MFR0623 Axial resistors are only 1/4W rated

[Faringdon]

Hi,
MFR0623 axial resistors are only 1/4W rated.
However, they are 6mm x 2.3mm in dimension, and have 0.6mm metal leads.

When you think that a 2512 SMD resistor is only 6.4mm x 3.2mm, and less than 0.5mm thick,  and its 1W rated...how is it that the MFR0623 performs so poorly with power?

MFR0623
https://www.digikey.co.uk/en/products/detail/viking-tech/MFR0623FTEV1651/12141860

And strangely, the SFR25 axial resistor, is the same size as the MFR0623, and yet is rated at 400mW.

SFR25 resistor
https://www.vishay.com/docs/28722/sfr16s25.pdf

Something just doesnt add up...do you know what it is?

[dmills]

Rated at what ambient temperature? With how long leads? At what case temperature?

Seriously, modern resistors can, and often are, rated to run way hotter then you would want the FR4 to reach, think way above the glass transition temperature.... The resistors will often take it, but will the PCB?

Any sane datasheet will have curves for derating and temperature rise, it is on the engineer to take that information and use it.

It is not a case that you should take the number on the first page of the datasheet at face value (Ever, for anything), you been playing in this game for many years, you should know this. 

[Siwastaja]

Internal design of the resistor is of paramount importance:
* how much temperature can it take without changing in resistance?
* does it hotspot?
* over how large area is heat actually generated?
* how well does it couple into leads?

This is why you see large differences in resistor power ratings even given exact same surface area. Best first-order approximation is to trust the datasheet.

[ConKbot]

For the SMD 1W resistors, read the datasheet. How many square inches of 2oz copper do they want in the footprint? Even Vishay CRCW series are cheeky and just say "it depends on the thermal resistance not the assembled resistor together with the PCB" not particularly great.

[Siwastaja]

Yeah, for SMD resistors layout is important. Datasheets usually assume a 2-layer design, so one can see unexpectedly good performance with a 4-layer PCB where the internal ground plane sucks heat through the thin prepreg from the top layer pads. But even then the pads should connect to some larger fills, and 2oz definitely helps over 1oz.

I have tested a small 0603 resistor at some ridiculously high power level like 1-2W (I forgot the actual value) and surprisingly no damage for hours, on a 4-layer board.

[Gyro]

This is just another example of Faringdon/treez working up to finding some percieved 'validation' for using the wire ended axial part outside its specification. I'm sure the follow-up post will be along shortly.

[DavidAlfa]

Carbon or metal film? Metal ones can work at much higher temperatures.
I've seen small ceramic resistors, in a size normally rated for 1/4W, rated for 2W, only needing long legs to prevent the pcb from being damaged by the heat.

[tggzzz]

This is just another example of Faringdon/treez working up to finding some percieved 'validation' for using the wire ended axial part outside its specification. I'm sure the follow-up post will be along shortly.

Seconded
... except that there will probably be (if there isn't already) a very near duplicate post somewhere.

Today's examples are
https://www.eevblog.com/forum/projects/probing-noisy-signals-to-the-scope/msg5145303/#msg5145303
https://www.eevblog.com/forum/beginners/wound-torroids-not-useable-as-scope-lead-common-mode-chokes/msg5145261/#msg5145261

For background, see https://www.eevblog.com/forum/renewable-energy/mains-zero-cross-detector-pulse-issue/msg5127057/#msg5127057

[Faringdon]

Thanks, we have to re-do a cct but not change the PCB....i'd like to use a 39k axial resistor which is approx 6mm x 2.3mm but its only rated 250mW...and it would have 280mW in the application.......the "250mW" rating sounds so incredibly low, that i was wondering if this is more to do with the "must run at 250mW or less if its tolerance is required to stay within 1.1% of the  stated value", over 15 years, etc etc" type of thing.......its like sometimes, they want you to run it at low power so it stays within some marvellous tolerance bandwidth...but in one's application, this is not important.

