Resistence of linear halogen bulb when cold

[JS]

I am working out the wattage of some of my spare 118mm linear halogen
bulbs. I have access to only a regular meter.

So I have measured the resistence of the bulb when cold.

(Q1) Is the resistence likely to change significantly from my cold
reading compared to when the bulb is at operating temperature? Is
there a rule-of-thumb multiplier for such bulbs.

(Q2) Is the filament material used likely to vary from manufacturer
to manufacturer in a way that noticeably affects the relationship
between the cold resistence and hot resistence of a bulb? (If you see
what I mean.)

FWIW I am in the UK with 230 volt mains and my cold resistence
readings are 12.1 ohms, 13.5 ohms, and 8.1 ohms. Presumably the
first two are 300W bulbs and the third is 500W.

 

[Victor Roberts]

I am working out the wattage of some of my spare 118mm linear halogen
bulbs. I have access to only a regular meter.

So I have measured the resistence of the bulb when cold.

(Q1) Is the resistence likely to change significantly from my cold
reading compared to when the bulb is at operating temperature? Is
there a rule-of-thumb multiplier for such bulbs.

Yes, there is a huge change in resistance from cold to hot.
The ratio is 16.44:1 between 20 C and 2727 C, but since it
is a strong function of the filament operating temperature,
any rule-of-thumb that does not specify the operating
temperature will have an error. If my math is correct, a
300-watt 230-volt lamp would have a hot resistance of 176
ohms. If it operated at 2727 C, then the cold resistance
should be about 10.73 ohms.

(Q2) Is the filament material used likely to vary from manufacturer
to manufacturer in a way that noticeably affects the relationship
between the cold resistence and hot resistence of a bulb? (If you see
what I mean.)

Most filaments are made from essentially pure tungsten, with
some added materials to make the wire easier to draw and to
strengthen the final filament. The change in resistance is
therefore dominated by the properties of tungsten. The
largest error will be knowing the operating temperature. In
fact, the change in resistance of tungsten is so large and
so predictable that the ratio of hot to cold resistance is
used to determine the operating temperature in many lamps.

FWIW I am in the UK with 230 volt mains and my cold resistence
readings are 12.1 ohms, 13.5 ohms, and 8.1 ohms. Presumably the
first two are 300W bulbs and the third is 500W.

--
Vic Roberts
http://www.RobertsResearchInc.com
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[David Lee]

JS wrote...

FWIW I am in the UK with 230 volt mains

No you aren't! That's a common misconcepetion but UK mains voltage is 240V
and will not be changed in the foreseeable future. 230V is the NOMINAL
European standard but the standard includes tolerances that encompass the
national standards of all the member countries. The point of the standard
is that a European appliance should be safely usable anywhere in the EU and
not that it will necessarily work properly. You can buy 230V lamps but if
used on UK 240V they will burn a bit brighter and have a lifetime degraded
to only 55% of their design life. Hence you should make sure that you are
being supplied with the correct product for Great Britain. Buying from a
reputable supplier is not necessarily a guarantee of this - not so long ago
I was supplied a batch of 230V halogen theatre lamps by one of the major UK
theatrical suppliers.

David

 

[Ben]

JS wrote...

No you aren't! That's a common misconcepetion but UK mains voltage is
240V and will not be changed in the foreseeable future. 230V is the
NOMINAL European standard but the standard includes tolerances that
encompass the national standards of all the member countries. The point
of the standard is that a European appliance should be safely usable
anywhere in the EU and not that it will necessarily work properly. You
can buy 230V lamps but if used on UK 240V they will burn a bit brighter
and have a lifetime degraded to only 55% of their design life. Hence you
should make sure that you are being supplied with the correct product for
Great Britain. Buying from a reputable supplier is not necessarily a
guarantee of this - not so long ago I was supplied a batch of 230V halogen
theatre lamps by one of the major UK theatrical suppliers.

David

Is that reversible ? Would it make sense to order, let's say, beamer lamps
in the UK, for 240 V, and have a lifetime of 1.5 times the "European" lamps
when used on 230 V AC ?

Thanks for your reply,
Ben

http://home.planet.nl/~benzandstra and www.pe2bz.nl

 

[Bremecker]

Is that reversible ? Would it make sense to order, let's say, beamer lamps
in the UK, for 240 V, and have a lifetime of 1.5 times the "European" lamps
when used on 230 V AC ?

Thanks for your reply,
Ben

http://home.planet.nl/~benzandstra and www.pe2bz.nl

Yes it is. But you will loose a lot of the luminous flux you need. There
is a nice diagram that shows the influence of the voltage to luminous
flux, power current and lifetime. You have to got too:

http://www.osram.com/service_corner/glossary/index.html

In the glossary list you have to scroll down to "incandescent lamp" In
the pop up window that opens have a look at the second diagram
"Operating characteristics of incandescent lamps".

The problem with dimming of incandescent lamps is, that you reduce power
just a little but you loose a lot of light output, that means the light
efficiency that is already very low decreases much more than the power
reduction.

Regards

Willi