Wiring GPS into "ignition on" wire

blackc2004

I'm adding a GPS unit to my trailer to keep track of where we go but I need to get power from an "Ignition On" Source. I was thinking about pulling this from the 12v power supply from the TV. Anyone have suggestions of the best place to "tap" into this? Or a better suggestion of where to pull an "Ignition On" source from the trailer?

MuttonChops

The TV +12VDC connects directly to the T@B Battery so when the TV is disconnected your GPS would continue to receive +12VDC from the T@B battery if you 'tap' the TV +12VDC.
You might consider tapping the T@B 7-pin running lights circuit and training yourself to always have the TV headlights on when traveling.  TV 7-pin connected, TV headlights on, gives you GPS on.

A more complex approach would be to use the TV Back-up Light circuit. The 7-pin cable has the back-up lights wire run to the T@B junction box while the T@B itself has no lights connected.  To use this circuit you would need to modify your TV 7-Pin connection socket . . . change the back-up wire power source from the TV back-up lights to an ignition on +12V source.

All that said, a simple solution could be:
  Use any +12VDC T@B source to power your GPS.
  Train yourself to Turn the GPS on & off as you travel.

WilliamA

Hmmm....what an interesting question!  I live for this kind of question.  As it so happens, this also is pertinent for other things. For example, I have a wireless backup camera on my trailer. I don't like the idea of vehicles drafting in my blind spot so rather than wiring the camera power to the backup light circuit, (I have backup lights on my trailer) as muttonchops relates, I wired my camera power to the running light circuit.  When I am traveling, turning the running lights on will power the camera as a rear-view mirror.  The monitor has much too wide of a field of view to see rearward very far, but I can tell if there is someone hiding in my blindspot "bumper-sticking". It's also great for not backing into picnic tables at night.  
But the problem is how to wire up a circuit that triggers an event when the trailer umbilical is plugged into the tow vehicle.  It should be as straightforward as tapping into the trailer battery charging line (the wire that comes from the tow vehicle to charge the battery). As this wire is electrically energized all the time from the trailer battery too, you'll have to add a diode in the circuit so the wire can't "see" the trailer battery but will sense voltage from the tow vehicle when the umbilical is hooked up.  There are thousands of diodes with multiple mounting options.  A diode for this sort of circuit is also commonly called a "block rectifier". It allows voltage to pass through in one direction (in this case, from the tow vehicle) but blocks current flow from the other (voltage from the trailer battery).  The sticky wicket is to find a waterproof block rectifier with two poles that has its own self-contained heat-sink and can handle 30 amps max.  This sounds complicated, but is a pretty common, straightforward thing in automotive circuits.  There are lots of diodes in automobiles but not many that can handle 30 amps.  Most are used for warning lights or feedback loops to the computer and aren't rated for more than a fraction of an amp.  
A bit more research is in order here but it's easily do-able...
WilliamA 

MuttonChops

WilliamA said:

. . . the problem is how to wire up a circuit that triggers an event when the trailer umbilical is plugged into the tow vehicle.  It should be as straightforward as tapping into the trailer battery charging line (the wire that comes from the tow vehicle to charge the battery).

As this wire is electrically energized all the time from the trailer battery too, you'll have to add a diode in the circuit so the wire can't "see" the trailer battery but will sense voltage from the tow vehicle when the umbilical is hooked up. 

Yes a diode is doable . . . I had not considered that approach  because the diode could have to be as high as 40A rated (my TV +12VDC circuit is 40A, others may be different) and would require heat sink mounting.  That's far more complex then just 'tapping' the running/marker lights circuit.

WilliamA

Nice diagram!  I was going to make one for this post but I took a nap instead.  
It would also be possible to add a voltage sensing circuit that would trigger on anything over, say, 12.8V or so.  They can be had "off the shelf" with adjustable potentiometers.  Could be worth looking into.  Such a circuit resides inside the Norcold and most autos that have load-sensing circuitry to protect minimum battery voltage for starting.  According to my math, a diode capable of 30A would need to have a heatsink big enough to deal with 14 watts of heat.  That's not too large, but it's a waste of an awful lot of power just to avoid turning a switch.  If one just had to have the tech, the voltage sensing gizmo would be a better way to do it.
WilliamA

blackc2004

Wow! Thanks for all the great feedback but this is quickly WAY over my head on the electrical side of things!

MuttonChops

blackc2004 said:

Wow! Thanks for all the great feedback but this is quickly WAY over my head on the electrical side of things!

The keep it simple principle says, use the running/maker lights as your power source with highlights as on/off switch or just turning the GPS on/off yourself would be best since there is no 'ignition on' power source in the T@B.

Am wondering why a GPS 'in the T@B'.  The TV will go everyplace the T@B goes and it is so much easier to locate and use an ignition on power source in the TV.