The "2512 SMD with cooling copper" post above made me smile, thanks for it.....i got slane at one place for my suggestion that a 2512 was only 1W if it had the right amount of PCB copper with it. the rant went like this... "i've had 20 years working on SMPS for space industry, so i know 2512 can do 1W with no extra cooling copper on PCB".

[dmills]

Anyone doing reliable design will be derating to something temperature dependent but well below 250mW, or picking a part with more power handling.

Personally I tend to think hard if power handling exceeds about 50% of datasheet most of the time, gets a bit more difficult when thermal derating and things like second breakdown come out to play, but over rated is always better then ragged edge.

[Gyro]

Thanks, we have to re-do a cct but not change the PCB....i'd like to use a 39k axial resistor which is approx 6mm x 2.3mm but its only rated 250mW...and it would have 280mW in the application.......the "250mW" rating sounds so incredibly low, that i was wondering if this is more to do with the "must run at 250mW or less if its tolerance is required to stay within 1.1% of the  stated value", over 15 years, etc etc" type of thing.......its like sometimes, they want you to run it at low power so it stays within some marvellous tolerance bandwidth...but in one's application, this is not important.
...

... and there's that follow-up post. Totally predictable!

Study and abide by the datasheet, unless you somehow know more about the part than the manufacturer does.

[ebastler]

i'd like to use a 39k axial resistor which is approx 6mm x 2.3mm but its only rated 250mW...and it would have 280mW in the application

You can get resistors of that size (or minimally larger, 6.5 mm * 2.5 mm) with 500 or 600 mW power ratings. Vishay MBB/SMA 0207 or Vishay MRS25, for example; I'm sure there are many others. You still need to look into derating, of course.

[Siwastaja]

When tempted to go over the ratings (or even very close to the ratings), never assume the ratings must be conservative because they seldom are. Do thorough testing. For example, if you feel like you could use a 250mW rated resistor at 300mW in your product at worst-case ambient temperature of +40degC, build a test unit where you run it at 500mW at ambient temperature of +60degC for a week. Then measure if it's still within spec. If not, you have too little margin.

After that test, don't assume similar (even nearly identical) parts behave the same. Don't second source / swap parts without new qualification. Even batch changes have some non-zero risk.

[sparkydog]

I'd like to use a 39k axial resistor which is approx 6mm x 2.3mm but its only rated 250mW

LR1F39K; 6.2mm × 2.3mm, rated for 600 mW. Here are some additional options.

Alternatively if this is for a low volume board, you might be able to solder leads to an SMD resistor...

[Siwastaja]

Alternatively if this is for a low volume board, you might be able to solder leads to an SMD resistor...

Yeah, although if these leads are thin, it's going to seriously reduce the power dissipation capability of the resistor, and if they are thick, the resistor body or solder joints will crack in no time no matter how careful you are with handling it.

[The Chump]

https://www.vishay.com/docs/28729/pr010203.pdf
job done

[Wil_Bloodworth]

I just got these (my first order ever) resistors from DigiKey and was very surprised that they were different sizes even thought they're all 1W resistors!



- Wil

[rsjsouza]

I just got these (my first order ever) resistors from DigiKey and was very surprised that they were different sizes even thought they're all 1W resistors!

- Wil

Completely different insulation resistance voltages; the Yageo is rated for up to 600V while the Vishay is rated for up to 10kV.

[floobydust]

Resistor power ratings are full of bullshit and tomfoolery nowadays.
Antique IEC standards manufacturers do pick and choose what clauses and test methods they want to mention about a part.

In the 80's I was taught to never run a resistor at rated power, derate by a half. Now, that rule of thumb is useless.

I got tired of buying resistors rated 1/2-3W that were tiny, 1/3 the size of what I was used to. 
Massive trap for noobs (or cheap OP) with "supermini" sized parts miraculously having the same power rating. But when you dig into the specs and charts, the smell of bull comes in.
And all manufacturers are playing the game now, stretch the specs to appear competitive.

