This circuit may be used to replace the single current limiting resistor
often found in dirt cheap battery chargers. The alternative shown here
will eventually pay off because you no longer have to throw away your
NiCds after three months or so of maltreatment in the original charger.
The circuit diagram shows an LM317 in constant-current configuration but
without the usual fixed or variable resistor at the ADJ pin to
determine the amount of output current. Also, there is no switch with an
array of different resistors to select the charge currents for three
cell or battery types we wish to charge: AAA, AA and PP3 (6F22). When,
for example, an empty AAA cell is connected, the voltage developed
across R1 causes T1 to be biased via voltage dropper D1.
Circuit Diagram
This results in about 50 µA flowing from the LM317’s ADJ pin into the
cell, activating the circuit into constant-current mode. D4 is included
to prevent the battery being discharged when the charger is switched
off or without a supply voltage. The charging current I is determined
by R1/R3/R3 as in R(n) = (1.25 + Vsat) / I where Vsat is 0.1 V. The
current should be one tenth of the nominal battery capacity — for
example, 170 mA for a 1700-mAh NiCd AA cell. It should be noted that
‘PP3’ rechargeable batteries usually contain seven NiCd cells so their
nominal voltage is 8.4 V and not 9V as is often thought.
If
relatively high currents are needed, the power dissipation in R1/R2/R3
becomes an issue. As a rule of thumb, the input voltage required by the
charger should be greater than three times the cell or battery (pack)
voltage. This is necessary to cover the LM317’s dropout voltage and the
voltage across R(n). Two final notes: the LM317 should be fitted with a
small heat sink. With electrical safety in mind the use of a
general-purpose mains adapter with DC output is preferred over a
dedicated mains transformer/rectifier combination.
Author: Myo Min
Source -http://www.extremecircuits.net/2010/06/switchless-nicd-nimh-battery-charger_20.html
