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What a resistor can handle 
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#1
Apr1212, 09:26 PM

P: 53

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 


#2
Apr1212, 09:31 PM

PF Gold
P: 7,120

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. 


#3
Apr1212, 09:43 PM

P: 53

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



#4
Apr1212, 10:00 PM

PF Gold
P: 7,120

What a resistor can handle



#5
Apr1312, 11:11 AM

Engineering
Sci Advisor
HW Helper
Thanks
P: 6,962

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. 


#6
Apr1312, 11:34 AM

Mentor
P: 7,315

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. 


#7
Apr1312, 12:24 PM

P: 102

Well, when you doubled the voltage, you quadrupled the power that the resistor had to dissipate, since P = V^{2}/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. 


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