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# Carbon resistors power

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#### sbayeta

##### New Member
Hi,
How do I tell the power of a carbon resistor from its size ? Is there any reference on the web I can consult ?

Thanks

#### Nostrafus

##### New Member
A Carbon film resistor can be identified by it's markings, there should be between 3-5 colored bands which are used to identify a resistor.

Also size doesn't always matter when it comes to electronics, although with carbon film resistors it does, the longer the piece of carbon film, the more resistance, though it usually doesn't take much.

Note: Look VERY CAREFULLY at the resistor, some colors are difficult to distinguish like blue/green, and violet/blue, I know the last one sounds kind of funky, but one time it took me about 1/2 an hour to decide which one it was, I looked under many different lights, but didn't actually decide until I went out into sunlight (The best to identify colors under, other lights emit a slightly different color, like a yellowish/pale color which can impare your ability to identify some colors) and some of these bands can be under 1mm in size so a magnifying glass may help.

#### sbayeta

##### New Member
Nostrafus said:
A Carbon film resistor can be identified by it's markings, there should be between 3-5 colored bands which are used to identify a resistor.

Also size doesn't always matter when it comes to electronics, although with carbon film resistors it does, the longer the piece of carbon film, the more resistance, though it usually doesn't take much.

Note: Look VERY CAREFULLY at the resistor, some colors are difficult to distinguish like blue/green, and violet/blue, I know the last one sounds kind of funky, but one time it took me about 1/2 an hour to decide which one it was, I looked under many different lights, but didn't actually decide until I went out into sunlight (The best to identify colors under, other lights emit a slightly different color, like a yellowish/pale color which can impare your ability to identify some colors) and some of these bands can be under 1mm in size so a magnifying glass may help.

Thanks for the info, but what I need to know is how to distinguish the power of a resistor, not its resistance.

Any ideas on this ?

#### bogdanfirst

##### New Member
take a look at **broken link removed**
or go **broken link removed** to select other categories.

#### subzero349

##### New Member
Nostrafus said:
Note: Look VERY CAREFULLY at the resistor, some colors are difficult to distinguish like blue/green, and violet/blue, I know the last one sounds kind of funky, but one time it took me about 1/2 an hour to decide which one it was, I looked under many different lights, but didn't actually decide until I went out into sunlight (The best to identify colors under, other lights emit a slightly different color, like a yellowish/pale color which can impare your ability to identify some colors) and some of these bands can be under 1mm in size so a magnifying glass may help.

a handy trick to finding out the resistance of a resistor if the colours are bad is to measure them with a DMM...

#### Noggin

##### Member
my lazy butt skips the bands and goes straight for the DMM. As for determining power, I dont know

#### daviddoria

##### New Member
haha i agree with skipping the bands... who is going to be playing with carbon resistors w/o a DMM nearby anyway?

#### bogdanfirst

##### New Member
i don't see any way to determine the power of the reistor exept from it's size.....
maybe the colour of the reistor means something.....?

#### sbayeta

##### New Member
bogdanfirst said:
i don't see any way to determine the power of the reistor exept from it's size.....
maybe the colour of the reistor means something.....?
I know that. What I need to know is if is there any reference to determine the power.
E.g.:
Lenght: 10mm, Diameter: 1mm ----> 1/4W
Lenght: 20mm, Diameter: 3mm ----> 1W
etc....

#### bogdanfirst

##### New Member
look at the links i posted......the first one.....it has a chart that contains the dimensions of different power resistors.
i think that the dimensions don't matter from different manufacturers

#### sbayeta

##### New Member
bogdanfirst said:
look at the links i posted......the first one.....it has a chart that contains the dimensions of different power resistors.
i think that the dimensions don't matter from different manufacturers
I saw that when you posted it, and was great. But my doubt remains if is there a standard reference

#### bogdanfirst

##### New Member
i don't know about standards....,but the fact is that you will not find a 2 W resistor from one manufacturer the same size as a 1/2W resistor from another manufacturer....if you know what i mean/////

#### Nostrafus

##### New Member
a handy trick to finding out the resistance of a resistor if the colours are bad is to measure them with a DMM...

Yeah, but not everyone has a DMM, like me, I just haven't had much spare time to go get one, nor the spare cash, because if I get one, I'm probably gonna get one of the $100 that can measure voltage, resistance, capacitance, and all the extras, because I don't want to have to spend a whole lot of time trying to identify components. #### subzero349 ##### New Member I'm probably gonna get one of the$100 that can measure voltage, resistance, capacitance, and all the extras, because I don't want to have to spend a whole lot of time trying to identify components.

Good idea, a DMM that can measure the values of caps (along with resistance, voltage, current, and all the other standard stuff) is very worth the money...

As for the power of carbon resistors I would bet there is no set standard. Obviously size plays a factor, so good luck sbeyeta!!

#### bogdanfirst

##### New Member
yes, it can do that, but you CAN NOT mesure the power of a resistor with a DMM
the best way is from it's size, and probably the only way

#### Dean Huster

##### Well-Known Member
Power and DMMs

Yes, the physical size is the only way to determing the power rating of a resistor. Although there is no set standard, most manufacturers pretty well stay close to each other in this regard. The "standard" sizes are for 1/16-watt through 2-watts only and then for carbon composition or carbon film only. Beyond 2 watts or if playing with metal film, wirewound or metal oxide, all "standard" bets are off!

I might add that although a 1/4-watt carbon film and 1/4-watt carbon composition resistor are about the same size and have the same power rating, they are NOT really the same in power rating. For continuous power dissipation, they'll both handle 1/4 watt. But for short bursts of power over the rating -- that is, pulsed power that may average out to 1/4 watt -- the composition resistor will handle it while the film resistor will smoke on you. The composition resistor has a lot more physical bulk in the resistance element and can absorb power surges while the film resistor is nothing more than a "skin" of carbon deposited on the surface of a ceramic slug and will flash with a surge of power.

Dean

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