Vishay Resistor Power Rating Question

I am trying to decide which resistors I need to choose from the Vishay CMF/RN series - should I be looking at the 70 deg C or at 125 deg C  power rating?

For example: if a BOM calls for a 1Kohm, 1/4W, Metal Film, 1% resistor; then in the document (http://www.vishay.com/docs/31018/cmfind.pdf)

- CMF50 is 1/4W at 70c and 1/8W at 125c
- CMF55 is 1/2W at 70c and 1/4W at 125c

To add more confusion - with the RN series (http://www.vishay.com/docs/31027/cmfmil.pdf)

RN60 (CMF60) is 1/4W at 70c and 1/8W at 125c
RN65 (CMF65) is 1/2W at 70c and 1/4W at 125c

Does one typically look at the upper limit at 125c (CMF55 / RN65)?

Any help would be much appreciated

You need to know the temperature each resistor will likely reach which depends on the temperature in the enclosure and the current flowing in the resistor. As a rule, unless it is in a power circuit. most resistors will never get anywhere near 70 degrees.

Cheers

Ian

ruffrecords said:

You need to know the temperature each resistor will likely reach which depends on the temperature in the enclosure and the current flowing in the resistor. As a rule, unless it is in a power circuit. most resistors will never get anywhere near 70 degrees.

Cheers

Ian

Click to expand...

Hi Ian,

Thank you very much for your insight, much appreciated!

Ok that makes sense, the problem is that there isn't usually information on what sort of temperatures these components reach in the circuit and I am not very clued up when it comes to knowing what sort of temperatures these components typically reach in audio circuits - and I didn't want to make a balls up with my projects - the confusion set in when I noticed Mouser and Digikey were quoting different power ratings for the same component - DigiKey seems to quote at 70 deg C and Mouser at 125 deg C

I will keep your rule of thumb in mind then and look at the 70 deg ratings, thank you so much!

dacapitan said:

ruffrecords said:

You need to know the temperature each resistor will likely reach which depends on the temperature in the enclosure and the current flowing in the resistor. As a rule, unless it is in a power circuit. most resistors will never get anywhere near 70 degrees.

Cheers

Ian

Click to expand...

Hi Ian,

Thank you very much for your insight, much appreciated!

Ok that makes sense, the problem is that there isn't usually information on what sort of temperatures these components reach in the circuit and I am not very clued up when it comes to knowing what sort of temperatures these components typically reach in audio circuits

Click to expand...

It's kinda simple if you think about it. Remember that 125C is well above the boiling point of water ... and what audio circuit runs that hot? Even 70C is much hotter than one would expect to see in an audio circuit. Consider that 20C is considered somewhat room temperature.

-a

I do not remember ever worrying about resistor temperature rating. Most resistors will run very near red-heat for a while. The cheaper ones, maybe only hours; some vitreous jobs can run hot enough to broil steaks.

70 degrees C will burn your skin in seconds. Much hotter, you will blister.

The ambient around the resistor is "never" that hot.

(It can be very close in too-tight high-power tube amps.)

It is possible (and usually un-wise) to have a cool chassis and one hot resistor. I built a headphone amp with "10 Watt" resistors dissipating 7 Watts. They got HOT. When working above half the power rating, resistor life may be less than a life-time. In this case, I should have got 20 Watt resistors, and a lot more air-space inside the box.