0
\$\begingroup\$

I have some carbon film resistors out of an electronics kit I got given and for a project I need 1/2 Watt of 4k7. Is there anyway I can test to see the power rating? I don't have any other resistors in stock atm so can't use 2 2k2 with 1/4 Watt.

\$\endgroup\$
4
  • \$\begingroup\$ You could put 1/2 w on it and see if it gets hot. If it does, unhook it and go to the store. \$\endgroup\$
    – Kellenjb
    Apr 25, 2011 at 14:57
  • \$\begingroup\$ Thats not the real problem I will if I need to I wanted to see if there was a quick test to see if you could calculate the power of a given resistor. \$\endgroup\$
    – Dean
    Apr 25, 2011 at 15:12
  • \$\begingroup\$ Standard run-of-the-mill carbon resistors seem to be 1/4W. \$\endgroup\$
    – starblue
    Apr 25, 2011 at 17:41
  • \$\begingroup\$ they appear bigger than my other 1/4 watt ones. I've bought some now and done the job. \$\endgroup\$
    – Dean
    Apr 25, 2011 at 18:00

2 Answers 2

2
\$\begingroup\$

Unfortunately answer is no. There is no quick test you can perform to figure out the wattage of the resistor. Based on the appearance you can make an educated guess, but that's all it is, a guess. The only way to be 100% sure is if it's marked or you happen to have the manufacturer's part number and can look it up in the datasheets.

\$\endgroup\$
2
  • \$\begingroup\$ Thats the problem they were just in a box loose. So I don't know the manufacturer so can't find a datasheet. I guess I will have to go buy 1 or maybe 100. \$\endgroup\$
    – Dean
    Apr 25, 2011 at 15:33
  • \$\begingroup\$ This is often a problem in labs when you have loose components, but no idea what the part number/specs are. I tend to throw these away - tractability or nothing. \$\endgroup\$
    – Oliver
    Sep 23, 2014 at 14:58
1
\$\begingroup\$

If you have a variable power supply, increase the voltage across it until it is dissipating 1/2W, and see if it is getting hot. About 17V will be required. You should be able to judge the power rating by the temperature rise at a given voltage (W = V^2/R)).

Otherwise, put it in the circuit, switch on for a short while and see if it gets hot. If the ones you have are about 10 mm long, they are probably 1/4W devices.

If you test one and it doesn't have a high enough power rating, you could put four in series-parallel, if you have them, to increase the wattage.

\$\endgroup\$
4
  • \$\begingroup\$ "Is there anyway I can test to see the power rating?" You didn't answer his only question. \$\endgroup\$
    – Kellenjb
    Apr 25, 2011 at 15:05
  • \$\begingroup\$ @JRobert Because you don't know what the max temp the resistor is designed for. Some resistors can handle temps that are much higher than others, so what's "hot" for one resistor might not be "hot" for another. Also, most resistors are too small to accurately measure their temperature without expensive gear so "see if it is getting hot" usually means touching it and saying "ouch". This is very inaccurate. \$\endgroup\$
    – user3624
    Apr 25, 2011 at 15:58
  • \$\begingroup\$ How is "see if it is getting hot" not a way to test it? It is a pretty good guide to whether the resistor sufficiently dissipates the applied power. And yes, too hot is subjective, at least until the the resistor destroys itself or something else. A "rating" is just that - someone's "estimate of goodness" for a particular purpose, not an inherent and measurable physical property. \$\endgroup\$
    – JRobert
    Apr 25, 2011 at 16:01
  • \$\begingroup\$ I would consider "ouch" to be hotter than I want a component to run (assuming it's not meant to be a heater). Not to be argumentative; just to point out that this can be judgement call. That's where I'd make mine in most cases. \$\endgroup\$
    – JRobert
    Apr 25, 2011 at 20:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.