Constant current source from battery and solar panel circuit

Projects General Guidance
1

I am trying to build a constant current source that will be able to handle the input of both a small solar panel (1.5w, typical volt 5.5, typical current 270 mA) see here:
http://www.seeedstudio.com/depot/15w-solar-panel-81x137-p-952.html

and three 1.2 v NiMh batteries in series (~2500 mAh capacity). I have some Dc to Dc boost converter breakouts available that are seen here:
http://imall.iteadstudio.com/im130801001.html

The problem I am having is that I need to power a heating coil with precisely .2 watts. I have resistance wire with a resistance around 50 ohms per foot. I am looking for a way to provide a constant voltage that will not change whether the load is coming from the solar panel or the batteries, obviously the length of the heater coil will not change.

The system is powering an arduino clone, which has a 3.3v output capable of sourcing up to 40 mA.

2

I would suggest that you use a shunt regulator to control the solar output to the batteries,
its the normal way of solar charging batteries.
Over voltage to a fully charged nicad will destroy it.

What voltage will you be running the heater at ?

3

That's the question I am trying to figure out, I need something that would provide a constant voltage from both the battery input and solar panel input.

4

Pololu sells adjustable step up/down regulators that provide a constant output voltage from a wide range of input voltages, above and below the output voltage. Pololu - Step-Up/Step-Down Voltage Regulators

5

The simplest way is to connect the solar panel to the battery with a diode and have a shunt regulator on the solar panel side of the diode.
Then take whatever power you need from the batery terminals to your convertor.
This way you only need one convertor.
Im a bit curious what the .2 w heater is for byw if you can say ?

6

@ jremington: Those are perfect!! Thank you!

@ Boardburner2: how about a simple zener diode for the regulator?

7

@ Boardburner2

The .2w heater is part of a system to measure sap flow, aka water movement through the xylem of plant stems, using an arduino-based system. The equation to calculate sap flow requires a constant heat source of .2 watts.

8

camdenl:
@ Boardburner2: how about a simple zener diode for the regulator?

Spot on.
3 x 1.2 v = 3.6 .
Add the voltage drop of the series diode say 0.3 v you get 3.9V.
The solar panel is 1.5 W so a 3 watt dissipation zener should suffice.

Being pedantic , you should look at data sheets for, - max trickle charge current for the nicads, precise fully chaged terminal voltage of said nicads, actual voltage drop of the series diode.