Hydraulic circuit for a construction machine

A hydraulic circuit for a construction machine comprising an open circuit and a closed circuit,

• wherein the open circuit includes:
• a variable-volume hydraulic pump configured to discharge hydraulic oil sucked from a hydraulic oil tank;
• a hydraulic actuator operated by the hydraulic oil discharged from the hydraulic pump;
• a directional switching valve configured to switch a flow direction of the hydraulic oil from the hydraulic pump to the hydraulic actuator;
• a pump line connecting the hydraulic pump and the directional switching valve;
• a return line connecting the directional switching valve and the hydraulic oil tank;
• a return check valve provided in the return line;
• a bypass line connecting the pump line and the return line;
• an electromagnetic proportional bypass valve provided in the bypass line, and
• the closed circuit includes:
• a variable-volume bidirectional hydraulic pump having a first port and a second port;
• a hydraulic motor operated by hydraulic oil discharged from the bidirectional hydraulic pump;
• a first line connecting the first port of the bidirectional hydraulic pump and the hydraulic motor;
• a second line connecting the second port of the bidirectional hydraulic pump and the hydraulic motor, and
• wherein a charge line is provided between the open circuit and the closed circuit that connects a portion of the return line on an upstream side of the return check valve to the first line via a first check valve, and connects a portion of the return line on an upstream side of the return check valve to the second line via a second check valve.