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G8MNY > TECH 20.08.18 09:02l 209 Lines 10206 Bytes #999 (0) @ WW
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Subj: Fluorescent Lamp Circuits
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From: G8MNY@GB7CIP.#32.GBR.EURO
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By G8MNY (Correction Mar 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
HOW TUBES WORK.
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).
The low pressure gas (mercury vapour) is ionised. The ionised gas glows in the
ultra violet range. The ultra violet light causes the material sprayed on the
inside of the glass tube to fluoresce. (correction from G4IJL)
To make the gas ions (Hg) a high voltage is applied after first heating the
tube with the end heaters to encourge the mercury to varourise. 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. For
a workshop work with fast rotary items, 3 tubes are normally used one per mains
phase so that strobe effects the can make fast spinning things look still or
slow, do not occur!
12V DC types are all electronic, high voltage DC types used to be used with
resistive ballast & needed the tubes regularly tuned around as the burned out
at one end.
STARTER TYPE (including the older heavy energy saver lamps)
Advantages..
1) 3-5 times more efficient than normal tungsten lamps once warmed up.
Disadvantages.
2) The light is difficult to focus with reflectors than tungsten lamps.
3) 100Hz flicker is more pronounced that tungsten Lamps.
Fluorescent coated glass
Live _o-o________())))_____ Ú------------------------------¿
Fuse ³ ÚÄÍÍÍÍÍ ÀÄ(Ä¿ Heaters ÚÄ)ÄÄÄÄÄ¿
3A ³ ³ Ballast ÚÄ(ÄÙ Neon/Hg gas ÀÄ)Ä¿ ³
ÚÄÄ´ ³ Choke ³ À------------------------------Ù ³ ³
Mains PF 1M === ³ ³ ============================== ³ ³
Correction ÀÄÄ´ ³ ³ ³ ³ ³
Cap + R ³ ³ ³ Starter --- ³ ³ ³
³ ³ ÀÄÄÄÄÄÄÄÄÄÄÄ(Ù'À)ÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÙ ³
³ ³ --- ³ ³
Neutral ÄÄÄÄÄÄÁÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
³ ³
Earth ÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
The ballast choke is designed to give the correct heater current (1A) 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/argon tube that immediately strikes & passes a
small current when mains is applied to it. It has a normally open bi-metal
contact inside starter that form the electrodes, these quickly heat up from the
gas plasma. 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. Often a small capacitor is included to reduce spark
QRM & contact pitting.
Starter Pins Paxalin
_T___T_/ Disk
1n 1kV ³ ³ ³ ³
Capacitor ³ ÃĴô ³
³ \__ / ³ Plastic
Neon/Argon ³ /³ ³\ ³ or Metal
Discharge ³³ ³<³ ³³ Case
Bulb ³ \___/ ³
ÀÄÄÄÄÄÄÄÙ
When the starter contacts cool down, the contact breaks & 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 confirm age.
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, touching tube with finger, & warming!
d) Welded starter contacts cause permanent glowing & no start cycle.
e) Burnt out choke due to shorted starter & silly fuse rating. Also a DC
component from old tubes can saturate the choke & lead to overheating
failure.
For b) & d) change/swap/remove starter. For a) & b) change tube.
ELECTRINIC STARTERS
In recent years new non mechanical starter has been available. It should give
longer starter live & some claim to give longer tube live as well.
INSTANT START TYPES
Advantages..
1) 3-5 times more efficient than normal tungsten lamps once warmed up.
2) No flashing starter, gentler on tubes.
3) May be able to run tubes when the heaters are burned out to give really long
useful tube lifetime & lower maintenance.
Disadvantages..
4) The light is difficult to focus with reflectors than tungsten.
5) 100Hz flicker is more pronounced than tungsten Lamps.
6) They are often more expensive than starter types.
7) They are often heavier than starter types.
