Flourescent bench lamp

[Radio Fixer]

I have put this here as it is a most useful part of my test gear with eyes that don't see so good these days.

Anyone got any bright ideas (no pun intended) on an old (maybe 25 years) fluorescent lamp that goes through tubes in about 3 months of limited use. If it were an ordinary fitting I would just dump it but it is a circular 5” magnifying lamp and very useful. It’s better than modern ones as it sits on a heavy base and can be moved around the bench. New ones are less flexible as they clamp to the bench.

The circuit is simple: in the base there are 3 chokes of 0.5H and 16 Ohms each, in series, as ballast once the tube has stuck and current is flowing through the ionised gas. There is no neon type starter but a push button that allows current to flow through the tube heaters if the gas doesn’t strike. When the tubes are new it is not necessary to use this. The heaters are always OK and so something is quitting in the gas: I've worn out the ions !

This lamp may originally have used a 30W tube but this is not available now in 200 mm size. So I have used metal clad resistors, mounted inside the large metal base, in series with the ballast, to use a 22W tube.

Total added resistance, arrived at by trying and measuring, is 300 Ohms and using one of those plug in power meters and a true RMS lab meter these are the figures the tube was running at when fitted:

Mains in 244V, I in 0.27A, W (as shown on the plug in meter) = 42, V across the 300 Ohms = 80V (measured with the lab meter).

So power taken by the resistors = 21.3W and so power in the tube should be

42 – 21.3 = 20.7W for a tube rated at 22W.

This seems all good to me so why do the tubes have such a short life ?

I cant re-measure the figures as the tube no longer strikes. But I'm usually careful about making measurements and don't suspect them.

thanks Gary

[AlanBeckett]

That all sounds very reasonable Gary. Perhaps the problem really is using Resistors instead of an Inductance.
Can you add some Inductance instead of the Resistors? I had a quick look for 21W Ballasts and they seem very thin on the ground.
A radical solution would be to replace the Tube with a string of LEDs.

Alan

[Skywave]

To compensate for the reduced wattage of your replacement lamp, you have installed additional resistance. But a quick calculation of the original arrangement (assuming that the given 30-watt is correct), shows that nearly all the volt-drop between the supply and the lamp appears across the series inductance: 1.5 H in total. (Series impedance = 475 Ω @ 84°; reactance = 471 Ω). On that basis, when you fit a 22-watt lamp, I would have thought that increasing the series inductance rather than the resistance would have been appropriate. My calcs. are as follows.

Old wattage ÷ new wattage = 30 ÷ 22 = 1.36.
So, inductance needs increasing by 1.36.
1.36 x 1.5 = 2 H.
That is an increase or 0.5 H.
So an additional choke of 0.5 H is required, with a resistance not exceeding approx. 50 Ω.

In effect, this is all about power factor. However, I am not at all familiar with the behaviour - incl. PF - of fluorescent lamps, so maybe someone can provide a better analysis and resultant calculation or explanation.

HTH,

Al.

[Radio Fixer]

It could be so Alan.

I rather like the LED idea if I can find suitable LEDs to string. Will look.

thanks Gary

[AlanBeckett]

I've got lots of High O/P LEDS but they all come from PCBs so have very short Leads.
Let me know if you want some.

Alan

[Skywave]

To obtain the required degree of illumination using LEDs, (even 'high-power' ones), isn't that approach going to require an awful lot of LEDs? And then there is the question of the wattage rating of a suitable dropping resistor. That rating could be quite large: a step-down transformer may be a better solution. OTOH, a 0.5 H choke is a fairly common item salvaged from a scrapped valve radio . . . .

Just a few thoughts.

Al.

[Radio Fixer]

How about an electronic ballast : http://www.amazon.co.uk/POWERSTAR-electr...?s=kitchen&ie=UTF8&qid=1409050043&sr=1-1&keywords=flourescent+light+ballast

OK it will no doubt stick out some 'hash' from the inverter but turn off when using a radio.

Thanks for the offer of LEDs.

And there are these: http://www.btflighting.co.uk/tridonic_pc_compact.htm

Even one specified for 22W. It is a multi-ballast but expensive at £21.50 + VAT + postage.


Gary

[Radio Fixer]

Thanks Al

I have just spotted your input and that is most likely what is required but mounting another choke in there could be difficult. Assuming I can find one of the right size.

The existing chokes measure 16 Ohms each, so quite low resistance.

The electronic ballast seems the way to go?

Gary

[Skywave]

Perhaps replacing all 3 existing chokes with one that is approx. 2H may be the solution. A typical 2H choke will have a resistance of about 75 Ω and its size will be usually in the range 3" x 2½" x 2½" to 4" x 3" x 3".
With regard to 'size of the base', I had a similar problem some time ago with a lamp. I simply replaced the base box with a larger one. As it was for workshop use, the 'cosmetic factor' was not relevant.

Al.

[Radio Fixer]

Well AL took the plunge and gone electronic as I couldn't find any chokes.

Sent for this one:

   

Sounds OK and instant start and flicker free will be an advantage.

Will post how it works out.

Gary