Constant current battery charger circuit diagram analysis

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

Photocoupler

description:

The battery is charged with a constant current, and the charging current is approximately one tenth of the battery's ampere-hour calculation. That is, a 4.5Ah battery, the charging current is about 450mA.

This constant current battery charger has the following features:

• 1. Can charge 6v, 9v, 12v batteries. Other rated voltage batteries can be charged by changing the voltage values ​​of the two Zener diodes ZD1 and ZD2.
• 2. The size of the constant current can be set arbitrarily according to the battery capacity by using a potentiometer and a multimeter in series with the battery.
• 3. Once the battery is sufficient. After it reaches a certain voltage (for example, a 12V battery reaches 13.5V to 14.2v), the circuit can give an indication and automatically cut off the charger without removing the battery from the circuit.
• 4. If the battery is discharged, the voltage is lower than a certain value. The circuit will give a "deep discharge" indication.
• 5. The circuit quiescent current is less than 5mA. Most of its current consumption comes from the Zener.
• 6. The DC supply voltage (Vcc) is available in a wide range from 9V to 24V.
• 7. The charger has a short circuit protection.

Under normal circumstances, the minimum DC power supply voltage should be "D1 voltage drop + battery voltage after full charge + Vdss + R2 terminal voltage", this voltage is close to "battery voltage +5v after full power". For example, if the battery voltage after fully charging a 12V battery is 14v, the DC power supply voltage should be 14V+5V=19V.


The charger circuit can be divided into three parts: constant current source, overcharge protection and deep discharge protection.

The constant current source portion is composed of a MosFET field tube T5, a transistor T1, diodes D1 and D2, resistors R1, R2, R10 and R11, and a potentiometer VR1. D2 is a low temperature coefficient, highly stable reference diode LM236-5. The gate-to-source voltage (Vcs) of T5 is set slightly higher than 4v with VR1. After setting Vcs, the charging current can be determined by the battery capacity. First, determine the charging current (one tenth of the battery's Ah capacity), then calculate R2 (take the most recent value from the resistance standard series): R2 = 0.7 / safe fail current.

R2 and T1 limit the charging current in the event of a fault or an unexpected short circuit at the battery terminals.

Overcharge and deep discharge protection circuits are indicated by dashed lines in the circuit diagram. All component values ​​in the dashed box are determined by the maximum battery voltage and not the DC supply voltage. such. The charger circuit can operate over a wide range of supply voltages. It does not affect the charging current value.

In the overcharge protection circuit. When the battery is sufficient (for example, the 12V battery voltage reaches 13.5V), adjust VR2 to set Vcs of T5 to zero, so the charging current stops flowing into the battery. At this time, LED1 is illuminated. Indicates that the battery is fully charged. During the LED1 lighting period. The internal LED of the optocoupler IC1 also emits light. Turn its internal transistor on. As a result, Vcs of T5 becomes zero and charging stops.

In the deep discharge protection circuit, ZD2 is normally turned on. It drives transistor T3 to turn it on, and the result. When T4 is turned off, LED2 does not emit light. If the battery terminal voltage drops to 11V (for a 12V battery), adjust VR3 to turn T3 off and T4 turn on. At this time LED2 shines. Indicates that the battery voltage is low.

The regulation values ​​of ZD1 and ZD2 are the same for 6V, 9V and 12V batteries. For batteries of other voltages, it is necessary to appropriately change the values ​​of ZD1 and ZD2. This circuit provides a charge current of 1mA to 1A. The T5 does not require the use of a heat sink. If the required maximum charging current is 5A. It is necessary to replace D2 with two LM236-5 and change the value of R11 (10kΩ) to 1kΩ. In addition, two SB560s must be used in parallel to replace D1. At the same time, T5 needs to add a heat sink. When T5 is packaged with TO-220, the maximum power consumption can reach 50w.