Halogen Lights in Electric Fire Blowing

Standard uk socket has ground right, 3 wires? Therefore no problem for surge. if your dad connects the 2 bulbs in series, it is the problem otherwise just improve heat transfer surrounding of the bulbs. Temperature increases resistance and resistance increases circuit current, thus you would yield that result definitely.

"Standard uk socket has ground right, 3 wires? Therefore no problem for surge"-A ground connection has NOTHING to do with the absence or presence of surges.

"Temperature increases resistance"-TRUE!

"resistance increases circuit current"-FALSE! The opposite is true (increasing resistance decreases current).

If one bulb burns out, its resistance rapidly rises to nearly infinity, and the current drops to nearly zero, regardless of the condition of the other bulb in series.

ALL NOW TRUE - well put!!

Thanks for the replies -

To clarify my original post, the reason for thinking it might be a power surge is that both bulbs go at exactly the same time.

They don't seem to go while the fire is on. They last for months, but then one day when the fire is switched on, both bulbs will have gone and need replaced. Which would imply they are being blown together, either when the fire is switched off, or when it is switched on.

Am I understanding rightly that if the bulbs were wired in series, then the one bulb going might cause the other to go at the same time, since it was now (as it were) taking the load that had previously been spread over the two?

No. If the bulbs are wired in series, when one bulb blows, the circuit is broken and the other buld will no longer light.

You are probably replacing two bulbs when only one is blown.

Absolutely correct, if these lamps are wired in series. Unfortunately the OP is already convinced that a power surge is causing these failures. They're so convinced of their solution, a real circuit inspection and design will likely not be brought to us.

What most filament novices don't realize is that the resistance of a filament significantly changes with temperature. As the link shows, the change is enough to be a useful college level lab experiment. When you add in the chemistry of the halogen cycle of filament redeposit in a halogen lamp the process becomes even more confusing.

"Absolutely correct, if these lamps are wired in series. Unfortunately the OP is already convinced that a power surge is causing these failures. They're so convinced of their solution, a real circuit inspection and design will likely not be brought to us."

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No, my own feeling was that it seemed unlikely to be a surge, given that the fire would probably be drawing far more juice for the heating elements than for the lightbulbs in any case. But not having any great knowledge of electrics I didn't have a convincing explanation to offer my dad of what else could cause them to go simultaneously.

When you guys were mentioning they might be wired in series, you were probably assuming the idea that while both bulbs were going out, only one bulb was actually blowing - but it took WJMFIRE stating it outright for me to get it!

Anyhow, now that I see it, I'm pretty confident that's what's happening - I'll take both the old bulbs out & put in a single new one, which should tell us series or parallel, yeah?

And if they're not both blowing, the only question is why one is going every 9 months or so - & that could be accounted for by underpowering causing deposition of the vapourised filament on the casing etc...

Heh - I do get there in the end!

"given that the fire would probably be drawing far more juice for the heating elements than for the lightbulbs".

Your original post did not mention heating elements. I, for one, assumed that this was a simulated fire, that provided only a visual effect, but no significant heat.

If there are heating elements in addition to the lamps, there may be a significant interaction between the heating elements, the thermostat, and the lamps.

I suspect that post #4 is correct. If not, then we may need a circuit diagram to troubleshoot further.

I might add here that you should be careful not to touch the bulbs with your hands or fingers, instead use gloves to handle these bulbs...fwiw

Everything what was said:

Halogen bulbs operating at a lower voltage have shorter life.

Do not touch envelope of the lamp with your fingers. The oils cause hot spots. Lamps I worked with were at 15 amospheres pressure or so,

Vibration is also a killer.

The filament resistance is 10-12x lower at startup.

Bulbs tend to blow on startup.

Correct, incandescent bulbs almost always blow on start up. Filaments tend to have one spot with higher resistance which means a higher temperature which means faster filament erosion/metal loss. When the metal gets thin enough (resistance gets high enough) the current surge at turn-on is high enough to cause the thin spot to melt in two and the bulb blows. Most halogen bulbs have a very short useful life because they run at such a high heat-load. That's why most projectors have a fan that continues to run for a set period (or temperature-controlled shutoff) to prolong bulb life.

You are not thinking straight. Let me explain, I assume you understand Ohm's Law?

If there is a thin spot, with a higher resistance, you can like it to say 3 resistors in series. The first one (good filament part) say is 10 Ohms, the second is 20 Ohms (bad filament part, thin and high resistance as you said), the third one (good filament part) is again 10 Ohms.

A normal bulb will have a (simplified) resistance cold of 30 Ohms.

The one with the "thin bit" will have a resistance of 40 Ohms.

Now you tell me which one is running the higher current? Use Ohm's Law!!

This is just to show you that your thinking is flawed and your method is mostly wrong!!

Also your statement of:-

That's why most projectors have a fan that continues to run for a set period (or temperature-controlled shutoff) to prolong bulb life.

Is also completely wrong!!

The fan is there mainly just to cool the bulb TO STOP THE RESIDUAL HEAT FROM IT DAMAGING OTHER EXPENSIVE PARTS OF THE OPTICS OF THE PROJECTOR!!!!!

The bulb can handle the heat just fine!!!! Its designed to do just that.....any life extension by cooling is more by accident than design.....the optics, often including a tiny, high quality LCD screen, are the parts that do not like heat....

I think you have not understood Graycav's post, because he did not give sufficient detail.

What he says is true: filaments do usually burn out at turn-on, and over time a filament is going to vaporize more from one or more regions, making that or those regions thinner and weaker.

