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G8MNY > TECH 25.12.05 13:13l 101 Lines 4868 Bytes #999 (0) @ WW
BID : 10294_GB7CIP
Read: DL1LCA GUEST OE7FMI
Subj: Fluorescent Lamp Circuits 1/2
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0RES<DK0WUE<7M3TJZ<ON4HU<GB7YFS<GB7CIP
Sent: 051225/1022Z @:GB7CIP.#32.GBR.EU #:10294 [Caterham] $:10294_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY
HOW TUBES WORK. (updated Sep 04)
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. For
rotary workshop work, 3 tubes are normally used one per mains phase so that
strobe effects do not occur with synchronous motors.
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
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
³ ³ Choke ³ À------------------------------Ù ³ ³
PF === ³ ³ ============================== ³ ³
Correction ³ ³ ³ ³ ³ ³
Cap 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 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 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.
In part 2 Instant start & electronic types.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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