# Overheating Resistor

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.

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).

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 ;)!).

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]

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]

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]

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

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.

Do you think this could be throwing my numbers off?

Definitely.

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 :P!

I think we need an expert.

Agreed! ;)

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.

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.

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.

originally it wasn't burning the transistor.

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

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!

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.

By transistor I meant resistor...

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.)

I can read the colours because I'm colourblind. But I tested it with a multimeter.

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 ?

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.

Resistor color codes are a pet peeve of mine. Color stripes were fine for the technology 50 years ago, but if they now have the technology to print digits on an 0603 (that’s 6 thousandths of an inch long and 3 wide) SMT chip resistor, they could print the value on the stinkin’ carbon resistors, too.

-j

Resistor color codes are a pet peeve of mine. Color stripes were fine for the technology 50 years ago, but if they now have the technology to print digits on an 0603 (that’s 6 thousandths of an inch long and 3 wide) SMT chip resistor, they could print the value on the stinkin’ carbon resistors, too.

-j

Agreed. I never got why they still use those darned colors… :-?

well, with the exception of the colourblind, looking at the 3rd stripe can quickly give you an idea of magnitude. There's only around 10 colours, I think it'd be manageable. It would likely bring the cost of resistors up if they had to print text on a cylindrical object.

I think a barcoding system would work but it'd require special equipment. With that said though, I wonder if someone could develop a system that would work off a webcam to detect the coloured stripes and spit back a value.

Because the codes are such a pain to read, my solution is to just keep them well-organized, labelled, and separated (one resistor type per drawer). I carefully put them back when I'm finished as well.

Its digital - you can do almost every thing with 1k and 10k resistors - brown black red and brown black orange......... :)

Well, figured out the problem. It was the resistor. Turns out the electrical engineer at work who showed me how to read a resistor with a multimeter either made something up to fuck with me or his multimeter is completely different. He had said that when you set it to a value, such as 20kohms, then touched the resistor you had to calculate the difference to get the resistor value. So if the multimeter then showed 15khoms that meant that 5kohms was being resisted, making it a 5kohm resistor. Made sense to me so I went on with that logic. After having all these issues I did some Googling and confirmed that this was BS and that if it said 15kohms it was 15kohms, no need to calculate a difference.

So after putting in the proper resistor there is no more burning smell :P

On top of that, it reads a proper value though it does seem a tad high. I have a digital thermometer that reads 26C in here when mine is saying 30C. But when I squeeze it with my fingers it heats up to 98-100F which is an optimal reading considering the human body is 97-100. So my other one might be off a few degrees.

Anyways, I appreciate all the help :D Despite programming for 10 years I'm a complete n00b when it comes to this side of the coin.