[Lab] Odd Question

[Micro]

It's a Rayovac alkaline. I don't know how old it is. The voltage is 8.87V without a load.

That's an enlightening document. I'm surprised to see such high temperatures. So what I figured would be "very warm" would really be "pretty darn hot".

Guy

--- On Sat, 1/21/12, Michael <krazatchu at hotmail.com> wrote:

From: Michael <krazatchu at hotmail.com>
Subject: Re: [Lab] Odd Question
To: lab at artengine.ca
Received: Saturday, January 21, 2012, 2:56 AM


  

    
  
  
    I'm curious about the battery you used to test this, new 9v alkaline
    or rechargeable or? 

    Using (Nominal V - Loaded V) * (Load R) / (Nominal V) to figure the
    internal resistance gives 5 ohms.

    That seems pretty high, also what brand is it?

    

    Temperature rise/heat dissipation is equally dependent on surface
    area as it's dependent on the thermal conductivity factor k. 

    Power resistors are designed to dissipate and have a high k, sugar
    maybe not so much...

    

    The common rule of thumb is to de-rate by 50%. But even at 1.5W into
    a 3W wire wound resistor will be too hot to touch without active
    cooling.

    The chart on the left side of page two of the following link, says
    it should reach at least 150C over ambient at 50% of it's rated
    load... ouch!

    http://www.electro-tech-online.com/custompdfs/2011/09/appnotes_res_select-1.pdf

    My rule of thumb is to de-rate by 4 or 5 times depending on PCB
    density and proximity to electrolytic caps, etc...

    

    Michael 

    http://n0m1.com/

        http://krazatchu.ca/

        

        

        

      On 1/20/2012 9:05 PM, Micro wrote:
    
      I tried a few resistors and found that took 15 ohms to drop the voltage to 6 volts. That works out to 400 milliamps and most importantly 2.4 Watts.

The temperature will mostly depend on how many Watts and how much surface area there is to be able to cool it off. To give you a rough idea, take a look at this 3 Watt resistor. 

http://www.wjoe.com/resistors/1.2k3watt5per.jpg

At that size it can usually get rid of enough heat to keep it from getting more than very warm. If the material you're heating is much bigger than that, it should also be much cooler.

Guy
 
 
--- On Fri, 1/20/12, Emily Daniels <emily.daniels at gmail.com> wrote:

From: Emily Daniels <emily.daniels at gmail.com>
Subject: [Lab] Odd Question
To: "lab" <lab at artengine.ca>
Received: Friday, January 20, 2012, 1:33 PM

Hi All,
Does anyone know how much heat is produced by a 9 volt battery when the resistance of the material it flows through reduces it to 6 volts? What would the temperature be inside the material? Thanks!


Emily

    
    

    

    -- 

      ----------------------
      http://NoMiDesign.net/
      http://krazatchu.ca/
    
  


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