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I'm curious about the battery you used to test this, new 9v alkaline
or rechargeable or? <br>
Using (Nominal V - Loaded V) * (Load R) / (Nominal V) to figure the
internal resistance gives 5 ohms.<br>
That seems pretty high, also what brand is it?<br>
<br>
Temperature rise/heat dissipation is equally dependent on surface
area as it's dependent on the thermal conductivity factor k. <br>
Power resistors are designed to dissipate and have a high k, sugar
maybe not so much...<br>
<br>
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.<br>
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!<br>
<a class="moz-txt-link-freetext"
href="http://www.electro-tech-online.com/custompdfs/2011/09/appnotes_res_select-1.pdf">http://www.electro-tech-online.com/custompdfs/2011/09/appnotes_res_select-1.pdf</a><br>
My rule of thumb is to de-rate by 4 or 5 times depending on PCB
density and proximity to electrolytic caps, etc...<br>
<br>
Michael <br>
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<br>
<br>
<br>
</font></span>On 1/20/2012 9:05 PM, Micro wrote:
<blockquote
cite="mid:1327111508.53056.YahooMailClassic@web121604.mail.ne1.yahoo.com"
type="cite">
<pre wrap="">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.
<a class="moz-txt-link-freetext" href="http://www.wjoe.com/resistors/1.2k3watt5per.jpg">http://www.wjoe.com/resistors/1.2k3watt5per.jpg</a>
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 <a class="moz-txt-link-rfc2396E" href="mailto:emily.daniels@gmail.com"><emily.daniels@gmail.com></a> wrote:
From: Emily Daniels <a class="moz-txt-link-rfc2396E" href="mailto:emily.daniels@gmail.com"><emily.daniels@gmail.com></a>
Subject: [Lab] Odd Question
To: "lab" <a class="moz-txt-link-rfc2396E" href="mailto:lab@artengine.ca"><lab@artengine.ca></a>
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
</pre>
</blockquote>
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