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Yanom

Sun Aug 19 2012, 06:59PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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I recently discovered that if you chop off the eraser part of a pencil and expose the lead on both ends of the pencil, you get a nice big resistor providing 4-5 ohms. This immediately begs the question - what's the wattage rating on a pencil? How much power do you think it could take?

Thomas W

Sun Aug 19 2012, 07:05PM

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i would think, ideally you want to take the wood of it, as that would keep it hotter, to test the wattage, just see how much power you can push through it until it glows hot, im sure you could extend its lasting by putting it in a small 1/2 inch plastic or metal tube, suspended in the middle and fill the rest with oil, metal would be better i guess as it would be able to dissapate the oils heat..
Thomas~~

Ash Small

Sun Aug 19 2012, 08:15PM

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__=|(:3)-|--{__ wrote ...

i would think, ideally you want to take the wood of it, as that would keep it hotter, to test the wattage, just see how much power you can push through it until it glows hot,

I think, for it to be useful, you'd want to establish at what power it's resistance increases by whatever percentage your acceptable tolerance is. Stripping the wood off and cooling it will help, though.

Steve Conner

Sun Aug 19 2012, 10:00PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
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I'm sure I remember playing with this years ago. If you apply enough power, smoke shoots out of the ends, then the wooden casing splits in half to reveal a brightly glowing pencil lead. I can't remember what voltage it took to do that, but a couple of SLA batteries in series would be a good starting point.

brandon3055

Sun Aug 19 2012, 10:42PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
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This reminds me of when I used to cut thin copper and steel plate with the carbon from a C battery a set of jumper leads and a car battery as I recall it Dosent take a lot of current before a pencil lead starts getting hot which could be a problem if you need a fairly constant resistance

Ben Solon

Sun Aug 19 2012, 11:59PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Steve Conner wrote ...

I'm sure I remember playing with this years ago. If you apply enough power, smoke shoots out of the ends, then the wooden casing splits in half to reveal a brightly glowing pencil lead. I can't remember what voltage it took to do that, but a couple of SLA batteries in series would be a good starting point.

I used to do the same thing

if you use mechanical pencil lead you get a higher resistance for the same length. But they don't sell then that long...

While they make great heating elements, I suspect the reason that they can't be used in all but the smallest sizes is that they have a very high thermal coefficient- not quite positive(I don't think) but close. Even the smallest currents tend to rapidly heat them. Then again, in some botch up open chassis power supply, you could possibly use them as shunts if you don't mind the glowing graphite!

HV Enthusiast

Mon Aug 20 2012, 12:23AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Want a better resistor? Just hook up the spiral part of a spiral notebook up to 115VAC.

Dr. Dark Current

Mon Aug 20 2012, 10:00AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
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I think power resistors are so cheap that a smoking pencil lead is unnecessary. The working temperature of ceramic power resistors is up to 275 Â°C, wood surely can't handle this and a bare pencil lead will have just a little surface to dissipate heat - will get much hotter than a "proper" resistor.

Sulaiman

Mon Aug 20 2012, 12:59PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If you work out how to
- protectt the rod
- dissipate the heat from the rod
- reliably connect leads to the end
............
then you'd have a commercial solid carbon resistor,
together with wire wound, the earliest components, early 20th Century

Hazmatt_(The Underdog)

Tue Aug 21 2012, 01:24AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
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Posts: 1735

If you really want a high power resistor, go buy a box of carbon cutting electrodes.
They're already dipped in copper, aka your contact.
They're designed to handle a couple hundred amps.
They're machinable
AND... if you etch the copper away to different degrees, you can vary the resistance. After that, you can shave down the carbon, increasing the resistance.

And if you want to know how to etch them, use a paint pen and standard Ferric Chloride.

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