[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.  
This one:


[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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