A charger circuit that can automatically identify the battery polarity and has protection function

The circuit is shown in the attached figure. After the charger power is turned on, if the charger output terminal is not connected to the battery, the relays J1 and J2 will not work. Since J1 does not work, there is no voltage output at the charger output terminal. As we all know, even after the battery oil is completely discharged, there is a residual voltage of 8-10V at both ends of its electrodes (it cannot provide current).

When point A of the charger output is connected to the negative pole of the battery and point B is connected to the positive pole of the battery, the photocoupler U2 is cut off, Q4 is cut off, and the relay J2 does not work. The photocoupler U1 is turned on. After U1 is turned on, the circuit outputs a voltage of negative A and positive B, which just meets the battery charging conditions and can charge the battery normally.

When point B of the charger output is connected to the negative pole of the battery, and point A is connected to the positive pole of the battery, the photocoupler U2 is turned on, Q4 is also turned on, and the relay J2 is activated. At this time, the photocoupler U1 is turned on. After U1 is turned on, the circuit outputs a voltage of positive A and negative B. This just meets the battery charging conditions. The battery can be charged normally.

Improved supplementary content The overcurrent protection circuit of the charger circuit that was originally designed to automatically identify the battery polarity is connected in series in the original charger circuit through a current sampling resistor. For a large current charger, its sampling resistor will produce a large loss, which will reduce the efficiency of the charger. This time, the overcurrent protection circuit designed by the author uses a current transformer to detect the current of the whole machine. The detection circuit has almost no voltage drop on the charger power output, which improves the efficiency of the charger. The original charging polarity conversion part can be found in "Electronics News" 2009 No. 13.

The overcurrent protection circuit is mainly composed of current transformer T, potentiometer VR1, resistor R5, thyristor Q2 and other components. The primary coil of T is connected to the front circuit of the rectifier bridge, and the induced voltage of its secondary coil is divided by VR1. A voltage signal that changes with the load charging current is obtained, which is added to the thyristor control electrode through R5. When the current during the charging process is too large, the secondary coil voltage of T increases, and VR1 and R5 force the thyristor Q2 to turn on. After Q2 is turned on, Q1 is turned off, and relay J1 is disconnected. At this time, the charger has no output. In this way, the battery and charger are protected. After protection, the fault indicator LED2 lights up. The working indicator LED1 goes out. At this time, it is necessary to disconnect the charger power supply and the connection between the battery and the charger, plug in the charger power supply again, and connect the battery, and the whole machine will resume normal operation.

In the improved circuit, R4 is used for overcurrent protection of the whole machine. Its function is to speed up the discharge of capacitor C1 after turning off the charger power supply and disconnecting the connection between the battery and the charger. In this way, the thyristor can smoothly switch from the on state to the off state, thus shortening the time for the whole machine to switch from the protection state to the normal working state.

Component Selection

The diodes in the figure are all 1N4007, and the models of Q1, Q3, and Q4 are all C2482. If this model is not available, S8050 can be selected. The model of Q2 is MCRl00-6. The mutual inductor is modified from the mutual inductor of the induction cooker. The primary winding is selected according to the required current. The wire diameter is selected (generally the same as the low-voltage winding wire diameter of the charger, and multiple strands of enameled wire can be connected in parallel), and it is wound around its frame once (specifically, it should be three quarters of a circle). The secondary winding is wound 300 times with enameled wire with a diameter of about 0.09mm. The overcurrent protection control point is determined by adjusting VR1 according to the size of the required protection current. It is best to use a precision adjustable potentiometer for VR1.

J1 can choose JOX-13FDC12V, and use its normally open contact. J2 uses JOX-13FDC12V, JQX-13F relays, because the manufacturers are different. Its contact current varies from 10A to 30A. It is best to choose a contact current of 30A. Other device models have been marked in the figure. The fuse F in the figure is designed to prevent the relay contact from aging and causing a short circuit. The specific value should be determined according to the rated maximum output current of the charger. Generally, it is between 10 and 30A.

If it is a modified charger. The rectifier bridge in the figure is attached to the original charger, and generally there is no need to modify the secondary circuit of the original transformer. If it is a homemade charger, the 2 and 4 terminals of the rectifier bridge are connected to the secondary coil of the charger transformer, or to the secondary coil of the transformer adjusted by the voltage regulator. When homemade, the rectifier bridge is generally selected to be greater than the maximum output current of the charger.