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[aprssig] Kenwood TH-D7 GPS connector grounding

Greg D. ko6th_greg at hotmail.com
Sat Sep 13 23:26:34 UTC 2008

Hi Steve,

Thanks for the link - I had briefly considered such a thing, but didn't realize that they were available in this small size.  However, I think I need to pass on them.  The problem is that the input voltage range is going to be too great.  They appear to have two versions, one requiring 4.5 to 5.5 v, and the other 10.8 to 13.2v.  Neither matches what I have.

When running from the battery pack, I'm going to be running on 7.2 volts, while from a car adapter it could be up around 14v.  Dropping 7.2 v down to 5v at 50ma (what the GPS needs) with a 3-pin regulator will be wasting something like 1/8 watt, if I did the math right; not huge.  Dropping 14v to 5v is a don't care, because it's coming from the car...

For isolation, they'd be ideal.  But I don't appear to have that problem anymore.

Thanks anyway,

Greg  KO6TH

> Date: Sat, 13 Sep 2008 00:01:17 -0700
> From: wa8lmf2 at aol.com
> To: aprssig at lists.tapr.org
> Subject: Re: [aprssig] Kenwood TH-D7 GPS connector grounding
> 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 
> load.  
> Consider using a tiny switching-mode DC-DC converter.   Astrodyne 
>  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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