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

The measuring circuit is mainly composed of the steady current output by the constant current source and directly passes through the power transformer winding, without connecting to the standard resistance (connecting the resistance with the short relay), to measure the charging current. When the charging current reaches the set value, disconnect the relay and string the resistance into the measuring circuit. In order to ensure the stability of SCM power supply, relay control power supply is supplied separately.

Due to the existence of the winding inductance, residual current will pose a threat to the user and the device under test, therefore must have a current discharge circuit, current discharge circuit made up of discharge resistor and a reverse diode, diode shut off when charging, charging power supply to the winding, when power diode conduction, winding through the diode and the discharge resistance discharge.

At the end of the measurement, the relay K1 disconnected, because of transformer winding inductance, current can't immediately drop to zero, it will produce very big potential, so you must add a discharge circuit, in the discharge circuit, when the relay K1 disconnect, diode D1 conduction, winding current through the discharge resistance in R4 and diode loop formation, complete discharge process. The choice of the discharge resistance should be the larger the discharge resistance. The more power consumed on the resistance, the shorter the discharge time, and the higher the voltage at both ends of the discharge circuit. Therefore, the choice of the discharge resistance should not only control the discharge voltage to be safe, but also make the discharge time as short as possible.