Slightly OT. Heat and a Bench Light ...

In message , Arfa Daily writes

I haven't been following the thread, so apologies if this explanation has been offered earlier.

I'm guessing, but I reckon that smaller sized bulb may be hotter because: (a) The filament may be physically smaller (squeezed in more, and coiled up tighter). Some parts of the filament may be obstructing the path of the light from other parts. Similarly, the path for radiated heat may be being obstructed. As a result, the filament may be less efficient (less light output, but more heat). (a) The glass bulb is closer to the hot filament. (b) The glass bulb has quite a lot less surface area available to dissipate the heat.

What do you think?

Sorry. I misread your comments. I'm not sure where I got the 75 to

Despite my screwup, the rest is still applicable. Did you read:

If the smaller bulbs are halogen filled, that would explain the increased temperature.

Also, note that the easiest way to increase the temperature of a light bulb surface is to simply decrease the surface area of the bulb. It's exactly like a heat sink. If you have a heat source that beleches some number of watts, a large surface area will be cooler than a small surface area. Since the article indicates that halogen bulbs REQUIRE a much hotter bulb temperature, a smaller bulb diameter, with a corresponding smaller surface area, will be the way to accomplish that. Methinks it all fits.

Lost in this conversation is the fact that you claim that both the old and new bulbs have been 60 Watts. Now, if that's true, then there's no more power available from the new 60 W lamps than there was from the old 60 W lamps.

Halogen lamps, while they DO have much hotter envelopes, convert electrical power into visible lighte somewhat more efficiently, so that should make the shade run slightly cooler, if anything. I would expect this effect to be small.

Certainly a smaller 60 W bulb will run at a higher glass envelope temperature, but that envelope will be farther from the shade, assuming that they both got mounted on the same centers. The end result is that there will be no difference in the radiant heating of the shade.

The confusion here seems to be the common one between temperature and heat. Cram the same amount of heat into a smaller amount of material and you'll get a higher temperature, but in this case, the shade has remained the same, so the amount of heat energy collected should be the same, and the resulting temperature should also be the same.

It may help to think of heat and temperature as having electrical analogs in charge and voltage.

Really, there are only 2 possible conclusions: Either the new bulb is actually higher wattage than the old one, or the perception of a hotter shade is mistaken.

Separate from this is heat conduction thru the base of the bulb. If the bulb is shorter and hotter, then it will likely conduct more heat into the socket. Halogen sockets are generally ceramic or some rather special high temp plastic in order to deal with this, so putting a halogen bulb in a standard socket will always result in a destroyed socket.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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OK. The bulb does not claim to be a halogen type. It was sold as a 'bog standard' light bulb. It looks like a standard light bulb. It is stamped 60 watts on the packet, and on the bulb itself. The glass envelope and bulb in general, is identical in every way to what any of us would recognise as a 'standard' incandescent light bulb given, of course, the obvious difference between a UK bi-pad bayonet cap, and a U.S. edison screw cap. However, it has one major difference in that instead of the glass envelope being the size of a tennis ball, it's more like the size of a pool ball. When installed in my bench light, which is the only 'closed in' place that I've used one so far, I did not notice any change in light output from any other

So, if we believe the rating stamped on both the bulb itself, and its box, and you are prepared to believe me when I tell you that with this bulb fitted, the temperature of the shade was a whole heap hotter, then somewhere, there must be another explanation than the two that you believe are the only possibilities. There must be a greater degree of heat being conducted into the base cap, in order for the temperature to have been raised to the point where the insulation material within the lamp itself's base, to have started to fry itself and to have destroyed the connection pad, which is where this thread started from. There must be considerably more heat steaming off the bulb itself, to have raised the temperature in the upper part of the shade, to the point where the nylon insulation around the choc bloc which was located there, has fried. That piece of choc bloc had been there for a couple of years, and trust me, before fitting this bulb, it was not even discoloured, let alone crisped.

The physical contact area between the brass lamp holder, and the bracket to which it is attached, is small, so it would seem unlikely that heat conduction is playing much of a part in raising the temperature of the shade. So that would leave only radiation as the mechanism for raising the shade's temperature. I'm pretty sure that it must be a combination of the area of the glass envelope being - what, I don't know, 30% smaller maybe? - making for a less efficient radiator, and exacerbation of this by that glass being nearer to the filament.

It still seems to me that this has potentially far-reaching consequences under the right (wrong?) circumstances. The bulb was a B&Q own brand BTW. I don't have any more in stock at the moment, but I will try to get to the store and pick some more up, and do some further tests and measurements.

Arfa

Yes, all valid thoughts. See my further discussions of it all in my reply below to Jim

Arfa

I believe everything you've told us about the bulbs. I have no reason not to, but 60 Watts is still just 60 Watts.

There's no way to get more heat out of it than that. So either the old

One other possibility occurred to me, and that is that your socket chose this moment to become resistive, which could have raised the total power above 60 Watts and concentrated that extra power dissipation in the socket - consistent with your observations. I'm not familiar with the socket you describe, so maybe you can judge the likelyhood of this. I suspect that if this had happened, the light output would have been affected and you would have noticed.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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I've been following this thread rather loosely for the past several days, so please pardon me if I repeat previously addressed thoughts.

Arfa, I understand that you've stated that you're using lamps labeled as 60-watt in your fixture. Have you actually measured the power consumed (for instance, using a Kill-a-Watt or similar meter) by the lamps? It's a possibility that the lamps have been mislabeled, and you actually have 75- or 90-watt lamps instead of 60-watt lamps.

