Is this light circuit correct?

[slderiron]

I'm currently working on a project which has two 240v bulbs in series, however having done the necessary wiring I wanted to get an opinion as to whether this will work before I plug in. Somehow it doesn't look right to me or am I just being over cautious? If I'm wrong on this could someone point me in the right direction, I'd prefer to save on two bulbs and a fuse (and possibly burning the sockets out).

[Richard]

Hi

Those look to be in parallel, not in series.

Richard

[Red to black]

As richard said, these look to be in parallel rather than series.
The will both glow at full brightness connected in parallel, and half brightness when connected in series (provided the Wattage is the same).

Cheers,
Baz

[russell_w_b]

Quarter brightness, surely? Half-brightness at 1/√2 applied voltage per-lamp. (brightness / power characteristic not considered).

[Red to black]

Sorry yes,

Quarter brightness, surely?

As we have halved the voltage and 'roughly' halved the current = 'roughly' 1/4 the wattage,

I posted quickly

Cheers,
Baz

[Richard]

Assuming they are 240v bulbs, and he is not trying to run 2 x 120v bulbs on 240v

Richard

[Red to black]

and he is not trying to run 2 x 120v bulbs on 240v

Very true ! Richard

[slderiron]

Many thanks. The bulbs in question are both 240v. One is a small 15w bulb, the other is to be a neon bulb, I'm unsure of the wattage of this bulb. Is it essential that the wattage of both should be the same when run in parrallel/series?

The other question I have directly relates to neon bulbs; if a neon bulb is run on half its required power ie a 240v bulb run on 120v, does this have a detrimental effect on the neon bulb? I ask this because a few years ago I built a nixie clock and was advised at the time that running the nixie bulbs on less their required working voltage, would reduce their working life.

If the final output of both bulbs powered in series results in only 1/4 brightness, then in any case I'm inclined to stick with the parrallel method as pictured. Thanks for the advice on the pictured example.

[Red to black]

Hi,

I stand to be corrected here, but here goes,
Neon bulbs tend to have a highish value resistor in series (internally, sometimes), due to the low resistance of the ionised gas (when they strike), so I would have expected that if you place these two bulbs in series, the Neon would light normally, and the incandescent one would remain off, as it would not have enough current to light.

In parallel they would both light normally, provided they are, as you say both normally rated at 240V, which is the way you look to have wired them.

Cheers,
Baz

[Richard]

Hi

If they are in parallel and all 240V bulbs that will be fine.

Running the neon in series with a filament bulb will result in the neon lighting and the filament bulb doing nothing, the neon will draw much less current than the filament bulb.

Richard

I see Baz beat me to it LOL

[Herald1360]

I reckon agreed with, rather than corrected

Back to the subject of series and parallel, if both (ordinary filament type) bulbs are 240V, it won't cause any failures whatever wattage they are either in series or in parallel. That's not to say no problems, though....

In parallel, no problem, both will light to whatever their full brightness may be. In series, it will depend on the relative wattages. For one high (say 100W) and one low (say 15W), of the same filament material, the low wattage bulb will be at near enough full brilliance and the high wattage one will only glow dimly if at all. As the bulbs approach each other in wattage (this is a thought experiment, rather than physical one), the brightness of each bulb will converge until they are at equal, somewhat reduced from full, brightness when of equal wattage. The power actually dissipated in each under varying wattage ratios will be problematic to predict, since the filaments have a strongly positive temperature coefficient of resistance (tungsten filaments) or negative (carbon filaments), which means that with equal wattages, each tungsten filament bulb will dissipate more power at low voltage than might be expected and vice versa for carbon filament types.

What a mouthful! It would probably be easier to actually try it with a couple of batten holders and a box of assorted bulbs

[Dave UXB]

Originally Posted by slderiron

Many thanks. The bulbs in question are both 240v. One is a small 15w bulb, the other is to be a neon bulb, I'm unsure of the wattage of this bulb. Is it essential that the wattage of both should be the same when run in parrallel/series?

The other question I have directly relates to neon bulbs; if a neon bulb is run on half its required power ie a 240v bulb run on 120v, does this have a detrimental effect on the neon bulb? I ask this because a few years ago I built a nixie clock and was advised at the time that running the nixie bulbs on less their required working voltage, would reduce their working life.

If the final output of both bulbs powered in series results in only 1/4 brightness, then in any case I'm inclined to stick with the parrallel method as pictured. Thanks for the advice on the pictured example.

Neon lamps are a different kettle of fish. Theoretically, they draw no current, so power rating is nonsense.

If you'd care to say what you are trying to achieve, there may be a better (safer?) way.

[kalee20]

Originally Posted by Dave UXB

[Neon lamps are a different kettle of fish. Theoretically, they draw no current, so power rating is nonsense.

