# Resistor wattage measurement

I'm not an engineer professionally but I'm interested in. So my question is, how to recognise the wattage of a resister? And how to measure it using multimeter?

• You wonder how to find out the power rating? Read the resistor's datasheet. – Nick Alexeev Nov 1 '15 at 6:59
• For a resistor already removed from its packaging, it is not possible to tell. Only to sacrifice one and test it to destruction. – Mark Ch Nov 1 '15 at 9:23
• Mainly material and size (and leads) determines the power dissipation of a resistor and to keep it in a safe temperature (i.e 70 Celcius). The rated wattage vs size it is linear and for example wirewound has a proportion of 0,004*Volume of resistor body (mm^3). This is valid for DC only, since in AC the current density it is accumulated in surface area reducing current carrying capabilities. – GR Tech Nov 1 '15 at 20:09
• – Fizz Nov 23 '15 at 2:15

You cannot measure the wattage of a resistor with a meter. It is a function of its mechanical properties alone, how much heat it can dissipate, what temperature it must stay below.

Until you have some experience looking at resistors and saying 'oooo, that's about 0.1 watts for a chip resistor this small, or about 10 watts for a wire-wound, metal cased, heat-sunk resistor about that big', you must read the data sheet for the resistor, or the description on the site you're buying it from.

You must exercise care when trying to compare an unknown resistor in your hand to a picture on a data sheet. It's better than nothing, but maximum permissible temperature depends on the materials used as well, so it's not 100% foolproof. Any resistors you buy, keep them identifiable. Any random ones you get, guess what type it is from data sheet pictures, and then work to half that power for safety.

You could alternatively fit a thermometer to it, decide a permissible temperature rise, then gradually increase the power you supply to it until it reaches your permissible temperature.

Be aware that the headline figure for power is usually for an isolated resistor in a 25C ambient, so you need to read the data sheet for real world applications. If you are building commercial equipment, you will probaly derate the power they suggest further to allow for extra reliability.

Be aware that in some fonts, the character $\Omega$ is erroneously rendered as W (I've seen this painful error on so many supplier sites)

• I've never seen W instead of Ω but ω is the lower case of the Greek letter Ω (omega). Would that somewhat explain it? – Transistor Nov 1 '15 at 9:42
• @transistor yes – Neil_UK Nov 1 '15 at 10:47
• you can somewhat guess the wattage of a resistor by comparison with a resistor of known wattacge of similar size and construction. – Jasen Nov 1 '15 at 11:00
• This image can give you some idea resistor power rate visually. Click here – BloodOnMyBlade Nov 1 '15 at 12:52
• As @Jasen says you can get an reasonable idea by comparing data sheets of similar parts- best to assume the lowest plausible rating for a similar part. Sometimes a resistor that looks like a 1/4-W resistor can be made to dissipate 1/2-W (it will have to run very, very hot so special materials and it might tend to discolor the PCB) but it's best to assume that if it looks like most 1/4-W resistors it's good for 250 (or 200 if you're being cautious) mW at 70°C Ta. (note that power resistors can be rated at as low as 25°C Ta, as user44635 says- which means the usable rating is less. – Spehro Pefhany Nov 1 '15 at 13:00