Adjustable 3-Terminal Regulator for Low-Cost Battery Charging Systems LB-35

National Semiconductor

Linear Brief 35

August 1976

Adjustable 3-Terminal

Regulator for Low-Cost

Battery Charging Systems

With the introduction of the LM317, a 3-terminal adjustable

vent overcharging, especially with nickel cadmium cells. The

internal protection circuitry of the LM317 is helpful in pro-

tecting against accidental overload conditions commonly

the output current, protecting the regulator as well as the

transformer. A constant voltage charger circuit is shown in

. The output voltage is set with resistors R2 and R3

FIGURE 1. Constant Voltage Charging Circuit

Since, in low cost applications, no filter capacitors are used

on the output of the rectifier, the battery is only charged on

characteristics. Resistor R1 controls the output impedance

of the charger allowing a ‘‘taper-charge’’ characteristic to be

generated. The LM317 can also be used to limit the peak

charging current to a partially charged battery at a value

other than the regulator current limit. With R1 in the circuit,

Including R1 in the feedback loop decreases the value of

resistor needed for a particular output impedance reducing

conditions, the peak output current is still 2A for the

LM317K with the resistor dissipating 1.6A as opposed to 8W

ifa2Xresistor were used directly in series with the battery.

This regulator configuration provides some other important

features to the charger. If input power is removed and a fully

charged battery is connected to the charger output, there is

no damage. Under these conditions about 5 mA of current

will current limit and thermal limit normally, protecting the

DECREASING CURRENT LIMIT

with Peak Current Limiting

output voltage must be above about 0.6V for the external

current limiting to be active. The internal current limit of the

LM317, of course, is still operative. This is not usually a

problem since batteries charge to above 0.6V very quickly.

Resistors R4, R5 and R6 protect the regulator and transistor

for both direct short circuits or reverse battery connections.

FIGURE 3. Charger with No Battery Loading when

Power is ‘‘OFF’’

As illustrated in

When power is ‘‘OFF,’’ Q1 is open, eliminating any loading

on the battery. A separate pair of low current diodes D1, D2

are necessary to bias Q1, rather than the power bridge recti-

fier. If R1 was tied to the output of the bridge, reverse cur-

rent flow through the LM317 would keep Q1 ‘‘ON’’ and load-

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