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Sometimes a light will start to flicker because of some internal problem. The flickering is usually very frequent, but not as frequent as 50Hz or 60Hz.

What kind of problem causes a light (or other device) to flicker rather than losing power completely?

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Incandescent bulbs have an infrequent failure mode where they flicker just before going out for the final time. Probably some interaction between the magnetic field of the current flowing through the filament and the arc between ends of a broken filament flailing around. Once turned off they don't come on again. Individual LED laps can flicker at a few Hz, probably due to thermal effects and a bad bonding wire or die attach.

Fluorescent tubes, especially when new, can exhibit a kind of plasma instability that causes rotating patterns inside the tube.

External changes such as the momentary voltage drop caused by a fridge compressor, forced air blower, air conditioner or other load switching can cause visible flickering. Laser printers can cause cyclic flickering when active due to temperature control of the fusion roller.

Failing power supplies inside LED or fluorescent bulbs can cause flicker.

Some cheap decorative (eg. Xmas tree) LED bulb sets have half wave rectification and flicker at 25/30Hz.

Finally, a real possible cause for concern is flickering caused by a bad connection somewhere in the wiring. This could be caused by a loose connection like a wire nut or improperly tightened screw (aluminum wiring was a real issue). The heating caused by arcing is a cause of many electrical fires- to the point where arc detectors may be mandatory in some jurisdictions. You can sometimes hear a sizzle sound inside the box (or, worse, outside) coincident with the flickering. Sometimes it's in the fixture or lamp socket. If this is happening you should make sure it gets fixed promptly.

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LED lights often use PWM to adjust brightness. Reducing duty cycle makes switching more noticeable, because the gap between pulses becomes wider, making separate pulses distinguishable for human eye. This effect is especially noticeable when using relatively low frequencies like 200-300Hz.

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Flickering lights is a normal occurence. Uauslly the cause is usually benign. Here’s a look at the main causes of lights flickering in a house based on the type or pattern of flickering that’s occurring.

1.A situation where the same light or lights are flickering in one area and not throughout the home.

2.Lights in the home only flicker when a large appliance (such as the air conditioner) kicks on.

3.Lights are flickering throughout the house and not associated with large appliance operations.

4.Your neighbors are experiencing flickering lights, too.

For more, see:http://www.doityourself.com/stry/flickeringlights

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  • \$\begingroup\$ It would be good if you could list the likely causes as well as the situations you might observe flickering. \$\endgroup\$
    – Zaz
    Aug 12, 2016 at 10:36

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