The display power management doesn't work

Configure desktop => Display => Power Control tab => Enable display power management:

Standby after: 1min
Suspend after: 1min
Power off after: 1min

After 1 minute, nothing... the display is not in standby mode.

Should I activate something somewhere?

My system:
Slax 5
KDE 3.5.2
Kernel 2.6.20

Thank you for your help.