Aug 1999 15:53:06 -0500
Not bad except most unregulated supplies don't quite run 24, just 18 or
22 but in any case the energy efficiency is rotten. You need to protect
the world from the batteries being capable of a kilo amp or two of short
circuit current so fusing is in order.

The second computer that I built, back about 1978 did just that but with
a switching step down regulator. I used 3 6 volt gel cells, probably 8
or 10 ampere hour, maybe 12, in series, and my one unregulated supply
was about 18. The charged batteries made 21, so I had to disconnect them
from the load until power failed. I developed a little circuit that
switched after the second half cycle of 60 Hz was missing. I generated
+8 preregulated with a simple switching supply and rectified the AC at
the beginning of that switch to get -15 to run S-100 buss where each
board had its own regulators. With two sets of gel cells that computer
ran 24 hours a day (except when power outages were longer than half an
hour, limited by the battery capacity and the power hungriness of 1978
vintage RAM) continuously until 1997 when it was retired because the
data source changed it was programmed painfully in Z80 assembler without
any removable media. All static and dynamic ram with some EPROMs. half a
megabyte of RAM was a lot for a z80... There was about 40 hours of down
time in those 19 years from a pinched line wire and from moving from one
town to the next.

Point is that the two "12" volt batteries fully charged are going to be
at about 28.4 volts and dropping that in half with a linear regulator
means 50% efficiency at best. These days it would be better to run one
battery and a little boost switching regulator for the radio. Perhaps 85
or 90% energy efficiency. Which nearly doubles battery life, or gets the
same rig running time from one battery instead of two.

When charging batteries with a regulator following the batteries
(regulator or not!) the battery generally does a good enough job of
acting like a capacitor that filter capacitors are only needed when the
batteries are old and decrepit.

Or more simply, start with a 350 VA or larger computer UPS. Hook it into
the AC feed to the radio power supply. When AC goes away the UPS
supplies power for a while. When there is AC the UPS takes care of its
own batteries. No switches, no circuits to worry about... Battery energy
efficiency probably about 85% of the original power supply efficiency
which isn't changed. But if it was my FET regulated linear supply it
runs about 85% at 20 amps load... Unlike Astrons and the like at under
50% efficiency...

73, Jerry, K0CQ

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