[aprssig] Kenwood TH-D7 GPS connector grounding
Stephen H. Smith
wa8lmf2 at aol.com
Sat Sep 13 02:01:17 CDT 2008
Greg D. wrote:
> Hi all - Dave, Bob, Patrick, Steve,
> Wow, this is a bit more complicated than I thought it would be. A couple of items:
> 3. So, that diode (D3) also prevents any juice from the battery from being available at the power plug. But, I think this means that if I can get a wire to the positive side of the battery pack (way down deep inside), I can get juice from either the battery or external power (through the battery pack connection to the radio, since they're always connected), and I can pick up the ground side from the ground side of the GPS plug (NOT the battery pack!). Yes? And, at that voltage, I'll need a 5v regulator; a simple 3-terminal one should do.
The standard 3-terminal regulator is horribly inefficient. When you
drop 12VDC to 5VDC with the standard 3-terminal analog regulator (i.e.
78xx-type) you will be dissipating more power than you use. [7 VDC at
75mA = .525 watts wasted while 5 VDC at 75mA = .375 watts delivered to
Consider using a tiny switching-mode DC-DC converter. Astrodyne
<http://astrodyne.com> makes a huge variety of small DC-DC converters
that feature complete isolation between the input and output sides.
[Warning: Long link may wrap and get broken.] accepts 10.8-13.2VDC in
while producing a constant 5 VDC output at 150mA for only $12.00 .
Since the 5VDC output is totally isolated from the input (i.e. no common
ground or negative pin -- this is a four-terminal device), the issue of
whether the GPS data-in ground is common to the radio's chassis/power
negative goes away. This device is packaged in a case similar to a DIP
IC four-tenths of an inch wide and seven-tenths inch long. It is
intended for PCB mounting in standard tenth-inch-spaced rows of holes.
I have used many of these Astrodyne converters in various
voltage-in/voltage out combinations for powering GPS devices, digital
cameras, small CCTV cameras, and other electronics in my mobile HF/VHF
APRS/SSTV/VIDEO installations. Because of the complete isolation
between the input and output sides, they are invaluable in eliminating
ground-loop/alternator whine problems that arise when too many
12VDC-powered boxes are connected to each other and start sharing
common -12VDC ground returns.
I often mount the Astrodyne converters "dead bug" style (glue the back
of the case to my chassis box and then tack-solder wires to the pins
normally used for PCB mounting). Or I turn them into in-line "power
adapters" by connecting a PowerPole-equipped reb/black 24-gauge zipcord
to the 12VDC in side, and a suitable connector to the 3.3 or 5 or 6 VDC
output to mate with the device in question. (For devices like the Garmin
GPS18 or Byonics GPS, this will be a female in-line PS/2 connector
pigtail.) I then slip a piece of 5/8th-inch shrink sleeve over the
device to produce a a "cable with a goiter in the middle"-type assembly.
Stephen H. Smith wa8lmf (at) aol.com
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