Boom cylinder control circuit for construction machine

A boom cylinder control circuit for a construction machine that includes a boom cylinder having an ascending-side chamber and a descending-side chamber, the boom cylinder control circuit comprising:

• a boom control valve that supplies operational oil discharged from a main pump to the ascending-side chamber and the descending-side chamber of the boom cylinder so that the boom cylinder ascends or descends;
• a first floating valve that selectively connects a drain tank with the descending-side chamber and the ascending-side chamber of the boom cylinder;
• a second floating valve additionally disposed on an oil flow line, which connects the drain tank with the descending-side chamber through the first floating valve, the second floating valve selectively connecting the descending-side chamber with the drain tank;
• and
• a floating selection operation part that provides an operational signal such that the first floating valve and the second floating valve are switched to be opened or closed and,
• a floating selection valve that is selectively switched to a first spool position or a second spool position in accordance with the operational signal of the floating selection operation part, wherein in the first spool position a boom-down signal line of a boom operation part is connected with a pressure receiving portion of the first floating valve and a descending pressure receiving portion of the boom control valve is connected with the drain tank, and wherein in the second spool position the boom-down signal line of the boom operation part is connected with the descending pressure receiving portion of the boom control valve and the pressure receiving portion of the first floating valve is connected with the drain tank.