Couple of days ago I purchased on a hamfest one used Garmin's eTrex GPS receiver. When I looked to the price list of Garmin's accessories at a local dealer's shop, I decided to make my own power/data cable, because the price of the cable was almost one half of the price of the receiver at hamfest. Thanks God and Internet community there is rich and good documentation for this little jewel available on the Internet. OK, let's design and make our own power adapter (dropdown stabilizer) for eTrex.
Reading the the documents, I found that the eTrex box is probably very sensitive to overvoltage. So the power adapter should deliver voltage around 3-4V, not excessing to higher voltage under any circumstance. The current required is, according my measurements, somewhere around 100mA (GPS on), up to 200mA (backlit on). The adapter is primary designed for car use, so the input voltage is in a range 11..15V, negative grounded.
The choice of stabilizing circuit was very easy - LM317T. Cheap, reliable, resistant to external RF filed (I am a radioamateur), thermal compensated. The chip works in a "catalogue example" circuit, thus, there is nothing to explain. The 3,6V Zener diode at the output serves as "brake of last resort". If the output voltage rises above its Zener voltage, the diode opens and shunts the output. Because of high input current in this state, the fuse blows and disconnects all the things from incoming power. Thus, USE THE RECOMMENDED rating of the fuse.
The same protective circuit is at the beginning of the stabilizer. In a case of accidental reverse-polarisation of the input power, the input diode (DUS - diode universal silicone, 1N4147 does a good job) opens and blows the fuse.
All the circuit is carefully protected against RF field using bypass capacitors.
The physical realization of the circuit is shown on Fig.2, Fig.3, the schematics at Fig.4. All the passive components are SMDs, mounted in a "cuckoo nest" style. You just need a good, regulated soldering iron, good quality solder and a patiency, believe me. The whole circuitry is hidden under the eloxed aluminium cooling profile, tightened to an ordinary 9pin DSUB connector, making together a nice, compact block. After completion and testing(!), you can fill up all the "cavern" between the heatsink and the connector cap using some epoxy filler.