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Author Topic: Overheating Resistor  (Read 1494 times)
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I'm following this guide:
http://www.arduino.cc/playground/ComponentLib/Thermistor2

Except using a 2kohm resistor and thermistor.  The resistor gets extremely hot when I turn it on.  I've gone through about 3 now and they're all very darkened but still seem to have the proper resistance.  Any idea why this is happening?  They're 1/8 watt which is from what I'm told more than enough for an arduino.
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5v * 50mA = 5 * 0.05 = 0.25 W = 1/4 W

That's just my best guess. I've never gone lower than 1/4W resistors. The wattage isn't how much energy the resistor consumes, rather it's a maximum rating (as far as I understand it).

P = V*I

and

P = I^2 * R

are helpful equations. Without knowing how much current is running through the resistor I can't be very helpful. It looks like that 10K resistor (2k in your place) is a pulldown for the input.

EDIT: Actually, I have no idea what the 10K resistor is for. I do know that a 2k resistor will draw more current than a 10K though. (V = IR, with constant voltage, a decrease in resistance necessitates an increase in current). The store in my town sells 100 10k 1/4W 5% resistors for about 2 bucks. It's a worthwhile investment (10K is common).
« Last Edit: December 10, 2009, 07:17:26 pm by StkMtd » Logged

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EDIT: Actually, I have no idea what the 10K resistor is for.

It's used as a voltage divider (which I still don't really get smiley-wink!).

[edit]By the way, I'm pretty sure it's not a coincidence that it's a 10k Thermistor, as well as using a 10k Resistor.[/edit]

[edit]Also, why did you use 50 mA in your calculations?  Arduino pins can only supply a maximum of 40mA and even then I doubt it's using even that much.[/edit]

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The store in my town sells 100 10k 1/4W 5% resistors for about 2 bucks. It's a worthwhile investment (10K is common).

Nice deal!  What store is it/what country do you live in?[/edit]
« Last Edit: December 10, 2009, 07:21:02 pm by Tchnclfl » Logged

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Hmmmm... alright I'll see if I can find some 1/4w ones.  Do you think this could be throwing my numbers off?  Right now I'm reading 495*C which is a tad too hot :S
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It's used as a voltage divider (which I still don't really get Wink!).

So the voltage at the input will depend on the ratio of resistance between the 10K, and the sum of the 10K and the resistor that varies with temperature.....

I think we need an expert.
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Do you think this could be throwing my numbers off?

Definitely.

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So the voltage at the input will depend on the ratio of resistance between the 10K, and the sum of the 10K and the resistor that varies with temperature.....

Er...Something like that smiley-razz!

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I think we need an expert.

Agreed!  smiley-wink
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Also, why did you use 50 mA in your calculations?  Arduino pins can only supply a maximum of 40mA and even then I doubt it's using even that much.

I pulled that number out of my... head. Good catch.

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Do you think this could be throwing my numbers off?

Can't say for sure, but when things burn and turn black, I usually sit back a moment and think of how to avoid that.
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Can't say for sure, but when things burn and turn black, I usually sit back a moment and think of how to avoid that.

Why do you think I'm here?


As far as the 10k/10k goes, no its not coincidence.  I looked at a few other guides with different sized thermistors and they always matched the resistance.

The weird thing is, originally it wasn't burning the transistor.  I was still getting odd values but it wasn't until about a half hour in that it started happening.  But now when I toss in a new resistor it happens immediately.  
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originally it wasn't burning the transistor.

Woah woah woah...Why are you using Transistors?  :-?

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But now when I toss in a new resistor it happens immediately.

Yeah...I recommend you give that a rest until we figure this out ;D!
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As far as the 10k/10k goes, no its not coincidence.  I looked at a few other guides with different sized thermistors and they always matched the resistance.

What happens if you try subbing in something higher than 2k? I know with the digital thermometer kit I built, I had to adjust a trim pot to calibrate the readout. I'm sure the value of the resistor must have some significance. If you only have 2K resistors, you could string 5 of them in serial for a quick hack.
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By transistor I meant resistor...
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There is no way that a 2K resistor should get hot given a 5V supply.  That's 1/80 Watt.
Are you sure you read the resistor value correctly (should be red black red.)
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I can read the colours because I'm colourblind.  But I tested it with a multimeter.
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I'm with westfw, the worst case scenario (just the resistor, one end at 5v and the other end at ground) with a 5v supply and a 2k resistor is next to nothing.  Its either seriously more than 5 volts or the resistor is seriously less than 2k.  The meter isn't saying 20.00 (or worse still 2.000) is it ?
« Last Edit: December 11, 2009, 01:44:21 am by stephen_t » Logged


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I can read the colours because I'm colourblind

So best ask some one who isn't.

I had a friend when I was at university who was colour blind and I always read his resistors for him.
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