So, an electronic ballast has a rating, as well as, in some cases a lamp type. OK, thats simple. But then, there are so many lamp types and wattages, that keeping a stock of ballasts in the van for each type of lamp would be costly. Testing a few yesterday, it seems they cannot be reliably inter-changed between types. Why wouldn't a 36w linear ballast work with a 28w 2D lamp, and vice versa? OK, rating at 8w over rating may be asking too much, but why not run a
28w off a 36w ballast?
And a 2 x 36W works with one lamp lit, the other not working, but wouldnt work with a single, good, lamp fitted?
A fluorescent tube requires the correct power, or close to it, the right V = & i which depend on tube width, length and design features, and also the co= rrect heater power, v and i. Hence ballasts that will by design run more th= an one tube type are a minority.
A 36w ballast on a 28w lamp would overrun the lamp, reducing life expectanc= y.
Its certainly possible to do some ballast/tube type swapping. Iron ballasts= are easily adapted to run lower power tubes by adding the right capacitor,= but the calculation of its value is nontrivial and correct calculation is = essential. CFL ballasts often run small fl tubes happily enough. In princip= le modern electronic ballasts can run lower power tubes with 2 capacitors o= n the output, but its too far from trivial to be practical.
Perhaps someone should make a ballast with multiple settings.
which depend on tube width, length and design features, and also the correct heater power, v and i. Hence ballasts that will by design run more than one tube type are a minority.
ITYM it might. It depends on the tube. eg. you can get different length 28W T5 tubes for aquaria, so they require different ballasts to give 28W. The effective resistance of the tube will vary according to length, cross sectional area and the gas mix.
are easily adapted to run lower power tubes by adding the right capacitor, but the calculation of its value is nontrivial and correct calculation is essential. CFL ballasts often run small fl tubes happily enough. In principle modern electronic ballasts can run lower power tubes with 2 capacitors on the output, but its too far from trivial to be practical.
Its probably possible to design a ballast that has voltage and current sensors to measure the actual power delivered and adjust accordingly. It wouldn't sell unless it only cost an extra few pence.
A filament lamp is designed to run at a partcular power rating when connected to specific voltage, and this is governed by its operating resistance. To run it at the designed power rating, you feed it a constant voltage, and it will decide what current it's going to draw.
A fluorescent lamp is different in two respects. Firstly, they have to be run at a constant current in order to run at the designed power rating. Secondly, the resistance of a fluorescent tube decreases dramatically as the current increases, and in effect, the tube decides what voltage it will operate at. Given that our mains supply (and battery supplies) are effectively constant voltage supplies, they cannot be connected directly across a fluorscent tube, because as the current increased, the tube resistance would drop, and the current would increase, and the tube resistance would drop further, until something goes bang. Consequently, a tube requires something to limit the current.
Originally, each ballast was designed to run just a single tube, or possibly a small number of tubes with similar operating characteristics. This required a large number of different ballasts to be manufactured and stocked, which was not a good thing.
There are families of tubes which all operate at the same current and the same loading (watts per unit length) and vary in power rating proportionally to the tube length (and hence operating voltage). If you build a ballast which generates a truely constant current, you can run any of these because the tube will decide on its operating voltage which will give the correct rated power output, and that's what's done today. Thus you can get a ballast which will run all tubes in one family because they operate at the same current.
As ballasts have gone digital (controlled by micro controllers), they have been able to get even more sophisticated by looking at the characteristics of the lamp, and working out which family its from, and can thus drive different lamps from different families, adjusting the current appropriately. As an example, I just grabbed one off the shelf and it can drive: All the T5HE family lamps (14W, 21W, 28W, 35W) The longest T5H0 family lamps (49W, 80W) DL55W, DL80W (folded 55W and 80W tubes) FC55W (Circular 55W T5 tube)
My guess is that it initially drives the tube at the tube current of
170mA for T5HE, and checks the operating voltage. If it's not what's expected of the T5HE lamps, it will then increase the current to that of the T5HO lamps (don't know what that is off-hand), and possibly some more checks for the other 3 if they have different operating currents.
This probably replaced 9 separate ballasts in the catalogues.
However, back to your 28W and 36W examples, they may have quite different operating characteristics in terms of tube current and voltage. Even if they did both work, the lamp and/or ballast may be operating well outside it's designed rating.
Microcontroller ballasts check for open circuit filaments before trying to start the tubes, so they don't start one with worn out filaments and end up running it in cold cathode mode, which will overheat the tube ends.
The ability to operate less than the full compliment of tubes varies by ballast - most commonly they operate them all in series, so this usually won't work. Some 3 and 4 lamp ballasts have two series chains.
And there do seem to be what I'd call cold cathode starters that appear not to use the heaters at all, and just hike up the voltage till it breaks down and strikes, then reduce it to the nominal level again. I have even got one of those to run two small 8 watt tubes with the heaters of on ent connected together so it has to strike in both to operate. Good fun, but now I can't see its all a bit academic, and probably dangerous to do such things. Note I'm not advocating this mode, just saying it can be done. Brian
So if I have a tube that, when power is supplied, flashes a bit with a glow at both ends and then eventually comes on, is that the tube going, the ballast, or the starter?
snip. Cheers, confirming my experimentation that swapping around the cheap ones supplied with light fittings is a waste of time, and I need to buy the correct ones. Ta Alan.
If you're into odd ways to light a tube, try rubbing it with pads that prod= uce static electricity. Or for something more sensible, drive it with high = frequency via bits of foil on the glass, the capacitance acts as the curren= t limiting element. This can make most dead tubes work fine.
Today I used a filament lamp for testing of my multimeter and became a bit desperate when I found that its resistance did not agree with Ohms Law. Its reading was not only wrong, it was wrong by 800 %.
What do you do when a physical law like Ohms Law fails? I tried some Google searches and calmed down when I found this:
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A filament lamp does not obey Ohm's Law because the ratio of voltage
Ohms law is in any case more a rule of thumb for metallic conductors.
It never applied to gas discharge tubes or to semiconductors.
Or rather you can say that there are things whose resistance depends on current and on applied voltage. And in the case of valves and semiconductors, what's happening elsewhere in the device.
That's the trouble with these 'know a very little knowitalls'.
They stumble on something a bit more than basic and think they have won the Nobel Prize. They always believe in global warming, too.
On 30.12.2012 13:34, The Natural Philosopher wrote: ...
I believe in global warming because I was a very clever student. When I graduated from my university my qualifications in physics could not be measured. I broke the scale an got 1.0.
I told you about the misery of Ivar Giæver in an earlier message. It is repeated below because more research is needed to explain his failure.
More research is also needed to explain the failure of The Natural Philosopher.
Now back to Ohms law:
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Ohm's law has sometimes been stated as, "for a conductor in a given
Because of the excellent work of Ohm, Maxwell, Heaviside, Croll, Arrhenius, Milankovitch and others, our world become very predictable for those that have big brains.
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