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Does Frequent Switching On and Off Impair Devices?






 © 2009-2014
Christian Bachmann

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Switching off a device when it is not used is undoubtedly the most simple power saving measure. In connection with this the question whether frequent switching on and off would not impair the device is often asked. This question was examined at the Swiss Federal Institute of Technology (ETH) in Zurich with regard to PC monitors, fax machine appliances, video recorders and copiers.

PC monitors with cathode ray tubes (CRTs)

Switching on and off the monitor five times or more a day increases the frequency of faults in power transistors in the control and deflection parts only after the machine has been used 20 to 30 years. After about 20,000 switching cycles (17 to 20 years of service) emission of the cathode starts to weaken. The other components are not impaired by frequent switching. Based on the parameters measured and known from technical literature the critical operating time can be determined when wear caused by the switching cycle starts to impair the life cycle. For PC monitors this critical operating time is 15 minutes. This means that switching off the monitor is worthwile for breaks lasting more than 15 minutes [8].

Photocopiers

Most photocopiers currently being made are not suited well for frequent on/off operation. However, it would be possible to adapt photocopiers to automatic on/off operation with a few technical modifications and reduce stand-by losses by 90 percent or more. Among other things a circuit would have to ensure that the heat exhaust fan remains in operation as long as the device is hot. Some other components (fixation drum, pre exposure lamp) would also have to be slightly adjusted. Switching off during long breaks or over night does not have any considerable influence on the life cycle [8].

Fax machine

External devices which switch the fax off and activate it only when a signal arrives are available but have failed to catch on as yet. Among other things there are problems with the lithium type batteries, which in case of a power failure back up the memory containing the phone numbers. However, the introduction of a sleep mode might be useful. The problem of increased stress on components and failure probability can be technically resolved. Power consumption in stand-by would be reduced by about 70 to 80 percent [8].

Video recorders

Complete switching off is not sensible because of the built in clock and because the various tuning and programming procedures require a considerable amount of time. However, various functions which are not required could be switched off in a sleep mode that has yet to be developed: switching power supply, tuner, terminal etc. The tuner would be activated about two minutes in advance of the programmed starting time and would be available in time. The higher frequency of switching cycles would barely impair life cycle and reliability of the device. Power consumption in stand-by would be reduced by about 80 percent [8].

Is latch-up a problem in circuitry?

Latch-up means the presence of a signal without a supply voltage in integrated circuits (IC). Certain types of ICs (CMOS) may be destroyed by latch-up. This problem mainly concerns old fax machines and video recorders. The circuits of more modern devices are probably latch-up protected [8].

On balance: Switching off is possible.

The investigations show that sensible switching on and off is already problem-free. However, many of the concepts for building devices used today make frequent switching on/off more difficult. This concerns purely technical aspects (wear, life cycle, reliability) as well as operating convenience. In future generations of devices this problem should be taken into account and circuits be modified accordingly. Automatic power management will decrease power consumption and improve operating convenience.

© Christian Bachmann

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