devices which make it possible to supply emergency power to a load which is to be supplied in the event of mains failure the invention is based on the object of providing a device for supplying uninterruptible power which device can be operated in a more energy saving manner and in which the emergency power can be controlled in a rapid and safe manner to this end the uninterruptible power supply device has a power transistor which can be rapidly switched and can be used to connect a standby power source to the output connections during emergency power operation the output current can also be dynamically limited by driving the power transistor which can be rapidly switched in a corresponding manner in addition the input and output of the ups device are decoupled using a field effect transistor that acts as a power transistor application_number US 95912210 A description the single figure shows a device for supplying uninterruptible power ups which is referred to below as ups device for short the ups device is generally provided with the reference symbol 10 on the input side the ups device 10 has two input terminals 90 91 to which a primary power supply device 230 is connected said power supply device providing a dc voltage u e in the present example the primary power supply device 230 may be a 24 v power supply unit or a 24 v power system on the output side the ups device 10 has two output terminals.