Safe temperature range for Metal Film Resistors?

[RobAinscough]

I put a FLIR and temp probe and on my circuit and noticed one of my 470 Ohm resistors heated up to about 117F (47C). What are desirable temp ranges for metal film resistors?

Cheers, Rob.

EDIT: 9V 0.5A circuit using 555 IC and a few LEDs, caps, resistors

 

[KlausST]

Hi,

If I wanted to know I'd read the datasheet.

Klaus

 

[RobAinscough]

I don’t have a data sheet for the ones I purchased but they are from China and similar sources indicate max range -55C to 150C.

However no information is provided on “desirable” temp, per my original question.

Is your indirection out of frustration... my apologies if this gets asked frequently.

Cheers, Rob

 

[BigBoss]

Typical Power Derating Curve is as follows.This is a standard VISHAY Metal Film Resistor that has 150^oC max. Ambient Temperature.
47^o is a little bit warm but not dangerous as thought.

 

[Easy peasy]

it's very much designers choice - if you have a whole lot of parts running at a point where they would give 70 deg C on a pcb - then the middle components will be quite a bit hotter, etc, you get the idea, standard rule of thumb for 0805' 1206's is to run at o more than half power rating, e.g. 40-50mW for 0805 ...

 

[KlausST]

Hi,

However no information is provided on “desirable” temp, per my original question.

Is your indirection out of frustration... my apologies if this gets asked frequently.

No. No frustration. It was meant seriously.
We do have less information than you. The datasheet is one of the most reliable sources of information.

But I agree "desirable" is not written in the datasheet.
So my opinion on that:
At first I wanted to answer "as low as possible"(usually below 50°C), because I am a designer for "long life" industrial electronics. (Someties for clean room environment)
I've seen resistors that were specified for temperatures > 120°C. They probably never were >60°C but the PCB they were monted on looked like it was burnt with >300°C.
The problem is "time" (besides of PCB material). The resistor was "warm" 24/7 over ten years.

So if reliable long life is "desirable" on a continously hot resistor, then low temperature is desirable.
But also low cost may be desirable, short life time (planned obsolescence), heating of a PCB that is operated in cold environment, small size, or when the resistor is hot just for seconds a day...
I've seen sensors using hot resistors as inert gas sensor or as gas flow sensor..

So for most designers "<50°C" is not desirable.

Klaus

 

[schmitt trigger]

To elaborate on what Klaus mentions, I also have seen many burned out PWBAs by resistors which have running warm over a long period of time.

If you would like to run a thru hole resistor hot, use ceramic spacers on the leads, such that the resistor body is separated from the PWBA by at least one centimeter.

 

[RobAinscough]

Excellent information all, much appreciated. Looks like I'm "ok" on the resistor temp ... just curious are there "Brand" specific resistors that are better than other brands?

Cheers, Rob.

 

[c_mitra]

Device temp, by itself, does not mean much unless it is close to the absolute limit. For metal film resistors the absolute temp working limit will be well above 100C and you are safe to be around 47C. What is relevant is the effective dissipation which depends on the difference of the ambient and device temps.

If you have taken the measurement after sufficient time (all temps have reached a steady state) - from power up- then the resistor is effectively dissipating the heat produced internally into the ambient air. If the ambient temp is 25C and the device temp is 50C, you need to estimate the actual power lost in the device and compare that with the nominal rating of the device.

It is possible for a 0.5W resistor to dissipate 1W and still be within the safe temp limit (often happens when the ambient temp is low or a fan is used for cooling). But to prevent permanent damage, it is good to stay within the power handling range of the resistor.