Two points that Graycav did NOT make:

1. The resistance of a cold filament is roughly an order of magnitude lower than the resistance of the same filament when hot, so the initial current is roughly an order of magnitude higher than the hot current.

2. Most filaments, and all of the filaments I've ever seen in halogen bulbs, are wound in the form of a coil. Whenever a current flows through a coil of wire, the coil becomes a magnet, and every loop of the coil attracts its neighbors.

Thus the magnetic field of a filament tends to shorten the coil, and once the filament has weakened sufficiently by thinning due to vaporization, that large current at turn-on makes the stronger portions of the filament contract and break the thinnest point. Whether the ends at the break melt probably depends mostly on the voltage for AC lamps. At 12 VAC, there won't be much arcing, so not much melting.

What difference does your post make to the whole story?

Using the minimal amount of infos supplied I did my best, you have not added anything useful that I can see.

I have had 12 volt Halogens short out many times both at home and in the office. Mine were cheap DIY Market ones, I cannot talk about the office ones.....

The fault usually dropped a breaker......made worse because Germany has a primitive system of electrical supply, basically what the UK had before 1946....and what many other countries also have....

Maybe the "inrush" current on 12 volt Halogen transformers at switch on also plays a role when a bulb is already weakened.........but they can short, or better said, they can cause a breaker to drop.

By the way, all the Halogen bulbs I have ever looked at closely (still working at that point!), do NOT use a "coiled coil" as you appear to imply.

They use a simple coil. I have not looked at every design though.

I did find the following at the end of a very interesting article which seems to support most of what I said:-

Because tungsten-halogen lamp fabrication technology is so well advanced at this point, the life of a typical lamp ends suddenly, usually upon powering up a cold lamp filament. During the course of an average life span, advanced tungsten-halogen lamps do not blacken and undergo only minor changes in photometric output characteristics. Similar to other incandescent lamps, tungsten-halogen lamp lifetimes are determined by the vaporization rate of tungsten from the filament. If the filament does not have a constant temperature along the entire length of the wire, but instead has regions of much higher temperature produced by uneven thickness or internal structural variations, then the filament will usually fail due to premature breakage in these regions. Even though vaporized tungsten is returned to the filament by the halogen regenerative cycle (discussed above), the material is unfortunately deposited on cooler regions of the filament and not those critical hot spots where thinning usually occurs. As a result, it is virtually impossible to predict when any particular filament will fail in lamps that are operated continuously. In those lamps that are switched on and off frequently, it is safe to assume that they will fail at some point when being switched on.

The article can be found here:-

http://zeiss-campus.magnet.fsu.edu/articles/lightsources/tungstenhalogen.html

I trust that you find this of interest!!

Also, I found this link interesting, that even the Leviton Dimmer tech support do not understand when using the Sylvania 50W PAR20 Halogen bulbs with their dimmer (probably a bad idea anyway...).

WHEN ONE BURNS OUT, IT TAKES THE DIMMER WITH IT!!!!

This can ONLY be a short circuit to my mind.....let me know what you think!! If you read down, there is a comment that may attract your interest too:-

....everybody else is right; it's a result of the usual failure mode of halogens to be a brief s/c before going open.....

See here for the whole link:-

This-shouldn't-be-happening

Now do you believe me?

I am not sure of the used method to simulate the flicker of the flames in your case.

If it is by means of a varying voltage to the bulbs by a control circuit,the problem may be the control circuit itself.

Halogen bulbs must operate at the designed voltage to develop the required temperature of the envelope to facilitate the required chemical reactions between the Halogen and the tungsten, and to ensure that the Tungsten redeposits on the filament instead of the bulb's inner surface.

There are applications where the halogen bulb flashes constantly, such as the roof top light bar of a police car,but the bulbs are switched at a very fast rate(typically 180 Hz) to prevent the bulb from getting too cool between cycles.

Look at the bad bulbs.

Is there a lot of dark color on the inside of the bulbs?

If so, this is a sign that they are running too cool.

See if the fan is directing air onto the bulbs.

If so, deflect it from them.

The bulbs must be hot enough to allow the halogen to redeposit on the electrode,rather than the shell.

It is normal for any incandescent bulb to blow on start up because the cold element has very low resistance.

The resistance increases as the element heats up,and current decreases.

Any fingerprints on the bulb should be removed using alcohol and a lint free cloth.

The oils in skin can become very acidic when heated,which will discolor and or weaken the quartz,sometimes even causing an explosion.

If the flicker is created by a rotating wheel, I suggest switching to an LED array that will give similar results with a greatly reduced power demand, and extremely long life.

Good luck.

Thanks guys - I've now warned him to keep the bulb casing free of hand-oils.

Tried removing one of the two bulbs, but contrary to expectation, the other one stays lit - which suggests they're not in series with each other. Either way, it now seems they are actually blowing simultaneously, probably when the fire is switched on.

The bulbs are 28W 240V (nominal UK mains voltage is 230 these days). The bulbs he is buying are rated at 2000 hours, which is less than I had imagined, and I suspect he may be getting about 1500 hours out of them. Still, the fact they're blowing at the same time seems less than optimal.

There is indeed an internal fan, which blows a series of little strips upward - the light from the lamps thru these produces the flames effect. But air from the fan is also reaching the bulbs, which may be causing them to run too cool...

I have found a flame LED kit that can be had cheap.....replace the halogen bulbs and be done with it....until something else breaks that is....lol

http://www.easternvoltageresearch.com/digitalflame10.html