Erm - they do when they are glowing, you know. In theory and in practice. If they didn't, we would in theory have invented perpetual motion.

Consequently, they have a very real power rating.

They do have the complication that they have a very non-linear V/I characteristic (which is re-entrant), so in sinusoidal AC circuits current flow is very discontinuous.

Usual types are very low power and consume only a couple of milliamps. Larger, old-fashoned beehive types may run at 10mA. However, I have run miniature wire-ended types at a quarter of an amp (!), at this sort of curent they are really bright pink, and the voltage drop falls to about 30V from their initial 90V or so. Of course, they don't last long like this.

[broadgage]

Of course neon lamps draw current, though very little in the common sizes.
A neon lamp typicly uses from less than 1ma up to several ma.
In order to limit the current used by a neon lamp a series resistor is required, this may be built in to the base of the lamp, or it may be an external component.

As regards connection of lamps, lamps in parralel can be of any wattage, but must all be of the correct voltage voltage for the supply.
Consider normal domestic lighting, numerous lamps of different wattages are used, but all should be 230/240 volts in the UK.

Lamps in series must all be of exactly the same current rating, they can be of different voltages, but the total voltage must eqaul the supply voltage.
As an example, consider the older type of Christmas lights, these used 12 lamps each of 20 volts, 3 watts all in series.

Pairs of 120 volts lamps are sometimes used in series from UK 240 volt mains, but in general series burning of lamps for general illumination should be avoided. Fine for test or experimental purposes though.

If an incandescent lamp is worked on half voltage, then the light output will be about 14% of that produced on full voltage.

[slderiron]

Thanks, have learned quite a bit on this thread, very illuminating (pardon the pun). I had in mind to light up an old lamp with two of the bulbs mentioned earlier, but have now changed tack as the lamp in question was subject to a clumsy accident a lesson in keeping the workbench clear of obstruction. I have a passing interest in unusal light bulbs, mostly vintage neon bulbs and had an idea of making use of one with a flickering crucifix. Maybe divine intervention caused the demise of the lamp, though more likely the aforementioned clumsiness. I may go ahead with the project at a later date with another table lamp I have, but I at least now know that I need to have the bulbs parralel. Many thanks, john.

[Brigham]

...Pairs of 120 volts lamps are sometimes used in series from UK 240 volt mains, but in general series burning of lamps for general illumination should be avoided...

I can't quite grasp that one. 2x 120 volt bulbs on 240v are each burning at the correct voltage, surely?

[kalee20]

Originally Posted by Brigham

2x 120 volt bulbs on 240v are each burning at the correct voltage, surely?

If they are the same type, yes.

But if they are from different manufacturers there is a good chance they will not be matched due to production tolerances. Suppose you have two 120V bulbs in series, but one actually draws 95W at 120V, and the other 105W. Clearly the 105W bulb will have a lower resistance. So the voltage won't share equally, from a 240V supply, with the 105W bulb actually seeing less than 120V.

This will mean that the 95W bulb will run brighter than normal and the 105W bulb dimmer. After a while, the filament of the 95W bulb (running hotter) will lose more tungsten than the other, exacerbating the mismatch. You'll end up with a runaway situation with the weaker bulb failing at rather less than its claimed life.

[broadgage]

Originally Posted by Brigham

...Pairs of 120 volts lamps are sometimes used in series from UK 240 volt mains, but in general series burning of lamps for general illumination should be avoided...

I can't quite grasp that one. 2x 120 volt bulbs on 240v are each burning at the correct voltage, surely?

In theory it works fine, and sometimes works well in practice.
In practice though the lamps may not be perectly equall, and therefore one will be over run and one underun.
It works ok for small, cheap, and relatively long lasting lamps.
Year ago outdoor scenic or seasonal illuminations often used 120 volt, 25 watt, coloured GLS lamps in series pairs. This worked well and actually enhanced reliability compared to 240 volt lamps. A lamp failure extinguished only 2 lamps. With paraleled 240 volt lamps, a lamp failure might blow the fuse due to arcing within the lamp and therefore extinguish a whole circuit.

In the days of trams worked from a nominal 550 volt DC supply, it was usuall to provide internal lighting from 5 lamps each of 110 volts, connected in series, it was soon found that regular GLS lamps did not perform well as they varied a bit in actual wattage.
Lamp manufactuerers therefore produced special "traction lamps" these were more accuratly manufactured as regards running current, and therefore suited for series burning.

In the modern era, certain theatre lighting effects are produced using a bank of "aircraft landing lights" or ACLs.
These are 28 volt sealed beam lamps, 8 being used in series from the mains.
Frequent complaints are made re the short lamp life.
8 times 28 is 224 volts, from our mains which is still about 240 in practice.
The lamps used in practice will often be from different suppliers, and of different ages, and therefore of slightly different currents.
Not suprising that they dont last long !