LM338 battery charger circuit diagram
A 12V, 4Ah lead-acid battery was used in the design of a cross-connected test equipment controlled by a single-chip microcomputer. In order to make the test equipment more convenient to use, the device itself must have the function of charging the battery. In order to minimize the production cost, the volume and weight of the equipment are reduced. The three-terminal regulator LM338, which generates a constant current power supply that only works in the device test state, functions as a battery charging control tube when the device is in a charging state. Because of the use of a single-chip microcomputer, the "charging" and "working" states of the battery are switched. The following mainly introduces how to use the LM338 to realize the charging function of the battery.
When charging the battery, the device does not need to output current. Can the LM338 be used in the charging circuit through circuit switching? Practice has proved feasible. . The charging circuit for the 12V, 4Ah lead-acid battery using LM338 is as shown in the above figure. The battery in the above figure is in the state of external output current. When the battery is to be charged, the device is connected to the mains. Control K1 makes J2 pull in, the battery positive pole is connected to BA, and the battery enters the charging state. The key to this circuit is the LM338 adjustable three-terminal regulator. According to the different states in the battery charging process, the external circuit of the LM338 can be changed to have two functions of "constant voltage" and "constant current". In this circuit, when the battery voltage is lower than 15V, the LM338 constant current output; when the battery voltage is charged to 15V, the LM338 automatically becomes a constant voltage output. Thereby the entire electrification process can be completed well. How does this circuit achieve this goal?
The constant current and constant voltage switching circuit is composed of voltage comparator LM393 and Zener diode. The voltage regulation value of Z1 is 15V (to ensure reliable switching of the comparator, make the comparator IC1(2) pin slightly lower than 15V during debugging), when the battery voltage is high. At this value, the LM393 (3) pin outputs a high potential, and the J1 energization relay pulls in. When the battery voltage is lower than this value, the LM393(1) pin outputs a low potential. J1 loses power and is released.
When the button switch K1 is in the separated position, the external power is supplied. When charging is required, K1 is pressed, and the positive pole of the battery is connected to the second common contact 4 of the relay. The battery voltage at this time is lower than 15V. Comparator|1 pin output low potential, transistor G1 is cut off, relay J1 is released, (11) ~ (13) foot is on, (4) ~ (6) foot is on. At this time, the LM338 is in a constant current output state, and the current magnitude is 1.25/(R5∥R6)A, which provides a constant current of about 0.65A. When the voltage across the lead-acid battery reaches 15V, the IC1 1 pin outputs a high potential, and the triode G1 Turn on, relay J1 pulls in, (9) ~ (13) foot pass, (4) ~ (8) foot pass, the equivalent circuit is shown below.
As can be seen from the equivalent circuit, the LM338 is a regulated power supply. Because the current is small when charging, the effect of R6 on the charging current can be neglected. The regulation value is related to R2. Adjusting R2 can get a regulated output of 15V. At this point, the lead-acid battery is in a slow state of charge. It takes about 6 hours to charge the 12.4Ah lead-acid battery with this charger. It has been proved to achieve the desired effect after repeated use.
Because the device itself uses LM338 as a constant current source of 3A, it is equipped with a large heat sink. There is no problem of overheating damage in the state of charge. If you make a charger separately, take into account the heat dissipation problem. If R6 is 2Ω, the power requirement is 2W or more. The 20V voltage input uses a switching regulator power supply. The K1 uses a toggle switch with a self-locking function. When the switch is fully charged, the lead-acid battery is switched to the external power supply.
Hot Sale,High Quality Hot Sale,Hot Sale Details, CN
SHENZHEN CHONDEKUAI TECHNOLOGY CO.LTD , https://www.szfourinone.com