The LP2951 regulator is manufactured by National Semiconductors. The
choice of values is from an application note Battery Charging, written
by Chester Simpson. Diode D1 can be any diode from the 1N00x series,
whichever is conveniently available. It functions as a blocking diode,
to prevent a back flow of current from the battery into the LP2951 when
the input voltage is disconnected. Charging current is about 100+mA,
which is the internally-limited maximum current of the LP2951. For those
wondering, this is compatible with just about any single-cell li-ion
battery since li-ion can generally accept a charging current of up to
about 1c (i.e. charging current in mA equivalent to their capacity in
mAh, so a 1100mAh li-ion cell can be charged at up to 1100mA and so on).
A
lower charging current just brings about a correspondingly longer
charge time. IMHO 100mA is quite low, low enough that the circuit can be
used for an overnight charger for many typical single-cell li-ion
batteries. The resistors are deliberately kept at large orders of
magnitude (tens/hundred Kohm and Mohm range) to keep the off-state
current as low as possible, at about 2?A. Resistor tolerances should be
kept at 1% for output voltage accuracy. The 50k pot allows for an output
voltage range between 4.08V to 4.26V - thus allowing calibration as
well as a choice between a charging voltage of 4.1V or 4.2V depending on
the cell to be charged. The capacitors are for stability, especially C2
which prevents the output from ringing/oscillating.
Parts List
IC1 = LP2951, voltage regulator
D1 = 1N4002, General purpose diode
R1 = 2M, 1%, metal-film
R2 = 806K, 1%, metal-film
P1 = 50K, potentiometer
C1 = 0.1uF, polyester
C2 = 2.2uF/16V, electrolytic
C3 = 330pF, ceramic
A
lower charging current just brings about a correspondingly longer
charge time. IMHO 100mA is quite low, low enough that the circuit can be
used for an overnight charger for many typical single-cell li-ion
batteries. The resistors are deliberately kept at large orders of
magnitude (tens/hundred Kohm and Mohm range) to keep the off-state
current as low as possible, at about 2?A. Resistor tolerances should be
kept at 1% for output voltage accuracy. The 50k pot allows for an output
voltage range between 4.08V to 4.26V - thus allowing calibration as
well as a choice between a charging voltage of 4.1V or 4.2V depending on
the cell to be charged. The capacitors are for stability, especially C2
which prevents the output from ringing/oscillating.Parts List
IC1 = LP2951, voltage regulator
D1 = 1N4002, General purpose diode
R1 = 2M, 1%, metal-film
R2 = 806K, 1%, metal-film
P1 = 50K, potentiometer
C1 = 0.1uF, polyester
C2 = 2.2uF/16V, electrolytic
C3 = 330pF, ceramic
