Alt-BEAM Archive
Message #02191
To: beam@corp.sgi.com
From: Bob Shannon bshannon@tiac.net
Date: Sat, 10 Apr 1999 11:29:09 -0700
Subject: [alt-beam] Re: Constant current solar chargers -WAS- Microcore memory for 100Farad caps.
Chiu-Yuan Fang wrote:
>
> Do you have a schematic handy for a constant current solar charger? > Thanks.
>
> Chiu
For small (BEAM) applications, you can simply use a LM317L as a constant
current regulator.
I connect a small value cap, not more than 200 uf across the solar
cell.
You can use a smaller, lower valued part though.
Then connect the postive output of the cell and cap to the input
terminal of the LM317L regualator.
Connect the output of the LM317 to a series resistor, say 240 ohms (for
a 5 ma charge current). The other side of this resistor goes to the
postive
battery terminal.
Connect the adjustment lead of the LM317L to the positive battery
terminal.
To change the charge current, simply change the resistor value. Assume
a voltage of 1.2 volts, and select a resistor value for the desired
maximum
current (within the limits of the LM317 of course...). There are
several
schools of thought on how this charge current should be selected. I'm
going to reccomend that you shoot for C/10 for BEAM applications. Just
make sure that the 'bots activity level and behavior cause it to wander
away from the light enough (thats why you gave it a battery, right?) to
prevent overcharging.
If you want to be immune to overcharge, the simplest thing you can do is
to lower the charge current to C/20. (thats 3.5 ma charge current for a
70 maH battery).
Your solar cell does need to have a higher voltage than your battery
however. This circuit works very well for 3.6 volt applications with
small Varta Ni-MHD cells.
Larger capacity cells will probably want a higher charge current.
If you need to charge a higher voltage battery, say a 7.2 volt Ni-Cad or
Ni-MHD pack you will need to use a boost DC to DC converter.
Linear Technology makes some small, inductorless converters in SOT-23
and SO-8 packages. The LTC1517 series are packaged in a five lead
SOT-23 pack and only need three external caps, 6.8 uf, 3.3uf, and 0.1
uf.
The input current is as low as 6 microamps.
Maxim also makes a number of excellent parts for this application.
By placing a 220 uf cap across the solar cell, and using a comparitor
with histerisis (Nv net application?) to shut down the DC to DC
converter
unless the capacitors voltage was above 0.484 v / solar cell element,
you
will operate the DC to DC converter in small bursts un low light, always
keeping the solar cell and capacitor combination chaging at the peak
efficieny point for your solar cell. In full sun the converter will
operate full time while the battery always receives a constant current
charge.
These micro-switching power systems and the low voltage PIC design by
Terry Newton make a very powerful combination. I think that this could
more
than compete with any conventional SE based design.
Imagine a tiny creature built under a Panasonic VL2320 battery, with a
2224 solar cell on top and a pair of tiny pager motors. It could also
have a bitty piezo beeper hidden in there as well!
The solar charged battery can also be used to replace the gold series
capacitor in a BEAM design like Vore-n-more, or a (small) walker.
------------------------------------------------------------------------
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