"Ray Andraka" wrote in message
...
I wouldn't do it with just a dropping resistor. The current on the data
plate
is typically the maximum current. Additionally, if you look at the
current over
time you'll likely find that it fluctuates rapidly, especially in a
digital
radio like the XM radio. Instead, your cable needs a voltage regulator.
This
can be as simple as a zener diode and a resistor if the power dissipated
is
small. A better regulator would be one that uses a switching regulator as
a
DC-DC converter. You can get switching regulator modules with 28v input
and one
of a variety of output voltages from a number of vendors. I'm sure
Digikey has
a few of these modules. You'd have to add a case, wires, a fuse and
connectors.

I use Datel DC-DC converters in current avionics designs.

Your other option is to add a zener diode to the simple resistor circuit,
but now I am out in Jim Weir land and I will now retreat. (run away

Casey Wilson wrote:

"Viperdoc" wrote in message
...
I'm not sure I understand the discussion on DC voltage converters. I
currently have XM radio in my 28v plane, but the radio runs on 6
volts, so
I
use an inverter to connect to the AC plug. Is it possible to get a 28v
cigarette lighter adaptor to cut the voltage to 6 volts directly? It
would
be nice to eliminate the inverter.

I'm not sure you will find one on the market. If you wanted to
build it
yourself, you need to know the current flow in milliamperes (or amps)
used
by the radio. That may be written on the ID plate or in the unit's
specs.
For illustration, let's presume it is 150 milliamps. The supply voltage
is
28 (actually closer to 32 without a load) so you need to drop 22 volts
in a
resistor. You can calculate the required resistance by dividing the
voltage
to drop (22) by the current flow (0.150) = 146.67 ohms. Round it up to
150.
Since it is going to dissipate energy in the form of heat, you need to
know
the wattage or the product of current and voltage = 3.3 Watts. To be on
the
safe side, you need to at least double that to 6.6 and round up to 10W.
The
next safety feature would be an in-line fuse, but that's optional since
you
can yank the connector out of the lighter socket.
Cut one of the pair of wires coming out of the connector and put the
10W, 150 ohm resistor in series. The typical night-light for the kid's
bedroom is 7.5W. Stick your finger up against one that has been on for a
couple of minutes and imagine about half that heat. That's what the
resistor is going to heat up to. The point is, keep your fingers off of
it
when it's plugged in. Oh, that reminds me, don't wrap it up in foam --
the
heat needs to dissipate.

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759