Lossy (Leaky)
Fuse Transformer______________
Live _o-o_____________ ÉÍÍËÍÍ»(____________ ³
³ )º º º( -³-³-
³ )º º º( ³ ÀÄÙ ³|
Optional ³ )º º º( 500v AC ³tube ³|
PF Cap === )º º º( 80V ³ ³ÃÄ¿
³ )º º º( struck ³ ÚÄ¿ ³| ³
Neutral _____³__________)º º º(___________-³-³- ³
ÈÍÍÊÍͼ(______________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). There are also resonant transformer types
with a series tuning capacitor, this can cut down the size of the transformer
while also correcting the mains PF.
When powered a high voltage (>500VAC) appears across the tube & the heaters are
energised (approx 6VAC @1A). 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
a) 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. A sensible slow blow fuse rating or thermal cut out
would stop this!
b) The heaters are not needed if in warm environment, so tubes can often be run
until they "drop", but fail in the cold.
ELECTRONIC BALLAST TYPE (including most Saver lamps)
Advantages..
1) Slightly brighter & more efficient lamp than normal ballasted types, as HF
excitation gives more light!
2) 4-6 times more efficient than normal tungsten lamps once warmed up.
3) No Mains 100Hz flicker, the fast pulsing is merged by the tube persistence.
4) AC/DC operation.
5) Lightweight.
Disadvantages..
6) The light is difficult to focus with reflectors than tungsten lamps.
7) Radio QRM radiated down mains. (especialy if the electrolytic cap ages!)
8) Direct QRM from tube acting as an aerial.
9) Cross modulation (Mixing) of nearby RF fields in the tube.
10)Interference to IR remote controls sensors from pulsing IR arcs at the
heater ends.
_______________
Fuse Surge 300V DC Ferrite³ÚÄÄÄÄÄÄÄÄÄÄÄÄÄ¿³
Live ÄÄo-oÄÄRÄÄÂÄLÄÂÄÄÄÂÄÄÄ¿ ÚÄÄÂÄÄÄÄÄÄÄÄÄÄ¿ ())))))))) _³³_
³ ³ _³_ _³_ ³ ³ - - -³- - - - -³³ ³ \/ ³
Mains ³ ³ \_/ /_\ ³ ³ _())))ÄÅÄ())))_ ³³ ³ ³
Filter === === ÃÄÄÄ)ÄÄÙ +³ ³ ()) R ()) ³ ³³ ³tube³
³ ³ ³ ÃÄÄ¿ === \³ ³ ÃÄÁÄ´ ³ ³/ ³³ ³ ³
³ ³ _³_ _³_ ³ ³ ÃÄÙ ³ _³_ÀÄ´ ³³ ³_/\_³
³ ³ /_\ \_/ ³ ³ e/³ === /_\ ³\e ³³_____³³
Neutral ÄÄÄÄÄÄÄÁÄLÄÁÄÄÄÁÄÄÄÙ ÀÄÄÁÄÄÁÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÙ
Power Inverter
The mains is protected from the large HF inverter QRM by the mains filter. The
Surge R has to handle turn on surges (>340V peak). The "always live" 300V DC is
roughly filtered by the electrolytic cap & the supplies the high frequency
current path needed by the inverter. Here I have shown a simple push-pull high
power inverter, some use driver control ICs & MOSFETS etc.
Push-pull positive feedback drive the 2 output hard, initially biased by a pull
up R from +300V (sometimes from the mains side to reduce R wattage), then they
self bias from the diode once oscillation starts. Frequency of oscillation is
usually 15-30kHz & changes on depending on the tube status, mains voltage etc.
The tube drive may have heater taps as shown or not, of the ferrite output
transformer secondary produces over 1000V AC before the tube strikes.
N.B. It may not be safe to run the unit without a tube!
COMMON FAULTS
i/ Not working, mains spike killed bridge/inverter, surge R blown up.
ii/ Loud singing, cause .. ferrite core chattering glue dried out.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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