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G8MNY > TECH 29.06.04 22:37l 107 Lines 5496 Bytes #999 (0) @ WW
BID : 64676_GB7CIP
Read: DO4SN GUEST OE7FMI
Subj: Fluorescent Lamp Circuits
Path: DB0FHN<DB0FOR<DB0MRW<OK0PPL<DB0RES<ON0AR<7M3TJZ<IK1ZNW<ZL2TZE<GB7PZT<
GB7YFS<GB7CIP
Sent: 040629/2028Z @:GB7CIP.#32.GBR.EU #:64676 [Caterham] $:64676_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY
HOW TUBES WORK. (updated Mar 04)
They are about 3 to 5 times more efficient than normal tungsten lamps once
warmed up, but the light is difficult to focus with reflectors & the 100Hz
flicker is more pronounced. For rotary workshop work 3 tubes are normally used
one per mains phase so that strobe effects do not occur with syncronous motors.
The tubes are coated with some fluorescent powders on the inside that glow to
make up the colour balance for that type of tube (Warm White, White, Daylight)
when the coating is bombarded with fast moving gas ions in a very low pressure.
To make the gas ions (Hg) a high voltage is applied after first heating the
tube with the end heaters. This heating encourages the low pressure gas to
strike an arc (plasma) when a high voltage is applied across the tube. The
heaters stay hot with the very high gas temperature of the arc, but due to the
low pressure there is little heat transference to the glass.
As the tube ages the heaters eventually burn out leaving tungsten blackening on
the glass, & an arc can no longer be initiated or maintained. To increase the
heater lifetime heater guard plates reduce the ion velocity near the heaters.
To encourage the arc striking it is important that the fitting be earthed as
this helps propagate the initial arc down the tube. To aid this some makers put
a metal strip down the outside of the tubes that is connected to the end caps.
With the right type of matching holder these are earthed, otherwise just the
nearby body of the lamp fitting has to do.
There are 2 mains iron ballast types other than electronic ballast types.
1/ STARTER TYPE
Fluorescent coated glass
3 Live _o-o_________())))____ Ú------------------------------¿
Fuse ³ ===== ÀÄ(Ä¿ Heaters ÚÄ)ÄÄÄÄÄ¿
³ Ballast ÚÄ(ÄÙ Neon/Hg gas ÀÄ)Ä¿ ³
³ Choke ³ À------------------------------Ù ³ ³
PF === ³ ============================== ³ ³
Correction ³ ³ Starter --- ³ ³ ³
Cap ³ ÀÄÄÄÄÄÄÄÄÄÄÄ(Ù'À)ÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÙ ³
³ --- ³ ³
Neutral ÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
³
Earth ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
The ballast choke is designed to give the correct heater current when the
starter shorts & also give the correct arc current (lamp rating) when running.
The capacitor is optional depending if the mains power factor (due to inductive
ballast) needs to be corrected, & even then usually only to a PF of 0.85-0.9.
The starter is a small neon tube that immediately strikes when mains is applied
it has a bi-metal contact inside starter that heats up. When it is hot after a
few mains cycles the contacts close for 1-2 seconds, this puts high current
from the choke through the heaters, which should cause them to glow. When the
starter contacts cool down the break & if there is still mains current flowing
(not at the wrong part of the mains cycle) high back EMF from the choke is
applied via the mains across the tube. If the tube fails to strike the start
cycle repeats.
COMMON FAULTS
a) Burnt out heaters causes no start cycle. Blackened ends.
b) Old tubes with blackened ends need higher than normal strike voltage & can
fail to start with a starter of too lower strike a voltage (wattage).
c) Fail to start when cold, try earths!
d) Welded starter contacts cause permanent glowing & no start cycle.
e) Burnt out choke due to shorted starter & silly fuse rating.
For b) & d) change/swap/remove starter. For a) & b) change tube.
2/ INSTANT START TYPES
Fuse ____________
Live _o-o_____________ ||======||(__________ ³
³ )|| || ||( -³-³-
³ )|| || ||( ³ ÀÄÙ ³|
Optional ³ )|| || ||( 500v ³ ³|
PF Cap === )|| || ||( 80v ³ ³ÃÄ¿
³ )|| || ||( struck ³ ÚÄ¿ ³| ³
Neutral _____³__________)|| || ||(_________-³-³- ³
||======||(____________³ ³
6v ³
Earth________________________________________________³
These use either a non resonant lossy transformer with a magnetic shut that
allows the flux to bypass the secondary. (An isolating one is shown here but
auto-transformer types are common). Or a resonant transformer with a series
tuning capacitor, this can cut down the size of the transformer while also
correcting the PF.
When powered a high voltage (>500VAC) appears across the tube & the heaters are
energised (approx 6VAC). When the tube strikes the 500V falls to about 80V @
the current rated for that tube's power. The heater voltage also falls
dramatically.
COMMON FAULTS
1/ Burnt out Transformers, due mainly to uneven tube striking (50Hz flicker) on
old tubes can causes DC through transformer & shorten transformer life due
to over heating. Sensible rated slow blow fuse or thermal cut out would stop
this!
2/ The Heaters are not needed if in warm environment, so tubes can often be run
till they "drop", but fail in the cold.
Why Don't U send an interesting bul?
/QSL
73 De John, G8MNY @ GB7CIP
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