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Resistor too hot?

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donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Sat Jun 12, 2010 12:27 pm Post subject: Resistor too hot?

I'm planning on using some LED strips for courtesy lighting. I found a 19" strip with 30 LEDs that draw 130mA at 12v. Since I'm going to using a 24v system, I'm going to need to use a resistor. 



So using 28v as the supply with a voltage drop of 12v and a current of 130mA gave me a 123 ohm 2 watt resistor.  I have access to some 123 ohm, 2 watt, metal film resistors so I tried those.



When I set it up on the breadboard the current was reading 100ma but the resistor was almost immediately too hot to touch. Using a point and shoot thermometer (not the most accurate for this) showed 120 degrees within about 1 minute and after 5 minutes it was up to 140 degrees. 



Now I don't expect these lights will stay on much longer than 5 minutes as they will most likely be used during loading and unloading but that heat doesn't seem right.



How hot is too hot for a resistor?



Should I bump up to a 5 watt resistor?



Did I miscalculate?



Don

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N20DG

Joined: 02 Jan 2008
Posts: 61
Location: lancaster, texas

Posted: Sat Jun 12, 2010 1:06 pm Post subject: Resistor too hot?

In a message dated 6/12/2010 3:32:06 P.M. Central Daylight Time,  don(at)numa.aero writes:

  	  Quote: 	
		  I'm    planning on using some LED strips for courtesy lighting. I found a 19" strip    with 30 LEDs that draw 130mA at 12v. 	
 >>>>>>>>>>>

 Where

 Tks

 Dick

  [quote][b]

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nuckolls.bob(at)aeroelect
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Posted: Sat Jun 12, 2010 1:11 pm Post subject: Resistor too hot?

At 03:27 PM 6/12/2010, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "donjohnston" <don(at)numa.aero>



 I'm planning on using some LED strips for courtesy lighting. I found a 19" strip with 30 LEDs that draw 130mA at 12v. Since I'm going to using a 24v system, I'm going to need to drop use a resistor. 



 So using 28v as the supply with a voltage drop of 12v and a current of 130mA gave me a 123 ohm 2 watt resistor.  I have access to some 123 ohm, 2 watt, metal film resistors so I tried those.



 When I set it up on the breadboard the current was reading 100ma but the resistor was almost immediately too hot to touch. Using a point and shoot thermometer (not the most accurate for this) showed 120 degrees within about 1 minute and after 5 minutes it was up to 140 degrees. 



 Now I don't expect these lights will stay on much longer than 5 minutes as they will most likely be used during loading and unloading but that heat doesn't seem right.



 How hot is too hot for a resistor?



 Should I bump up to a 5 watt resistor?



 Did I miscalculate?

 	


   No. The normal "full power" operating temperature

   for some power resistors is pretty toasty. Typically

   over 100C (will sizzle spit).



   For example:



 [img]cid:.0[/img]

   Here'a de-rating curve for a typical metal film power resistor.

   Note that it's good for 100% of power rating at 70C ambient.

   When ambient rises to about 225 degrees C (that's

   really toasty), you cannot call upon the device to handle

   ANY power. I.e., At full power in a 70C environment, one

   can expect the surface temperature to be over 200C.



   The specifications for your resistor of choice should

   be available on the 'net . . . but it's unlikely that

   you've got a problem with respect to the resistor itself.

   A significant risk for incorporating high power devices

   into a system may not be for the device itself but for

   surrounding devices and/or materials that will degrade

   in performance when subject to local heating.



 

   Bob . . .

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mdnanwelch7(at)hotmail.co
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Posted: Sat Jun 12, 2010 1:26 pm Post subject: Resistor too hot?

 	  Quote: 	
		   Did I miscalculate?

 Don


	


 



 



Don,



 



  From what I see, and according to the information you stated, no, you didn't miscalculate.  A 12 volt forward voltage does sound high, though.  Most LEDs have a forward voltage in the 2 volt range.  Hmm?



 



  From my experience with LEDs, why not just give a higher resistor a try?  The only thing



a higher resistor will affect is the luminosity of the LED, and from my experience it takes a lot more resistance to reduce the brightness any significant degree.   Why not try 330 ohm resistors, or maybe even some 470's?   Just a thought....



 



Mike Welch

 

 		 	   		  

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donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Sat Jun 12, 2010 1:34 pm Post subject: Re: Resistor too hot?

 	  Quote: 	
		  From what I see, and according to the information you stated, no, you didn't miscalculate. A 12 volt forward voltage does sound high, though. Most LEDs have a forward voltage in the 2 volt range. Hmm? 	




Mike,



It's not a single LED, it's a strip of 30 LEDs. According the information, they're designed for a 12v supply.



Spec's are here:

http://www.superbrightleds.com/cgi-bin/store/index.cgi?action=DispPage&Page2Disp=%2Fspecs%2FFLS.htm



It's the first one one the list (xFLS-CW30).



Sounds like I'm okay. I just need to figure out where to mount the resistor so the heat isn't an issue.



-Don

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rparigoris

Joined: 24 Nov 2009
Posts: 792

Posted: Sat Jun 12, 2010 3:07 pm Post subject: Resistor too hot?

Hi DonJust a thought, can you find two light strips you can put in series? Do you have a Pep Boys automotive store near you? They sell various length LED light strips, I have one that is bout half the length you want to use. There is a good chance on your light strip you have if it is like picture on website, there are 3 LEDs in series with a resistor. If you made a cut of wires at the 15 LED mark and put them in series instead of parallel you could get rid of your resistor all together.Ron Parigoris   [quote][b]

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enginerdy(at)gmail.com
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Posted: Sat Jun 12, 2010 5:30 pm Post subject: Resistor too hot?

Or, use a resistor like this:



http://mouser.com/ProductDetail/Ohmite/TBH25P150RJE/?qs=sGAEpiMZZMvhlCB8CTbT5P3ZZcE9WVSjJn%252bo%252b5MXx0c%3d

Screw that down to some convenient large piece of metal and it will stay much cooler. Just be sure to heat-shrink the leads so they don't short to the heat sink surface. The tab is electrically insulated, so there's no need for a mica sheet underneath it.

--Daniel



On Jun 12, 2010, at 6:04 PM, rparigor(at)suffolk.lib.ny.us (rparigor(at)suffolk.lib.ny.us) wrote:

 	  Quote: 	
		  Hi Don



Just a thought, can you find two light strips you can put in series? Do you have a Pep Boys automotive store near you? They sell various length LED light strips, I have one that is bout half the length you want to use. There is a good chance on your light strip you have if it is like picture on website, there are 3 LEDs in series with a resistor. If you made a cut of wires at the 15 LED mark and put them in series instead of parallel you could get rid of your resistor all together.

Ron Parigoris



    	  Quote: 	
		  



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