Resistors Difference between 1/2 watt and 1/4 watt?

starmanmm · Post by **starmanmm** » Sun Feb 28, 2010 11:41 am

Ok, picked up some resistors and in my haste, picked two different sets of 100 Ohm resisitors. One is listed at 1/2 watt at 5% tolerance and the other is 1/4 watt at 5% tolerance.

So my question is how are they different and how would you figure them into being used with leds?

Scott Hasty · Post by **Scott Hasty** » Sun Feb 28, 2010 4:12 pm

What and how many LED's are you using? What are your voltage and current values? The wattage rating of any resistor can be calculated by Ohm's Law. If you are using 12 volts at 20mA, you can calculate the wattage with Power(W)=Current(I)*Voltage(V). Using the values above, 12*.020=.24W. So you are right at your 1/4 watt value. It would be okay to use a 1/4 watt resistor, but I would step up to 1/2 watt to alleviate any heat.

Hope this helps...

starmanmm · Post by **starmanmm** » Sun Feb 28, 2010 4:31 pm

Scott, my basic question was what is the difference between the two... which I think you answered.

If I understand correctly, what ever the number of leds, etc. what matters is the Heat the resistor will generate.

If I don't care, then using the 1/4 is fine. But it will generate more heat from the resistor.

If heat does matters, then use the 1/2 to keep the resistor cooler.

USS Atlantis · Post by **USS Atlantis** » Sun Feb 28, 2010 4:38 pm

And if there's a borderline heat issue - go to the 1/2 watt

Heat is the killer of resistors -

less heat=longer life
too much heat=dead resistor&dead circuit

tetsujin · Post by **tetsujin** » Sun Feb 28, 2010 9:34 pm

Scott Hasty wrote:What and how many LED's are you using? What are your voltage and current values? The wattage rating of any resistor can be calculated by Ohm's Law. If you are using 12 volts at 20mA, you can calculate the wattage with Power(W)=Current(I)*Voltage(V). Using the values above, 12*.020=.24W. So you are right at your 1/4 watt value. It would be okay to use a 1/4 watt resistor, but I would step up to 1/2 watt to alleviate any heat.

Hope this helps...

But remember that the voltage that matters in this calculation is the voltage across the resistor.

So if you had a 12V power source in series with a resistor and then two white LEDs all running at 20mA, you wouldn't be looking at 12V * 20mA, you'd be looking at the voltage drop of the resistor * 20mA... If the LEDs each drop 3V, then it'll be something like 6V across the resistor, with the resistor dissipating a total of 120mW - half the safe maximum for a quarter-Watt resistor.

Scott Hasty · Post by **Scott Hasty** » Mon Mar 01, 2010 9:07 am

starmanmm wrote:
If I understand correctly, what ever the number of leds, etc. what matters is the Heat the resistor will generate.

The value is the total amount of energy [in watts] the resistor can handle before it fails. Of course, as you have surmised, the energy given off in this case is heat.

Scott Hasty · Post by **Scott Hasty** » Mon Mar 01, 2010 9:11 am

tetsujin wrote:
Scott Hasty wrote:What and how many LED's are you using? What are your voltage and current values? The wattage rating of any resistor can be calculated by Ohm's Law. If you are using 12 volts at 20mA, you can calculate the wattage with Power(W)=Current(I)*Voltage(V). Using the values above, 12*.020=.24W. So you are right at your 1/4 watt value. It would be okay to use a 1/4 watt resistor, but I would step up to 1/2 watt to alleviate any heat.

Hope this helps...
But remember that the voltage that matters in this calculation is the voltage across the resistor.

So if you had a 12V power source in series with a resistor and then two white LEDs all running at 20mA, you wouldn't be looking at 12V * 20mA, you'd be looking at the voltage drop of the resistor * 20mA... If the LEDs each drop 3V, then it'll be something like 6V across the resistor, with the resistor dissipating a total of 120mW - half the safe maximum for a quarter-Watt resistor.

My solution was a SWAG due to not knowing how he set his circuit is set-up. Your assertion is true for series and parallel circuits (in which you have current drops at each circuit branch).

starmanmm · Post by **starmanmm** » Mon Mar 01, 2010 8:49 pm

Ok, lets take a step back... what applications would one want to use 1/4 over 1/2?

Series or Parallel... pick your set up for.... let's say 3 leds at 20 mA.

USS Atlantis · Post by **USS Atlantis** » Mon Mar 01, 2010 8:57 pm

starmanmm wrote:Ok, lets take a step back... what applications would one want to use 1/4 over 1/2?

Series or Parallel... pick your set up for.... let's say 3 leds at 20 mA.

If they're White, Green or Blue

3v input - parallel - no resistor
6v input - parallel - 3x 150ohm resistors
9v input - series - no resistor
12v input - series - 1x 150ohm resistor

If they're Red or Yellow
3v input - parallel - 3x56ohm resistors
6v input - series - no resistor
9v input - series - 1x150ohm resistor
12v input - series - 1x330ohm resistor

All arrays use 1/4 watt without a problem

These are using the LED's I get from Unique