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Topic Title: Bulb flicker
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Created On: 11 August 2017 10:04 AM
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 11 August 2017 10:04 AM
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goldenboy1818

Posts: 812
Joined: 22 February 2011

Hi guys do you know about a fluorescent bulb flickering when off ? Its a standard ceiling rose/pendant and someone was asking about it. Im sure someone spoke about it on here before ? Thanks guys
 11 August 2017 11:10 AM
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AJJewsbury

Posts: 15838
Joined: 13 August 2003

Most likely capacitive coupling between the L and SL (but don't dismiss other causes like damp in the switch). Capacitive coupling is more likely to be a problem where things are looped into the light and the switch drop is long - so 2-way switching circuits are a favourite (sometimes operating both switches reduces the problem where triple+earth has been used). If it's a problem some manufacturers sell a capacitor to be wired across SL/N (e.g. in the ceiling rose) to shunt the leakage current around the lamp.
- Andy.
 11 August 2017 03:30 PM
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goldenboy1818

Posts: 812
Joined: 22 February 2011

It was only on a fluorescent bulb once changed to a normal bulb it was fine apparently
 11 August 2017 04:03 PM
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Dreckly

Posts: 41
Joined: 03 December 2014

That will be because the leakage current is insufficient to light the "normal" incandescent bulb.

- Dave
 11 August 2017 05:30 PM
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mapj1

Posts: 9371
Joined: 22 July 2004

a simple supression capacitor r
hidden in the rose or the lamp holder across the lamp normally does the trick.
The effect is seen in many designs of lamp with electronic controls, both LED and CFL, as the electronics trickle charges the internal DC supply at a few micro-amps, (via the stray inter-wiring capacitances and the bridge rectifier that is internal to the electronics) up to the voltage where it tries to start, (normally about 50 v DC) but almost immediately discharges all the stored energy, causing a short flash of light and then trickle charging starts again. The flash rate varies with the wiring arrangements.
The shunt C across the lamp forms a capacitive voltage divider with the stray C of the wiring (about 60-100pF per metre core to core for 1mm T and E, say 1000pF for 10m of wiring, a typical figure for a 2 way light circuit)

A 100nF capacitor as linked above is 100 time higher, so the voltage division in the off state is 100:1, and at 2.3V RMS, the electronics never gets up to the striking voltage. (unless you have hundreds of meters where unswitched and switched wires run along side.)

An RC is kinder to the switch and gives less EMI (is less likely to click the radio and false trigger a PIR )than a pure C, as when you operate the switch there is a surge to charge the capacitor, with the RC the current is limited regardless of when in the mains cycle it occurs.

-------------------------
regards Mike
 11 August 2017 06:50 PM
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kellyselectric

Posts: 164
Joined: 22 July 2016

I hope you've checked to mare sure the switch is in the live side and not neutral obvious I know but someones got to mention it
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