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Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
I made a rather strange observation whilst tinkering with my home-made infra-red LED array circuit the other day.
Whilst modding it to work with a 12v battery I noticed that the resistance of one of the resistors actually dropped when heat from a soldering iron was applied to the leads. I have always though the opposite should happen (which it did for the other resistor on the adjacent array of LEDs).
Anyone have any explanation as to why it did this? I am not sure what types of resistors they were as they were ones salvaged out of old electronic boards.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
If you remember back to the first light bulbs with carbon filament they needed series resistors to avoid thermal runaway because of their negative temperature coefficient. That is probably a hint about what your resistor is made out of.
A very simplified way of thinking about it is this, if the material have low electron mobility and high specific resistance, higher temperature will knock the electrons around and make them more mobile. In a low resistance material increased temperature will have the opposite effect and slightly disrupt the already easy flow.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yeah, and no decent scientist does that, they leave it to economists.
If I remember Semiconductor Physics 101 right, negative temperature coefficient of resistance is caused by electrons being bumped up into the conduction band by thermal agitation. This can free a lot of electrons. A related phenomenon is electrical breakdown in gases.
But no matter how many electrons are in the conduction band (a metal has all of them there) they are still subject to thermal jostling by other electrons and collisions with other atoms as they move. This is what causes electrical resistance.
To get zero resistance, you have to start with a metal (lots of free electrons) and then cool it to a very low temperature. As the thermal agitation decreases, interesting quantum effects appear, allowing the conduction electrons to move without bashing into anything. The result is a superconductor, which does indeed have literally zero resistance.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546
Do another test. Switch the multimeter to DC volts, clip it on to the resistor, then heat the resistors to see if some thermo-electric effect exists. If it does, that is changing the resistance as read by the meter, because of the phantom current.
It is common with metalic resistors where the lead wires are different than the resistance element metal.
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Resistor resistance decreases as the temperature is increased?
