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Author Topic: 1/2 resistors maxxed out okay?  (Read 550 times)
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Is it okay to max out 1/2 resistors? I have some leds powered at~40ma with 220ohm 1/2 resistors
the average power dissapated is about .44 watts each
the resistors get uncomfortably hot to the touch, but its gonna be in the open so is that okay? Would the heat cause them to fail over time or is just fine?
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in short NO it isn't OK...
If the part is Hot to the touch or Smells Hot... GET THE RIGHT PART... what you are currently doing is a recipe for a disaster which will usually pop up at the worst time...
 IMO

Doc
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Everything fails eventually and heat usually helps to accelerate it.   The real question is how long you want it to be operational.  Passive components are sometimes rated for a couple thousand (e.g. 2000) at rated use conditions.  You are running close to the rated power, so it'll probably be operational for a couple of months.

Also consider the dangers of having a hot component and other things (e.g. People) coming in contact with it.  

Lastly, hot is relative.  You can't rely on your sense of touch.  Components can withstand more heat than your skin. It's better to measure, not guess.
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Yeah id prefer to keep it for a while
how high rating should I get? Would 1w mean it doesn't get half as warm?
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440mW?

P = I^2 * R = 40mA * 40mA * 220ohm = 352mW
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Would 1w mean it doesn't get half as warm?
No it burns off exactly the same ammount of heat. It might not get as hot because the thermal resistance of the case might be lower because it is bigger, but this does not equate to half the temprature. The resistor will have a temprature rating as well as a power rating, look up how hot it can get.
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Yeah that's the estimate ~40ma actually its 45ish when I put an ampmeter on the circuit and divided by the number of leds
but some leds have like 2.8 vdrop some 3.1 so it varies a little
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...but its gonna be in the open so is that okay?
The power rating of a resistor relates to how much heat it can dissipate in free air.  As I recall the kicker is that free air means 1 foot away from everything else.

Don
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So say soldered an inch from an led and a large piece of plastic, with others 2 inches away doesn't count?
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So say soldered an inch from an led and a large piece of plastic, with others 2 inches away doesn't count?

It depends on how well they get cooled. I've just looked up the datasheet for some 0.5W metal film resistors and the rating was for up to 70C ambient temperature. If there is good airflow and the resistors dissipating nearly 0.5W are 2 inches away from each other and from anything else generating heat, then it will be OK. But if you are unsure, or if the LED is for a warning light or other critical function, then go for 1W resistors instead. You will still need to make sure that the heat can escape.

I used to fly a light aircraft that, according the the schematic, was supposed to have a 50 ohm power resistor in the alternator failure circuit. The manufacturer had used five 220 ohm resistors in parallel instead instead. From their size I would guess they were rated at 2W each. The resistors had about 15v across them continuously. After 3300 hours of use, all the resistors were charred and some had gone open circuit. They were dissipating only 1W each which must have been within their free-air rating, but because they were in a bundle, they were heating each other and getting too hot.
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Do you think if I go with the 1w then It would be okay with just say being 5 inches off the wall without any fan/active cooling?
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I
Do you think if I go with the 1w then It would be okay with just say being 5 inches off the wall without any fan/active cooling?

Yes, unless you are planning on putting the whole thing in a sealed, thermally-insulated box. However, it might be worth looking for ways to reduce the power dissipation, or moving it to other components where it is easier to get rid of the heat. It sounds like you are driving your LEDs from a 12v supply. Can you use 2x20mA LEDs in series instead of 1x40mA LED? If the supply is a wall wart, can you use a 9v supply instead? If it's a car battery, you could use a DC-DC switched mode converter to generate a 5v supply efficiently for powering the LEDs (and the Arduino). Or, if your project uses a metal case, you could use a 7805 regulator to produce a 5v supply and use the metal case as the heatsink for the regulator.
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