LEAD ACID BATTERY CHARGER

DESCRIPTION

(A) BASIC TRANSFORMER
Voltage is transformed by the transformer turns ratio N2/N1. For instance if 20VAC secondary voltage is required with 120VAC input, a 6:1 ratio would be needed. When full load current is drawn from the secondary winding, the voltage may drop from 5 to 25%, this effect is known as voltage regulation. Addition of a thermal fuse to the primary circuit insures safe operation in the event of an output short circuit or overload.

(B) UNREGULATED POWER SUPPLY
Capacitor C1 will charge up to 1.414 times the RMS secondary voltage minus the diode drop. Applying a loading current will cause the output voltage to drop by about 20 to 30%, the percentage drop is referred to as the voltage regulation. The circuit shown uses a center-tapped winding and 2 diodes, a non- center-tapped winding with 4 diodes is probably the most common configuration.

(C) LINEAR MODE REGULATED POWER SUPPLY
Unregulated DC power supplied from C1 powers the linear regulator. The regulator will precisely control the output voltage for critical circuit applications. Capacitor C2 is used to prevent oscillations in the regulator, it also improves regulator transient response characteristics. When the voltage at C1 minus the minimum regulator drop falls below the output voltage rating, the regulator is at the low-line point.

(D) SWITCHMODE REGULATED POWER SUPPLY
This type of regulator utilizes hi frequency switching techniques to minimize transformer size and maximize efficiency. Various converter topologies are the forward, boost, flyback and resonant. The flyback converter shown at left uses PWM (Pulse Width Modulation) circuitry to regulate the output. By modulating the switch at a fixed frequency with a given duty cycle the output voltage can be expressed as:
Vout= Vin (D.C./1-D.C.) x (N2/N1)
Where D.C.=Duty Cycle
Additional circuits are needed in order to comply with EMC Requirements.

(E) NICAD BATTERY CHARGER
The most common charging technique is the C/10 charger. C is the Amp/HR capacity of the battery. This type of charger requires approximately 12 to 14 hours to replenish a fully depleted battery. The current limiter can either be a resistive element or an electronic circuit. An indicating LED is useful for providing visual means to show charging is taking place.

(F) LEAD ACID BATTERY CHARGER
Voltage limited charging techniques are essential to extend battery life. At room temperatures lead acid cells require 2.3V per cell to stay at 100% charge. Voltage tolerance of the charging circuit should be kept to +/-1% by the use of voltage trimming. Recharge times of 3 to 5 hours can be realized with standard lead acid cells.