Heat bed max temperature 63*C

Hi i built a prusa i2 and I have no problems with the PLA. But when printing the ABS i have the problem that the heat bed doesn t pass 63 degrees C.
I divided the power source in two power supplies ( one for the motors and one for the bed). The power supply of the bed gives 24V 10A.
How can i solve the problem and reach 120 degrees?

What heater voltage do you measure at the bed terminals?

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].