"NiCad" battery charger
LM 317 is a good IC Voltage regulator for small power supplies.
FIG. 1
shows another application of LM317 as "Constat Current source" for
"NiCd" battery charging
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FIG. 1.
Example: we need a NiCd battery charger for a 700 mAh battery case.
How is it the "R" value?
Answer: NiCad batteries must be charged with 10 percent of their capacity...
that means 700 x 10% = 70 mA or 0.07 A... =>
R = 1.25 / 0.07 => R = 17.85 OHM
The commercial resistors have standard values 10, 12, 15, 18, 22, 27, 33 ....
so, the closer value for 17.85 is a 18 Ohm resistor .
Imprortant notes:
1) input voltage must be 5-6 Volts higher... i.e. for a 7.2 V battery
case the input voltage must be 12.2 V or higher.
2) For currents above 50 mA, LM317 needs a Hotsink but keep in mind:
the central conductor has an internal - connection with metal- body of IC .
DON'T put the IC-body direct to a GND-metal box for hotsink without isolation!
In this case we have short-circuit between pin2 of LM317 and ground (--)
Finally, during charging "NiCd" batteries have a thermal-loss of about 40-50 %.
That means a higher total charging period... i.e. for a 700 mAh battery case it is
not enough a 10 hours period (10 x 70 mA =700 mAh) but a 14-15 hours period
(total battery capacity "plus" thermal loss).
The following table has various Battery capacities and "R" values
