Thanks, Barry. As I digest your info, I had also thought about using a
fixed value LM7805 VR. However, the concern of the voltage drop used by
that device bothers me. I'm wondering if it would even work when the
generating system is at the 6vdc resting voltage ..being such a low
threshold, would that even operate the 7805? I surmise 8.5 volts would
though.

I believe your surmise is correct, in both cases. The LM7805 states
that the regulator has a 2.0-volt forward voltage drop at 1 ampere...
it won't regulate properly at below about 7.0 volts of input.

From the internal schematic it looks to me as if the dropout is
probably two diode drops (about 1.4 volts) plus the saturation voltage
of the pass transistor.

There are three-terminal regulators which have lower dropout
voltage... down to 1.5 or below, I believe... but even those may not
work properly in an application with so little "headroom".

One alternative approach is as other folks have suggested - use a big,
hefty zener diode shunted across the radio, and feed the radio (and
zener) through a current-limiting resistor. It's likely to be tricky
to pick a resistor whose value is low enough that it doesn't starve
the radio for current when the auto system voltage is low, and still
limits the zener current adequately when the system voltage is high.
Adding a big, hefty filter capacitor (say, 10,000 uF 12 WVDC) across
the zener would probably be an excellent idea.

Another approach is to build a custom very-low-dropout voltage
regulation circuit. This could probably be done using a PNP (or
P-MOSFET) pass transistor, with the base/gate being driven from the
output of an operational amplifier which takes its supply from the
(unregulated) auto voltage rails (perhaps filtered somewhat) and which
compares a divided-down version of the output voltage with a reference
voltage. You'd have to play around with the gain, and frequency
response of the op amp's feedback loop in order to ensure proper
stability of the circuit.

And while on the subject how about this alternative. If I wanted to just
stick in a (stand alone) 12 volt am/fm radio, how could I boost the 6 vdc to
12 vdc (just for the radio and nothing else powered in the truck)?

For this, you would need a "boost-mode" switching voltage converter (a
"DC/DC converter"), and possibly an additional noise-filtering or
post-regulation stage. These are not inexpensive (especially as you'd
probably need several amps of current out) but may be available
surplus.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

I would either convert the vehicle to 12 volts or fix the original
tube radio. There are rpl parts readily available if you know where to
look. A solid state vibrator is easy to build.

Ted Azito wrote:

I would either convert the vehicle to 12 volts or fix the original
tube radio. There are rpl parts readily available if you know where to
look. A solid state vibrator is easy to build.

Re : "solid state vibrator".... isn't that a contradiction in terms ? You
mean inverter I assume ? There are some pretty fancy adult toys these days
OTOH. ;-)

Graham

In , on 09/12/04
at 06:58 PM, (Ted Azito) said:

I would either convert the vehicle to 12 volts or fix the original
tube radio. There are rpl parts readily available if you know where to
look. A solid state vibrator is easy to build.

Wouldn't conversion cost more in $$ and trouble than buying an inverter
to step up the 6V for a few devices? During the conversion one would
have to tinker with the instruments, starter, ignition system, horn,
all the lamps, and the generator.

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wordgame:123(abc):14 9 20 5 2 9 18 4 at 22 15 9 3 5 14 5 20 dot 3 15
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