Eeyore writes:

Seán O'Leathlóbhair wrote:

What is going on? Is a tiny current leaking, building up a charge in
a capacitor somewhere until a sufficient voltage builds up to spark in
the bulb and discharge the capacitor, and then the cycle repeats.

That would appear to be the case.

This is at least partly true.

Low energy lamps including compact fluorescents and normal fluorescents
on electronic ballasts use a switching circuit to produce the high voltage for
the fluorescent lamp. The input is a bridge rectifier and filter capacitor.
Any source of AC even a small amount of leakage through a defective swtich,
a switch with a neon lamp night light feature (lighted when off), an
electronic timer, a motion sensor-controlled yard light, or
a dimmer that isn't fully off, may cause the voltage to build up
on the filter capacitor until the "startup circuit" kicks in. This usually
has some sort of threshold de
vice like a zener diode or diac that won't
pass current until the voltage across it exceeds a spec'd value. When
that happens, the lamp starts up and strikes but just for an instant since
there isn't enough current available on the input to maintain it.

I'm somewhat skeptical of the explanation with respect to inductive or
capacitive coupling (though possible under just the right - or wrong -
conditions) but it doesn't take that much leakage from some other fault
to do this.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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