[aprssig] Kenwood TH-D7 GPS - gnd CORRECTION

Steve Noskowicz noskosteve at yahoo.com
Fri Sep 12 20:59:52 CDT 2008

I decided to draw a shematic and not shoot from the hip.  See *****
73, Steve, K9DCI

--- On Fri, 9/12/08, Steve Noskowicz <noskosteve at yahoo.com> wrote:

> From: Steve Noskowicz <noskosteve at yahoo.com>
> Subject: Re: [aprssig] Kenwood TH-D7 GPS - grounding
> To: "TAPR APRS Mailing List" <aprssig at lists.tapr.org>
> Date: Friday, September 12, 2008, 7:08 PM
> --- On Fri, 9/12/08, Greg D. <ko6th_greg at hotmail.com>
> wrote:
> > Subject: [aprssig] Kenwood TH-D7 GPS connector
> grounding
> > To: "'TAPR APRS Mailing List'"
> <aprssig at lists.tapr.org>
> > 
> > So, I'm still working on mounting a GPS unit on
> top of
> > my TH-D7.  ...  EM-406A from Sparkfun.com; runs on 4.5
> to
> > 6.5v.  [50ma ] The TTL output is inverted with an NPN
> transistor,
> > and then direct into the TH-D7.
> > 
> > ...So, I want to tap into the ... Batteries America
> BT-11h
> >... I took an ohmmeter ...ground side of the GPS port
> ...
> > tied to the negative side of the battery.  
> > Instead I get no connection at all.  ...
> Greg,
>   You, unfortunately, have a double, or is it triple,
> whammy here.  *AND* a good piece of information for me (The
> D7 accepts unipolar RS-232] cool!
>   First, there is a diode in the negative side of the
> BT-11h.  If you're meter has a diode test setting, it
> will show .5-.6 volt in one direction, open the other. 
> It's hard to see, inside, down by the neg contact, but
> you can see the black wire running up along that side to
> that end of the cells. It prevents charging non-rechargables
> through the radio charger. 

    With this diode in place, your transistor inverter (when low) will go about 0.7 volts NEGATIVE for the GPS input of the radio.  This should work - no sweat as long as the positive voltage out of the inverter is positive enough for the D7 GPS input.   Try this first.

>   IF you connect the emitter of the inverter to the GPS
> connector ground, you'll just have to allow for this
> difference in your base drive arrangement.  A slightly
> smaller base-emitter resistor.  I'd probably do it this
> way.
>    I guess with NiMh cells you could short the diode and
> just use the radio charger for the required time (57 hour
> full charge - due to 70 ma charge current).  However, as
> previously posted here, this requires ship-in-bottle tools.
>   Second, thanks to Dave telling that the current regulator
> is on the ground side.  This means that when charging the
> radio, the GPS will be several volts more above ground
> (charger voltage minus battery voltage), relative to the
> radio's GPS input.    I'd have to review the circuit
> and think more, but off hand, I don't think that this is
> a problem, in itself.
>   Then, I see there is already another post from Patric,
> but I don't think his comment about other radios applies
> to the D7 given the above.
>   Lastly, I don't believe the drain imbalance is
> something to sneeze at.  The numbers were posted here a
> while back and IIR the radio+APRS TNC drain is somewhat more
> than the 70 ma charge. I think it is around 120-140ma.
>    50 ma is a significant part of that  30% - meaning some
> cells will either be 30% over or under charged.  A switching
> regulator, off the whole battery, would be nice if you can
> and keep it quiet.
> Good luck
> --
> 73, Steve, K9DCI
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