Vishay - Dale RN55 power rating

I am a bit confused of the power rating of these brown Dale resistors.
If looking at data sheet for RN55 series there is a P70C and P125C rating:
https://docs.rs-online.com/0711/0900766b81410816.pdf

For RN55 the P70C is 1/8W while P125C is only 1/10 W ?
I read it as if you dissipate 1/8W then temperature reach 70 C?
Has the two columns been reversed or how should I interpret the numbers?

For the Wayne linestage I looked for a 15 ohm 0.5W P70C and found a Koa Speer MFS type. When I got them it has approx same physical size as RN55 but can handle 4 times the power without reaching higher temperature than RN55?

It that the technology used? ….e.g. Koa Speer has end caps which could contribute to better power dissipation?

Koa Speer MFS also looks like "good stuff"?
 

Sangram

Moderator
Paid Member
2002-09-25 11:01 am
India
The way to read power ratings off that datasheet is simple:

With an ambient temperature of 70C, you can dissipate a maximum of .125W in a RN55D.

With an ambient temperature of 125C, you can dissipate 0.1W - but you cannot use the RN55D, either a C or an E type.

If you want a resistor that will be able to withstand 1/4W at 125C, use RN65. If you need 0.5W, use RN70. See that datasheet as a selection chart, not a guide to resistor body temperature.

However at these dissipation levels the resistor body temperature will be quite a bit above ambient, IIRC 30 or 40 degrees - i.e, very very hot. Though the resistor will survive those temperatures it will affect components close to it, and the PCB itself will discolour over time.

At 50% continuous dissipation, resistor body temperature is usually 20C above ambient. If possible, continuous dissipation should be 25% of resistor rating for safe overall temperatures, low tempco-related variations, and a good overload envelope. For parts that see high peak to continuous power demands, you can run lower rated resistors as long as total peak power does not exceed the overall resistor rating.
 

coolnose

Member
2009-09-12 9:14 am
MEPER,

take some time and compare CMF vs. RN datasheets, they are the same (at least to me) and the former are derated for meeting various MIL specs...

EDIT: Audiophonics is a french reseller, I would not rely on the specs they provide vs. the Vishay Datasheet - not saying it's a bad company though
 
Last edited:

Dennis Hui

Member
Paid Member
2002-08-27 7:19 pm
Toronto, Canada
As coolnose suggested, please have a look at the CMF datasheet:

http://www.vishay.com/docs/31018/cmfind.pdf

"Except for marking, the Industrial and Military versions are exactly the same."

(According to :
https://docs.rs-online.com/0711/0900766b81410816.pdf

Global model CMF55 corresponds to mil style RN55)

Also the CMF datasheet shows a P_70 rating of 0.5W for CMF55 (so the dealer didn't just make that up).
The derating example section then describes what can happen when the part is run at various
power ratings (up to 0.5W) at 70C ambient.
 

MEPER

Member
2017-11-25 11:50 pm
Copenhagen
Ok.....it makes sense that if run at higher temperatures then if it is just for commercial use then you can tolerate a higher failure rate rather than if it is for military, medical etc.
They could have the same markings on them and just specify in data sheet military or commercial use…..if resistor is exactly the same......then no need for two different markings. It may be a "compliance" issue that there are some strict requirements…..we sometimes see that….even that it seems to make no sense.
 

Sangram

Moderator
Paid Member
2002-09-25 11:01 am
India
Ok....thank you…..so it is ambient.....125C ambient......that is a lot. For our use in audio with ambient maybe 40C max inside a chassis then a RN55 could maybe take 1/4 W without problems.

Ambient is not just the air temperature inside a chassis.

For example, resistors mounted close to power devices using a reasonably thick PCB trace will actually be at a higher local temperature than (for example) one near an exhaust fan. Ambient has to be approximated based on all local conditions.

An example of such applications are source/emitter resistors in power amplifiers, sometimes even gate resistors if they're close to power devices (as they should be).

It is reasonable to estimate all local fault conditions and hotspot possibilities to provide for good reliability.