[TenTec] Battery Operation

[Dr. Gerald N. Johnson, P.E.]

By the time you get a lead acid battery down to 10.5 volts there's not
much capacity left.

The problem with many radios seems to be in the regulators. The
7800/LM340 family of regulators needs a couple volts headroom because
they will drop that much as a minimum. Probably the radios with the
greatest voltage requirements try to run the regulated line at 10 volts,
then those a little more tolerant try for 9 volts and so on. At least
from Japanese vendors these three terminal regulators are common with
output voltages of 8, 9, and 10 volts.

It might be of benefit to replace the 7800 chips with LM2930 family
chips that have a minimum voltage drop of half a volt. For some voltages
there may be three lead chips that will drop in directly. Otherwise it
may be necessary to use the 5 lead LM2931 with setting resistors. Its
important for all such regulators to have a .1 ceramic capacitor as
close as possible to the input and ground pins to keep it from
oscillating when run close to the margins of its operating region.

There is another problem with radios producing significant power. The
output transistors will have a couple volts drop at saturation and the
collector load is usually set for maximum power at 13.8 volts. when the
supply drops to 12 volts, that means the collector swing (from supply to
Vsat) has dropped from 12 volts to 10 volts and since the load impedance
was optimized for 12 volts swing the power output is reduced by the
square of the voltage drop. So 80% collector swing means 64% power.

I've not tried operating my Corsair II at reduced voltage or direct from
a battery yet. When I made the cable, I put in an automotive type fuse
(which I've blown once from key down and no load) holder with 15 amp
fuse (blade type fuses, buy only BUSS fuses, not imported fuses of
questionable repute). And I installed a relay since the front panel
power switch can't handle the PA current. And to protect from reversed
polarity I put a small diode in series with the relay coil so the relay
won't pull in with reversed polarity. I also put a diode across the
relay coil to absorb inductive kick which with a series resistor would
accomplish the same thing. I used a 30 amp automotive style relay that I
taped to the power cable. So far it works fine.

73, Jerry, K0CQ

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