The constant current source circuit is mainly composed of a sampling circuit and a comparison circuit of the constant current source. The alternating current is converted into direct current through a rectifier bridge, and after current adjustment and current feedback, a steady current output in the range of 0.1A to 10A is finally achieved.

The measurement loop is mainly outputted by the constant current source, and the stable current is directly passed through the power transformer winding without connecting to the standard resistance (the relay connected in series with the resistance is short-circuited) to measure the charging current. When the charging current reaches the set value, the relay is disconnected and the resistance is connected into the measurement loop. In order to ensure the stability of the power supply of the single-chip microcomputer, the relay controls the power supply separately. Due to the existence of the winding inductance, the residual current will constitute the user and the test equipment. There is a threat, so there must be a current discharge circuit. The current discharge circuit consists of a discharge resistor and a reverse diode. The diode is turned off when charging, the power supply charges the winding, and the diode is turned on when the power is turned off. The winding discharges through the diode and the discharge resistor.

At the end of the measurement, the relay K1 is disconnected. Because the transformer winding has a large inductance, the current cannot be immediately reduced to zero, which will generate a large potential. Therefore, a discharge circuit must be added. In this discharge circuit, when the relay K1 is broken After opening, the diode D1 is turned on, and the current in the winding forms a loop through the discharge resistor R4 and the diode to complete the discharge process. The selection of the discharge resistor should be appropriate. The larger the discharge resistance, the more power is consumed on the resistor, and the shorter the discharge time. The higher the voltage at the terminal, the choice of the discharge resistance is not only to control the discharge voltage value is safe, but also to make the discharge time as short as possible.