[aprssig] Kenwood TH-D7 GPS connector grounding
ko6th_greg at hotmail.com
Fri Sep 12 00:54:16 CDT 2008
So, I'm still working on mounting a GPS unit on top of my TH-D7. I have it all running, somewhat lashed-up, and it's great not having all sorts of little things strung together. Currently the GPS is being powered by 4xAAA 900mAh NiMH cells, and they seem to work great. For the curious, pictures at:
http://home.wavecable.com/~ko6th/Ham_Radio-IMG_0233.jpg. The GPS unit is an EM-406A from Sparkfun.com; runs on 4.5 to 6.5v. The TTL output is inverted with an NPN transistor, and then direct into the TH-D7.
But, batteries make the attachment too big, and need to be charged separately from the unit. So, I want to tap into the main battery pack, which is a Batteries America BT-11h with 6xAA 2700mAh NiMH batteries. Then I can mount just the GPS part of the board on the radio.
But, I want to do this safely.
That's the puzzle. I took an ohmmeter to the unit, to be sure that I'm not introducing a ground loop by doing this. I expected that the ground side of the GPS port on the side of the radio would be tied to the negative side of the battery. Instead I get no connection at all. Infinite resistance. I guess it's possible that they went to the trouble of isolating the two; perhaps they had to. But my mental warning alarm is going off...
Does anyone know if grounding the GPS port common terminal to the battery negative will cause a problem? My plan is to tap into the BT-11h, picking up the negative from the top of the right-hand column of cells (as you look at the pack with the lid open); that's the same as the negative of the overall pack. Then, 4 cells-worth up the 6-cell string is right next to it, at the top of the middle column. Easy to pick that up too. Drill a small hole, and out come the wires with about 5v on them. The result will be a little uneven in the discharge, with an extra 50ma load on 4 of the 6 batteries when the GPS is on, but I figure that overall it won't be noticed. The alternative is to add a regulator to the GPS board, but then I'd need to dig deep into the battery pack to get at the positive terminal. I like simple.
But either way, the negative of the battery pack will be connected to the ground side of the GPS port. Is that going to be ok?
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