Things to keep in mind and good tutorial

1>Diodes always have a .6V drop because once their breakdown voltage is met there is no additional resistance therefore even if the voltage changes, the current changes proportionally regulating the voltage.

2>Solar panels (i think?) have a higher output impedance than line power so voltage and amperage are not directly related but have to be figured with the impedance factored in.

Power source regulation using Zener diodes.
1.The first diagram shows the basic zener circuit it works by dumping current back across the zener diode when the reverse voltage threshold is hit. So when vIn goes above a certain point it shorts to ground keeping the voltage below the zener breakdown voltage.The resistor is there to prevent the zener from blowing out and should be set to a resistance that would limit the current to the specks of the zener.

2. Is another circuit based on the zener diodes breakdown voltage. In this case a npn bipolar transistor is used to regulate the current allowing for a much higher current gain while still regulating vOut. It is based on the emitter follower setup of a transistor.Using this setup the emitter voltage "follows" the inputs base voltage (which is regulated by the zener breakdown voltage) with a .6V diode drop from the pn junction in the transistor. So voltage out will be whatever the zener diode breakdown voltage is minus .6V with a current rating of (Base Current)(max gain "hfe" of transistor). you can also add a resistor capacitor circuit to help regulate the voltage ripple ( i usually guess at the values till it is smooth).

3. The third option uses a series of diodes to regulate Vout it is a great way to regulate your voltage if you are shooting for a multiple of six for your Vout. it works by combining the six voltage drops of .6V each to get a total of 3.6v at Vout. R is used to limit current flow across the diodes.

We tested all of these configurations with bench power and using solar cell, and found a slight difference between the two power sources. the bench power regulation was pretty spot on but with the solar power the voltage tended to float higher than expected. I think it is because of the impedance issues with the solar cells.

4.So the actual circuit we ended up with was a slight variation on the first with a small capacitor added in to smooth out the power. We could do this because the max solar output of mA (132mA) is well below the zener max mA rating of 220mA. So the only regulation we had to worry about was voltage and with a 3.3 zener diode with a accuracy rating of +-10% we ended up with a vOut of 3.8 max. this voltage is just outside of the gps nominal operating range of 3.0-3.6 but it doesn't seem to care that much and runs ok.