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### Topic: Overheating Resistor (Read 2969 times)previous topic - next topic

#### psilokan

##### Dec 11, 2009, 01:00 am
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.

#### StkMtd

#1
##### Dec 11, 2009, 01:10 amLast Edit: Dec 11, 2009, 01:17 am by StkMtd Reason: 1
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).

#### TchnclFl

#2
##### Dec 11, 2009, 01:14 amLast Edit: Dec 11, 2009, 01:21 am by Tchnclfl Reason: 1
Quote
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]

Quote
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]

#### psilokan

#3
##### Dec 11, 2009, 01:20 am
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

#### StkMtd

#4
##### Dec 11, 2009, 01:20 am
Quote
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.

#### TchnclFl

#5
##### Dec 11, 2009, 01:21 am
Quote
Do you think this could be throwing my numbers off?

Definitely.

Quote
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 !

Quote
I think we need an expert.

Agreed!

#### StkMtd

#6
##### Dec 11, 2009, 01:23 am
Quote
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.

Quote
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.

#### psilokan

#7
##### Dec 11, 2009, 01:27 am
Quote
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.

#### TchnclFl

#8
##### Dec 11, 2009, 01:29 am
Quote
originally it wasn't burning the transistor.

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

Quote
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 !

#### StkMtd

#9
##### Dec 11, 2009, 01:51 am
Quote
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.

#### psilokan

#10
##### Dec 11, 2009, 03:00 am
By transistor I meant resistor...

#### westfw

#11
##### Dec 11, 2009, 03:31 am
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.)

#### psilokan

#12
##### Dec 11, 2009, 03:52 am
I can read the colours because I'm colourblind.  But I tested it with a multimeter.

#### pluggy

#13
##### Dec 11, 2009, 07:42 amLast Edit: Dec 11, 2009, 07:44 am by stephen_t Reason: 1
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 ?
http://pluggy.is-a-geek.com/index.html

#### Grumpy_Mike

#14
##### Dec 11, 2009, 09:18 am
Quote
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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