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From the Q&A

With TJ Byers

Plug It In, Plug It In


I’ve heard that consistently turning on and off electronic components, such as a television or computer, will create a small amount of damage to the circuitry. Because of this, it is actually better to leave these devices on all the time. Is there any truth to this?

Gene Bozek
via Internet


What you describe is called in-rush current, and it applies to all forms of electronics. In incandescent lamps and CRT filaments, it’s caused by the difference in resistance of the heating element, which has less resistance when cold than hot. Hence, there is a big surge of current followed by reduced current as the filament heats. Destructive in-rush current also applies to power supplies, where the surge current is caused by the initial charging current of the capacitors.

A fully discharged cap behaves like a short circuit when voltage is applied — current that has to be absorbed by the rectifier diodes and pass transistors. And like a hot wire, the in-rush current decreases with time.

As to leaving the electronics on or turning them off, that’s a topic for debate. While the electronic device is running, it generates heat. Heat is an enemy of electronic components and shortens their life proportional to ambient temperature. Frequent on/off power cycling of the device, on the other hand, will take its toll in in-rush current. So which to choose?

The ideal solution is to limit the in-rush current to a safe level, then operate the device for only as long as you need it. This can be done using an in-rush current limiter (ICL — a negative temperature coefficient thermistor). From he Table 1, select the correct ICL from the Steady Amps or Rated Watts columns that most closely matches your equipment requirements. ICLs can be purchased from several sources, including Digi-Key.