AVR-Chat

On Sun, Mar 14, 2010 at 6:45 AM, syberraith <syberraith@yahoo.com> wrote:
> I just read Amtel's AVR-based Constant Current Supply app note, which left me wondering why would you prefer an external chip for the current regulation.  What difficulty did you have with using just one chip?



Codespace for one, and accuracy for another.  The T26's "precision
reference" ended up at about 10%, I would have been better off using
VCC as a reference.   Silicon processes that make good digital
products generally make lousy analog.


I would advise a bootloader, and a serial port.  You'll want to know
what the chip is seeing, and why it's making the decisions that it
does.  A terminal program can log the output for you, and help
demystify things.  You could send temperature, current, and voltage in
CSV format and plot in Excel.

IIRC the 15 minute chargers are only supposed to be used with their
cells that are designed for it.  For most NIMH cells, the sweet spot
is 1C, where the charge termination signals (rapid rise in
temperature, voltage plateau, absolute voltage) are expressed well.
HOWEVER: Cheap chinese cells can drive you NUTS because they don't
have operational vents, cheaped out on the catalyst material, or got
the electrolyte fill slightly wrong.  For your development, use only
cells that you are absolutely sure come from a quality Japanese
source. (Panasonic, Sanyo, etc) Definitely get the data sheets on
those particular cells.

When you have a problem cell, you may notice that the temperature
curve is more or less linear across the charge, without the pronounced
spike at the end which it should have.
You don't necessarily need a lot of precision for this, a thermistor
on an ADC port will do, but for bonus points, look up the Hart
Steinhart equation, and apply that to your thermistor for maximum
accuracy. (Or implement a lookup table, after working that out in
excel. MUCH faster!)