"zed wit"
** Why do so many fools suggest bulbs in place of resistors ??Incandescent bulbs use tungsten wire that has a strong positive tempco - so much so that the effective resistance value changes over a ratio of about 12 times from room temp to full brightness.
OTOH, high power resistors use wire that has almost no tempco and the same goes for heating appliances ( jugs, toasters room heaters etc).
... Phil
Phil Allison schrieb:
Hello,
the fools are not aware of the dramatic change of resistance between a cold and hot bulb...
Bye
"Uwe Hercksen"
** They should measure the resistance of a cold bulb some time....... Phil
Maybe the fools just understand what they need and how a light bulb might be useful in that application.
Sometimes, easy and good enough is just good enough. And the fools understand that.
When I was I kid, my dad go me introduced (indoctrinated) into electronics. He gave me some switches and 12V bulbs to play with. He then added a large 12V bulb in series with the battery, so a short in the succeeding circuit would be quite indicative. Bulbs can sometimes be put to good use :-)
Cheers
Klaus
Phil Allison schrieb:
Hello,
I did that some years ago and compared it with the resistance computed from voltage and power of the bulb.
Bye
"Klaus Kragelund" Phil Allison schrieb:
When I was I kid, my dad go me introduced (indoctrinated) into electronics. He gave me some switches and 12V bulbs to play with. He then added a large
12V bulb in series with the battery, so a short in the succeeding circuit would be quite indicative. Bulbs can sometimes be put to good use :-) ** 12V auto bulbs are often found in model train speed controllers as current limiters and overload indicators.Another use is with tweeters in high power speakers - one or two 12V bulbs in series protects them from being over powered and damaged.
In both cases it's the fact that bulbs are NOT like resistors that makes them useful.
... Phil
mike schrieb:
Hello,
but do the fools know if the source used is compatible to the high current flowing through a cold bulb?
Bye
I only used gold plated, cryo treated OFHC copper filaments in my tweeter protecting lamps.
I prefer color-coordinated powder-coated filaments myself >:-} ...Jim Thompson
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| James E.Thompson | mens |
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I love to cook with wine. Sometimes I even put it in the food.
Maybe the source is used to power a light bulb?
several metres of nichrome wire, say, a couple of toasters in series
-- ?? 100% natural --- news://freenews.netfront.net/ - complaints: news@netfront.net ---
Interesting.. thanks Phil. So do toasters and such use nichrome wire?
I was thinking (erroneously) that since a toaster gets 'red hot', that the resistance of the element would also change a bunch, like tungsten.
I'm going to have to measure a piece... I've got a drawer at home filled with old heating elements.
George H.
** Generally - yes. ** Draw a heap of amps at switch on and blow the fuse - wouldn't it?
Lamps get hot in fraction of a second but heaters do nothing of the sort.
Plus tungsten lamp filaments burn fiercely in air.
** Submersible heating elements may have a small positive tempco - as may soldering irons.... Phil.
-- Yes, though toasters use nichrome "ribbon".
"John Fields"
** Arrant nonsense.See table at bottom of this link:
The actual tempco is slightly non linear and WAAAY less than tungsten or copper.
Plus 6% at 2000C.
Tungsten is nearly plus 1200 % at the same temp.
... Phil
Hmm, the numbers for copper and silver look a bit high. I always figure that the temp coef for pure metals should be something like 1/temperature (in degrees K) so something like 0.0033 at 300 K. The numbers on the wiki page look 'better' to me.
George H.
(well F not C... )
But that's pretty neat, it's basically flat from 500 C to 1000 C. (more fun will be to measure some.)
Beware, Nichrome lengthens when hot. Years ago I made a Plexiglas bender, Nichrome wire in a Bakelite slot so I could get a nice heated line. Had to use spring-loading to keep the Nichrome from drooping. ...Jim Thompson
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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
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I love to cook with wine. Sometimes I even put it in the food.
--- Note that the resistance you refer to is at about 1100C, while a tungsten filament runs at about 3000C:
With that in mind we can say:
Rt = Ro (1 + alpha T)
Where Rt is the resistance at the desired temperature, Rt is the resistance at 0C, alpha is the thermal coefficient of resistance, and T is the desired temperature.
From the hyperphysics link at:
We find that Nichrome A has an alpha of 0.0004, while tungsten's is
0.0045.For a 10 ohm sample of Nichrome A wire, then, we have:
Rt = Ro (1 + alpha T) = 10 * (1 + (0.0004 * 1100)) = 14.4 ohms.
In the same vein, for a 10 ohm sample of tungsten wire, we have:
Rt = Ro (1 + alpha T) = 10 * (1 + (0.0045 * 3000) = 145 ohms.
"Arrant nonsense"?
Hardly. :-)
-- JF
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