I have a "typical" 5 ohm resistor. Is there any way I can know the max power it can handle without the smoke test?

Question

I need a 5 ohm resistor that can handle 5 watts. I don't know if the one that I have (a typical one, if that can be said) can handle it. Here's an image of one:

I do not have the datasheet otherwise I could check there.

Answer 1

For through hole carbon film resistors (usually light brown body) there is a conventional power rating for each physical size. According to some datasheets I just checked (links at end of post) 1/8W is 3.2mm long and 1.5mm diameter. 1/4W is 6mm long and 2.3mm diameter, 1/2W is 8.5mm long and 2.7mm diameter. 1W is 12mm long and 5mm diameter. In my experiance by far the most common size is 1/4 watt.

Metal film resistors (usually light blue body) sometimes have a higher rating for a given size. It's fairly common to see metal film 0.4W in the same body size as a carbon film 1/8W and metal film 0.6W in the same body size as a carbon film 1/4 watt. However this is not always the case.

The conventional through hole film resistors seem to stop at just over 2W.

Beyond that you tend to get into wirewound resistors. Smaller wirewound resistors are often blocks of a cement-like material. Downside of these is there is no good way of cooling them. Larger wirewound resistors often have a metal outer case designed to be bolted to a heatsink. Be aware with this style that they actually do expect you to bolt them to a heatsink and if you simply operate them in free air their power handling is singificantly reduced.

Resistors will usually tolerate some overload in the short term. Just how much I don't know.

http://www.rapidonline.com/pdf/64-0014e.pdf http://www.rapidonline.com/pdf/62-0310e.pdf http://www.rapidonline.com/pdf/62-0500e.pdf http://www.farnell.com/datasheets/91309.pdf

Answer 2

Typically, 5W might be a bit much. If it's just a regular axial lead, you're probably not gonna handle 1A through it.

But, to be more scientific, you would just want to put enough current in it to get to something like 75C. You can measure this with a thermocouple, an infrared camera, or "heat wax". If you're nowhere near 1A when you get to that point, I would seek out a different resistor.

I use these all the time and they can handle plenty of power.

Answer 3

What you are probably calling a "typical" one is probably 0.25W or thereabout. A 5W resistor is normally a wirewound resistor that will look like this and be marked with its power rating:

Photo taken from http://www.me.umn.edu.

Answer 4

There is no one "typical" resistor. Common packages are 0805, which are good for around 1/8 W, and 0603 which are limited to less.

A 5 W resistor is going to be physically big and should not by "typical" in most senses. Even just 2 W resistors come in 1210 packages, and then can only do 2 W with proper pads and heat management. At 5 W probably implies thru hole. I have some 2 W thru hole resistors here that are over 1/2 inch long and maybe 1/4 inch in diameter. A 5 W resistor will be bigger.

Note that you can make a effective 5 W resistor by parallel and series combinations of smaller (power wise) resistors.

Added:

Now that you have posted a picture, we can see your resistor isn't typical at all. That is a old thru-hole 1/4 Watt resistor, which hasn't been "typical" for a long time. 1/4 W thru-hole resistors are only used in rare cases today, like old legacy products where it is not worth the expense to redesign them with modern parts and by hobbyists. I don't know when the volume of SMD resistors exceeded that of 1/4 W thru-hole, but I suspect it was at least 20 years ago.

Answer 5

Re your picture, that looks more like 5.1K ohms to me! Unless I'm missing a band. And that means you'll have to put 160v across it for it to have to dissipate 5w...