Hydraulic circuit for a construction machine

A hydraulic circuit of a construction machine, comprising:

• a hydraulic pump having variable capacity,
• a work tool operated by operating oil discharged by the hydraulic pump,
• a control valve controlling supply of operating oil from the hydraulic pump to the work tool;
• a pair of pipelines connecting the control valve and the work tool;
• a first return pipeline connecting the control valve and the hydraulic oil tank;
• a first check valve disposed in the first return pipeline;
• an oil cooler disposed in the first return pipeline on a downstream side of the first check valve;
• a second return pipeline that branches from the first return line and extends into the operating oil tank without passing through the oil cooler;
• a second check valve disposed in the second return pipeline;
• a third return pipeline that branches from one of the pair of pipelines and extends to the first return pipeline between the first check valve and the oil cooler;
• a solenoid valve disposed in the third return pipeline;
• an operating device to output a signal for operating the work tool,
• a controller that opens the control valve and the solenoid valve based on a signal output from the operating device;
• and
• a temperature sensor electrically connected to the controller to detect a temperature of the operating oil,
• wherein the controller reduces the amount of operating oil passing through the oil cooler in the case where the temperature detected by the temperature sensor is less than a predetermined value.