Another tube query

[Brian Gaff]

A small 8 watt one stopped working in a mains fitting o f the old Thorn type, flashing and messing about, but it still runs in a battery torch. Any idea why?

Brian

[John Williamson]

Low emission on the heaters.

Mains fittings running at 50 Hz need more emissivity to make a tube work than the HF inverters in the battery powered torches, which run the tubes in cold cathode mode, rather than needing to heat the heaters first to allow the arc to strike.

If you replace the choke in the Thorn unit with an HF inverter type ballast, then the tube will work there as well.

[Johny B Good]

Possibly condensation from a high RH in the mains fitting. I've now come to the conclusion that this was why I used to experience starting difficulties with my QuickStart fittings (I was having to literally 'lend a hand' to the quite reachable basement fitting to persuade the tube to light up until I opened the window to clear out the damp).

The kitchen fitting could occasionally be a bit temperamental until a section of the ceiling plasterboard fell down from the weight of the soggy mass of insulation due to a slow leak in the flat roof a few months ago. Clearing out the sodden insulation and allowing the rafter space to dry out seems to have completely cured that light fitting of any signs of starting trouble.

The only common factor between the two cases being abnormally high Humidity levels (Cold alone doesn't seem to be a factor for the basement fitting).

[Dave Plowman (News)]

Battery types only have one connection to the tube at each end - ie don't use the heaters.

[Johny B Good]

That seems to be true of the cheaper battery ballast units. Better designs do actually have heater 'overwinds' to improve lamp life and eliminate the risk of the lamp going into rectification mode.

[Brian Gaff]

Rectification mode, not heard of that one. I think you are right as I'm told although it works in the torch, the ends look a bit blackened.

So the torches just raise the voltage until it breaks down then, I suppose and once its ionising, it just keeps going on a lower voltage, much like those old orange neon's used to. Brian

[Johny B Good]

That's the nature of arc discharge lamps. They need some sort of current limiting to prevent runaway conduction. With a simple DC circuit, this is usually a dropper resistor. With AC circuits, this can be a less wasteful reactance, usually a ballast choke in the case of fluorescent tubes.

IIRC, neons strike around the 90v mark and run at 60v, Fluorescents can strike at 200 odd volts when the cathodes are heated and typically run at about 108v. The cathode filaments are designed to run on the preheat current on switch start due to the ballst choke current that results when the tube is shorted out by the switch to complete the heater circuit.

The high voltage pulse that results when the bi-metal switch contacts open also helps the tube to strike but if the this happens just when the current has fallen close to zero, this inductive kick fails to occur, hence the repeat starter cycle(s) that sometimes occur.

The cheap battery powered inverters rely on a very high leakage transformer to generate a kilovolt or more on open circuit when the lamp has not yet struck, this voltage being high enough on its own to initiate a cold cathode discharge, the running current of which will provide heat at the cathodes. Unfortunately, this extended startup period in cold cathode discharge mode places more stress on the cathode emitter material hence the more rapid blackening at the tube ends due to this material being sputtered off and deposited onto the envelope.

The cathodes don't have identical characteristics and one will drop in emitivity before the other causing the tube to 'rectify' the current which causes a DC component to be superimposed which will then cause a conventional ballast choke to magnetize and also run into saturation which aggravates this condition. If left to operate for long enough in this state, it will fail to conduct in one direction altogether causing a marked drop in light output.

Normally, a fluorescent tube should be changed long before it reaches this stage of decrepitude. A modern electronic ballast can actually force the user to change lamps well before they reach this state of wear by detecting this and other problems which is is no bad thing.

The quoted life of tubes (7500 hours back in the late 70s and the current value of 16000 hours for modern tubes) is calculated not from total failure but on the basis of the gradual fall off in luminous efficacy to around 90% of the 'Design Lumens' output which itself is a value expected after a run time of 2000 hours (the initial luminous output from new being typically some 15% above the 'Design Lumens' figure).

The domestic user is more likely to postpone lamp change out to the point well beyond the 10% reduction from its 'design lumens' output, typically when the lamp starts flickering due to exhaustion of the cathode emitter materials. This is a false economy (if you ignore the Russ Andrews Pricing Model used by the likes of Tool Station and their ilk). It's better to replace the lamp as soon as it shows the first signs of reduced lumen output rather than wait for it to start flickering, especially when you can't recall when it was last changed.

In commercial premises, they normally schedule lamp replacements at an interval based on the stated service life since the cost per lamp replacement is much cheaper than when it is done on an ad-hoc basis to replace individual lamps as they fail.

A few of the changed out lamps, selected for least wear, can be retained as spares to cover the odd random total failure that would be expected to occur in any large batch of lamps. Obviously the much cheaper unit price for the lamps and the labour costs of relamping (and scheduling the job to minimse dirsuption) are what makes this a "No Brainer" strategy in the commercial world.

[meow2222]

I added relay starting to a conventional ballasted 5' luminaire years ago, and found it never failed to strike when the relay opened. I can only concl ude that mains voltage alone was enough, and that glowstarter flashing is d ue to a starter design issue. Apparently this flashing at start didnt happe n with glowstarters in the 1930s & 40s.

Its only a false economy if the extra light has some monetary value to the end user. In domestic situations generally it doesnt.

NT