Since an operating incandescent lamp is essentially a resistive element, the power consumed should be RMS power, which is the heating value of the power. Sixty watts RMS into a resistive element should produce the same amount of heat, whether it's in a large glass bulb or a small glass bulb. The temperature of the bulb might be higher in a small lamp because it has a smaller radiating surface. Power that is concentrated in a small area will have a higher temperature than the same power that has been spread over a larger area.

Is the radiating surface area of the new lamps small enough to cause the increase in temperature that you're experiencing?? I suppose the math could be done to derive the rise in temperature of your new lamps... would be interesting to see the results.

My ending thought on the issue is that, IMHO, the telling story would be in the reading of a power meter in the circuit. If you don't have a power meter such a the Kill-a-Watt, then you could measure it directly with a voltmeter and ammeter. V * I = P no need to measure phase since it's essentially a resistor.

Does it have one of these labels?

If so, what are the numbers? We don't have anything similar in the US so it would helpful if you could supply the numbers for an "ordinary" light 60w bulb. Note that halogen are in class "D" while ordinary bulbs are "E" or "F".

Could I trouble you to measure the approximate diameters? I want to calculate the relative surface areas.

I'd not appreciated that added effect with Ro80 "Apollo lander" shape bulbs. The filament is perhaps twice the distance from the socket as the same wattage but globular lamp. As well as reflecting forwards most of the rearward-going heat and light.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Presumably though, temperature will rise, both at the glass surface and the surrounding air, if that heat is not being radiated away as quickly as with the older design ?

Plus I did say earlier that the socket was replaced fairly recently, and is working just fine again now without one of those bulbs in it.

Here is a picture of the actual item so you will now know for ever, what a UK bayonet cap looks like !

and here's one on a CFL. which shows the connection pads very clearly.

The B&Q bulb got hot enough to fry that black resin stuff, and completely destroy one of the connector pads which are made from (lead-free now, I guess) solder.

Arfa

Hi Dave I haven't measured the actual power consumption of the new bulbs. A few years ago, I would have been surprised if the figure quoted on a lamp was not at least within say 5% accuracy, but these days, with the reduction in integrity of just about everything, and the fact that much manufacturing is now done in 'emerging' industrial countries, I would be less sure of that. As to whether the new bulb's envelope is small enough to reduce the dissipation of the heat by the amount noted, I'm not really a good enough theoretical physicist to make a call of any real value, but given that this appears to be the only mechanism by which this could be happening, I would have to say that is probably the answer.

I don't dispute what has been said about 60 watts being 60 watts, or that heat and temperature are not the same thing. However, radiation efficiency is key to the relationship between the two, as we both know. A 60 watt amp with a small silver heatsink on its output transistors will not get rid of the unwanted heat anything like as well as if that heatsink is black, and force air cooled.

For all I know, my bench lamp may be a 'special case', and the fitting of the smaller bulb might just screw up the dynamics of the air circulation within the shade. As anyone who knows the Terry's Anglepoise will agree, the shade is not particularly well ventilated. Perhaps it would benefit from having a ring of 5mm holes drilled around its top ... ??

Arfa

The things I do for science !! I just had to rootle thru' the workshop bin to locate the bulb which caused all the trouble in the first place. It is clearly marked 60 watts, and has a CE approval also. I can't tell you what the energy rating letter is, as I don't have any more on the shelf. I have two other types from the same stable, and one of the boxes has an energy rating on it, and one doesn't, so I'll have to look next time I am in the store.

OK. On the diameters. Consider them for all practical purposes, to be a sphere. The old was 60mm almost exactly, and the new, 50mm almost exactly, measured with an electronic calliper, so reasonably accurate figures.

Arfa

The R80 sounds the way to go from all angles. I'll pick one up next time I'm in B&Q

Arfa

Ah, you see, what you really need is something that'll automatically sense when the lamp is on, and turn on a cooling fan. I have just the circuit... %^)

Cease complaining. It's your question that I'm trying to answer. Never mind that I don't know anything about light bulbs.

Methinks there will be a difference in efficiency rating. If true, that implies that the newer smaller bulb is really a Halogen filled bulb, even though it's not marked as such.

Surface area of a sphere is 4Pi*r^2 Old = 4 * 3.14 * 3.0^2 = 113.0 sq cm New = 4 * 3.14 * 2.5^2 = 78.5 sq cm The new bulb has about 70% of the surface area as the old bulb. However, I gotta dust off the college thermodyamics texts before I can figure out the expected temperature rise given identical power dissipations. I also gotta lookup the IR transmissivity of glass. The things I do for science...

Quit complainin' !! I'm helping you figure all about light bulbs with my question .... :-)

Arfa

The other unknown factor is the lamp efficiency. So how much is going out as light, and how much as heat. A small change in this ratio will certainly have an effect on the heating effect even with identical sized globes.

Peter

I'd be inclined to think a bigger difference could be the relative heat dissipation via conduction through the base vs. convection through the glass envelope. If the newer bulb has significantly heftier and/or shorter leads for the filament, the metal base might be getting significantly hotter and conducting more heat to the socket.

Incandescent bulbs are inefficient enough, regardless of construction, that any moderate efficiency gains would have no readily observable impact on heat output--or, put another way, any improvements that would reduce the heat output by a nontrivial amount would also produce a bulb that is obviously much brighter. According to Wikipedia, something like

Peter, Andrew. Thanks for your input. Both good thoughts.

Arfa