Powering a Garmin GPS with Solar
I have a Garmin eTrex Vista GPS that I use to navigate when I tour. A GPS is more useful in some parts of the world than others. Generally a GPS is most useful in those areas where batteries are hard to find. Having a self sufficient power system is ideal. I have built and used the following circuit to power my GPS for 2 months while in Mongolia in 2002. There was plenty of sun and the circuit worked well.
Disclaimer
I don't know much about electronics. Use at your own risk. If you fry your Garmin try sending it in for warranty but be sure not to tell them you built a power supply for it! If they won't take it, it is your own fault and not mine.
Parts Needed
You will need the following:
- A small project box with a PC Board.
- A battery holder that holds two AA batteries.
- 6 AA NiCad rechargeable batteries.
- A solar panel.
- An electrolytic capacitor (50v, 20,000 uF).
- 2 blocking diodes (1N4002).
- LM317 Variable Voltage Regulator.
- 100 Ohm resistor.
- 140 Ohm resistor.
- Garmin GPS power cable.
- Garmin GPS handlebar mounting kit.
- Weatherproof couplers (i.e. the radioshack trailer brake light couplers.)
- A heatsink that fits on the LM317.
You can find almost all of the above items at Radioshack. I used a Siemens SM6 solar panel. It provides enough power, is indestructible, mounts easily to the bike, and is fairly lightweight. I mount mine to my rear rack with two small C clamps.
The Circuit
The circuit is very simple. The solar panel provides anywhere from 0-30v depending on the sun. The capacitor is used to smooth the input into the voltage regulator. The LM317 is responsible for maintaining the 3v output that the Garmin GPS needs. The resistors R1 and R2 are used to determine the output voltage satisfying the equation: Vout=1.25*(1 + R2/R1). In our case R1 = 100 Ohms and R2 = 140 Ohms.You can use any values of R1 and R2 as long as the ratio is the same provided that you draw enough power. If the values of R1 and R2 are two large then the circuit won't work.
The diodes are used for protection of the voltage regulator and to prevent the batteries from discharging into the circuit during the night.
The circuit performed well. Provided enough sunlight it was easy to both charge the batteries and power the GPS at the same time. I was able to use the charged batteries in the GPS during the early mornings and evenings when the sun was too low to power the GPS directly. On a good day I could fully charge one set of batteries. For the majority of the tour I was able to be self-sufficient and did not need to purchase any batteries at all to power my GPS.
The only problem I encountered was when the outside temperature was around 45 C and the circuit became hot. The LM317 has built in thermal shutdown. Since the box was sitting in the top pocket of my pannier in the sun, the circuit became quite hot and shutdown automatically. In the evening it worked fine again.
The circuit is not really designed to allow the batteries to power the GPS in the evening. Since the batteries are NiCad they only provide at most 2.4v. The Garmin will only run from the fully charged batteries for about 1 hr or so. In the evening the Garmin will begin to complain about "External power lost." However, removing the batteries from the circuit and placing them in the Garmin works fine. This is what I usually did.
Additions
If you are paranoid you may want to use an over-voltage protection system on the output. If the circuit fails you will want to prevent a large voltage going into the Garmin and frying it. There is an example circuit here.
If you would like to charge the batteries better, or charge NiMH batteries you may want to use a battery charging circuit.
Powering a Garmin GPS with a Dynamo
I have built another circuit that I used to power my garmin from my Schmidt dynamo. I had limited success with this circuit and plan to build a solar/dynamo power system to power a GPS and charge batteries. If you want a good start you can simply build this wonderful dynamo standlight circuit and simply add the above circuit to it. I plan on building a combination dynamo/solar power supply sometime in the near future. When I do I will post the complete schematic here.
