Electromagnetic radiation > Light, Lasers and Optics

Cathodes in Compact Fluorescent Lamps

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klugesmith:
One barrier to recycling dead CFL bulbs is that they invite experiments.
Reverse engineering the HF drive?  Energizing the tube directly with NST etc.?

It looks like the coated-tungsten filaments are designed for electric preheating.  Is that only for 120-volt applications, and unnecessary in 240-volt land?
Typically the filaments at both ends of tube are connected together, in series with a capacitor.


Here's what one tube looks like with filaments directly in series, with DC power of 12 V, 0.35 A.


So how do they get powered (for preheat) in normal operation of the lamp? 
Easier to ask on forum than to trace the circuit or search on Internet.

HiVi:
Preheating is always needed, not matter what voltage land you are in.
It is needed for mercury to vaporize so the right mixture of gasses is in tube.
Inductive ballast takes care of current regulation.
In startup operation, usually, there is thermal switch, that in series connects the filaments, so you heat them with resistive heat generation. When heated enough, the switch opens.
In normal operation, after preheating was done, the filaments are getting heated by electrons flowing though gas hitting the filament.

Hope I covered the basics of what you have asked.

klugesmith:
Thanks, but it's not what I was asking.  Plenty of fluorescent lamp ballasts do no preheating (see Rapid Start and Instant Start).
In fact there are fixtures and lamps with only one terminal on each end, not to be confused with cold cathode systems.


Electric preheating 1) reduces the arc-striking voltage and 2) greatly reduces the wear on cathodes in the first second of operation, if they are forced to emit while cold (a major factor in lifetime of lamps).   Hg concentration builds up as the whole tube gets up to operating temperature, and normally reaches about 1 atom of mercury vapor per 1000 atoms of argon.

Was just curious about the preheat circuit details, in the cheap ballast circuits built into screw-base CFL units.

klugesmith:
Had to play instead of throw away.   Lit up a few CFL tubes using very simple bench setup:
Wall plug - variac - MOT - curly lamp.  With AC voltmeter or ammeter, not at the same time.
This depends on the ballasting behavior of MOT because of its core shunts.


Starting from 0 on the variac, stepped up voltage reached 350 to 450 volts before the tube lit up.
With lit tube, the measured U immediately dropped to order of 100 volts.
Crept up in the course of a minute or two, as tube warmed up and got brighter.
Turning up variac, to increase current, made arc voltage lower (e.g. 85 V at 0.45 A). Soon the tube was much hotter than in normal operation.
Turning down variac, to reduce current, made arc voltage higher (e.g. 115 V at 0.08 A).

How about measuring the cathode filament temperature at different arc currents,
by DC resistance measurement at the same time we have AC current shared by the two leads at each end of tube?

davekni:
I've noticed the negative-resistance of fluorescent tubes too, mostly ~20 years ago playing with DIY battery-powered fluorescent lights.  My low-pressure sodium vapor tube is also negative-resistance like that.  I think it's common for almost all gas-discharge tubes, from small neon bulbs to xenon arc lamps.

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