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Topic: watts (Read 4439 times) previous topic - next topic


oK:) how much watts could i go over on a 10k resistor prob?


BTW is this Right???


ON R i ment to put 190 ohm's


Let's try and debunk this a little bit for you.  First off, some definitions of the various single characters that have been posted here.

E = Electromotive force.  Volts in plain English which is the measurement unit.
I = Current flowing through any given part of a circuit.  Measured in Amps
R = Resistance in Ohms
P = Power or Watts if you wish which is the measurement unit.

So, you can express watts as Watts = Volts x Current or,  Watts = Current squared x Resitance in Ohms.

This link is the Wikipedia entry for Ohms Law which is essential if you are to work with circuits.  As Grumpy_Mike showed, you also need a small amount of algebra to move the formulas about.

Resistors have several colored bands around them and knowing what these mean will allow you to know their values in Ohms.  Here's a link to the code http://en.wikipedia.org/wiki/Electronic_color_code

The codes are not immediately obvious but you can have some fun by learning one or more of these little mnemonics in this link http://en.wikipedia.org/wiki/List_of_electronic_color_code_mnemonics

If you have not yet got yourself a multimeter, then you need one quick.

I hope this helps.


how much watts could i go over on a 10k resistor prob?

Any amount of power that the resistor is rated for.
A rating is how much power you can use without destroying it.

This can be as little as 0.02 Watts to hundreds of thousands of watts, it depends on how the resistor is physically constructed. It is not something that is related to the voltage or current or resistance value in ohms.


Feb 04, 2012, 11:23 pm Last Edit: Feb 04, 2012, 11:25 pm by retrolefty Reason: 1
Think of it as this way. A resistor is 100% efficient at converting electrical power into heat power. The electrical power developed and consumed by a resistor is the product of the voltage across it times the current through it which in turn is determined by the resistance of the resistor. Once this electrical power is converted to heat power then the amount of heat the resistor can handle (dissipate) safely is determined by it's physical size, construction material used, ambient temperature and the lack of, or presence of, air flow helping to remove the heat from the resistor, and that is called it's wattage rating, which is independent of it's ohms value.

So the amount of electrical power is again determined by the voltage applied and the resistance in ohms of the resistor. The ability of a specific resistor to handle the heat produced is based on it's 'mechanical' properties stated, not directly by it's resistance in ohms.

Don't know if that helps, we may just be going around and around in a circle?  


I've just looked at a few resistors of various sorts and come up with a rough heuristic (a guess in the form of an equation) for the probable power rating of a resistor:

P = L^2        L = length in cm, P = power in watts.

Admittedly the larger resistors I looked at need to bolt onto a heatsink, but it sort of works as a sanity-check - the area of a resistor is roughly proportional to the length squared, and heat has to leave through surface area.  It even works for an electric fire where a 1kW element is usually about 30cm long (but runs rather hot as its too skinny for its length!)
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Heh guys;

I got one question related to the OP. I know about the standard "through hole" type power rating. Which is : 1/8 , 1/4, 1/2, 1, 2, 5, ... <-- about that, I think... But what about the SMT resistors ? What are the power rating of tthose small SMT resistors ?  That will be nice to know that info during design my circuits. Commun sense tell me SMT resistors must be under 1/8 Watt, correct ?

Sorry for the "noob" question.


It depends on the size generally a:-
1206 resistor is 0.3W
0805 resistor is 0.125W
0603 resistor is 0.063W although you can get 0.1W ones.
0402 resistor is 0.063W

But it also depends on what sort of construction they are, that is what they are made of.


1206, 0805, 0603, 0402 ?


That must be resistor type code ? Yep.


I found a datasheet about them.

Thank Grumpy_Mike.

I search it. under "1206 resistor" and click on the search results.

I did not know about them.

Sorry for the "noob" question.

Jack Christensen

If it is through-hole, you might be able to tell by the physical size of the resistor.  1/4W and 1/8W are generally about the same size, for example.

Respectfully disagree. At least for the ones kicking around my shop, color codes on 1/4W resistors can almost always be read by guys who have had too many birthdays, this is not always the case with the 1/8W parts. XD  The magnifier or the meter often get used to verify the 1/8W parts. Still, I have come to appreciate and in fact prefer the 1/8W parts as they do take up less real estate on boards.


1206, 0805, 0603, 0402 ?

These are sizes for surface mount components and apply to capacitors and inductors as well.
1206 - wow that's small
0805 - do you expect me to pick that up? I'll get my tweezers
0603 - that's not a component is it?
0402 - now you are just taking the piss.



LOL. Thank Mike. I did check the site I post. It explain the code. The resistors are easy to read ( need magnify glass ), but I don't know about the cap, it just a color like beige and not writting or values. To measure them ? A bit small...  :~


but I don't know about the cap, it just a color like beige and not writting or values.

Then you do know about the caps, that is what they are like.

So you keep them in a reel, or bag or pot with the value written on it. Once removed from the bag you can't tell unless you measure it. Which is a bit of a sod when you get a prototype board back and you want to check the capacitor. Basically you can't unless you un solder them and measure it. Yes it is a bit small but there are surface mount probes that look like a twin prong to allow you to stab each end for a measurement.


The raison I ask about the SMT caps, I harvested a lots of boards from the garbage, and it will be nice to know the value. And you right, the only way to know is : measure the cap...and I don't have a smt probe, so I am force to use what I have... the DMM from Canadian Tire - Mastercraft - that I have and it included a cap tester. OR I could build one using the Ardiuno.

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