N. Thornton wrote:
Ian Stirling wrote in message .. .
N. Thornton wrote:
(Andrew Gabriel) wrote in message ...

Good account there. I'd just add 2 things: electronic ballasts dont
always ensure perfectly symmetrical current flow in the 2 half cycles,
hence the line frequency element of flicker can still occur with some
types.

All the electronic balasts I've taken apart (probably 5-8 different models)
have a bridge rectifier/capacitor before everything else.
The only asymmetry could arise from differential heating of the diodes,
or manufacturing differences.
This is going to be at the very outside 30-50mv.
On a line going to +-340V, this is effectively zero.

A bridge rec isnt going to prevent asymmety though. A series cap will,
a reservoir cap wont. The first electronic fitting I had flickered
severely during start up, at 50Hz.

Combined with the fact that AC mains has very little DC component it will.
If there is a bridge rectifier, then there is no 50Hz component
in the output of it, only 100Hz.

and Andrew Gabriel wrote:

Please explain where you think there's any 50Hz signal in an
electronic control gear circuit.


OK, the factors stated above do not ensure no 50Hz current is present.
snip
With such a setup, if one emitter has total failure, the tube can
conduct current one way only, giving intolerably strong 50Hz flicker.

No, it can't.

An electronic ballast has a rectifier at the front of it.
The waveform after the rectifier is NOT a 50Hz sine wave.