What factors determine the maximum power a resistor can handle?

[WarrickF]

Hi Guys,

Given a resistor, I understand how to use the bands to calculate the resistance, but how does one tell what the resistor is able to handle.

For example, I have a 51ohm resistor that I connected to a DC power supply. At 5V all was fine, but when I increased the voltage to 10V the poor little resistor was glowing hot.

The last band is for tolerance, but I'm not sure if this helps in any way to determine what kind of current the resistor can handle.

Thanks
Warrick

 

[Pengwuino]

The tolerance doesn't tell you how much power the resistor will take, it tells you within what range your resistor will actually perform at. For example, a 1000 ohm resistor with a 10% tolerance won't be manufactured to have 1200 ohms of resistance for example.

When you purchased the resistor, it should say how much power it can handle safely.

 

[WarrickF]

mmm ... thanks. So what you're saying is that given a box of resistors there's no way of knowing?

 

[Pengwuino]

mmm ... thanks. So what you're saying is that given a box of resistors there's no way of knowing?

Probably, but I don't know. Depending on what material they're made of, I bet a general power rating can be determined by people who know more about it than I do.

 

[AlephZero]

mmm ... thanks. So what you're saying is that given a box of resistors there's no way of knowing?

By experience, you can make a good guess from the size and shape of the resistor, but you are correct there is no way of knowing for certain. The most common size modern resistors have a power rating of 0.6 watts.

Almost all resistors that can handle powers higher than 1 watt are not marked with color code bands, but have the resistance and power rating printed on them.

 

[Integral]

For the off the shelf axial resistor power rating is know by the size of the resistor. The bigger the resistor the more power it is able to dissipate. Once you have dealt with a few resistors you will be able to tell.

The standard wattages are 1/2 W, 1/4W and 1/8W.

 

[Jasso]

Well, when you doubled the voltage, you quadrupled the power that the resistor had to dissipate, since P = V²/R. At 5V, the resistor was dissipating 25/51 = 0.49W. But at 10V, it went to 100/51 = 1.96W.

And looking around I found this: http://www.instructables.com/file/FCSUQFCGJQEDD5L/. It should help give a general idea.