[tggzzz]

Resistor power ratings are full of bullshit and tomfoolery nowadays.
Antique IEC standards manufacturers do pick and choose what clauses and test methods they want to mention about a part.

In the 80's I was taught to never run a resistor at rated power, derate by a half. Now, that rule of thumb is useless.

I got tired of buying resistors rated 1/2-3W that were tiny, 1/3 the size of what I was used to. 
Massive trap for noobs (or cheap OP) with "supermini" sized parts miraculously having the same power rating. But when you dig into the specs and charts, the smell of bull comes in.
And all manufacturers are playing the game now, stretch the specs to appear competitive.

Oh, that's overstating it!

One aspect to consider is that the materials are better characterised and the manufacturing processes are tighter, which enables both tighter tolerances[1] and operating closer to a limit.

It has always been necessary to read and understand the specs in sufficient detail that you understand what isn't being guaranteed[2]. That's much like legal laws - and some lawyers make a good living from exploiting what isn't covered. Yes, there are similarities between the mental processes in experienced engineers and experienced lawyers

[1] e.g. electrolytic capacitors used to be -10% +200%, but now they are +-10%.

[2] e.g. MLCC capacitors with a DC bias

[floobydust]

Na, they hide a lot of it, it's a secret. Although OP could calculate the operating temperature of the resistor- assuming he can do math.

One big difference is the lead diameter and material - compare SFR25 with SFR25H and you can see tinned-copper lead option better conducts heat to the PCB giving the +20% difference.
Modern resistors have steel leads (do a magnet test) and the "Viking" lead diameter is -22% so a skinny, steel lead along with low tech epoxy.

[Siwastaja]

That old rule of thumb was useless to begin with. You can see so many failed products where designers used all sorts of rules of thumbs instead of proper design procedures. Thermal design is difficult, it requires both understanding and hard work. New resistors are not any more difficult to design in than the old ones.

[ebastler]

Resistor power ratings are full of bullshit and tomfoolery nowadays.
Antique IEC standards manufacturers do pick and choose what clauses and test methods they want to mention about a part.

In the 80's I was taught to never run a resistor at rated power, derate by a half. Now, that rule of thumb is useless.

I got tired of buying resistors rated 1/2-3W that were tiny, 1/3 the size of what I was used to. 
Massive trap for noobs (or cheap OP) with "supermini" sized parts miraculously having the same power rating. But when you dig into the specs and charts, the smell of bull comes in.
And all manufacturers are playing the game now, stretch the specs to appear competitive.

Could you be more specific? Despite "they are much smaller than what I was used to", what are the indications that the specs cannot be trusted? Which aspects of the specs are muddying the waters in your view?

[floobydust]

Thermodynamics gives the physical size (surface area) of the resistor related to the amount of convection cooling, the lead material (i.e. Cu vs Fe) and diameter for conduction cooling to the PC board.
So when a manufacturer has a magical small size resistor with the same power rating - it's basically bullshit as you can't break the laws of thermodynamics.

I entered the physical size, operating temp, thermal resistance, power at class etc. of many through-hole resistors in a spreadsheet because I am fed up with the game.
For the same power rating, surface area varies almost 5-10x. It's common to order a "1W" part and it's the size of what used to be "1/4W" 

Yes materials are "better" and can operate at higher temperatures which is largely driving the marketing bullshit. The IEC tests are in free-air as I also recall, so exploiting that loophole with high temp materials to give misleading ratings. But getting rid of heat, there is no magic there unless you use a fan lol.

Do any of us actually design resistors to operate at say 150°C? It's totally useless to make a board-mounted resistor run so hot, as to damage FR-4.
The Vishay SFR series are the worst in their supermini size and grandiose power ratings. Do the math, calculate their actual operating temp. at rated power. Look at the graphs very carefully.

[ebastler]

Thermodynamics gives the physical size (surface area) of the resistor related to the amount of convection cooling, the lead material (i.e. Cu vs Fe) and diameter for conduction cooling to the PC board.
So when a manufacturer has a magical small size resistor with the same power rating - it's basically bullshit as you can't break the laws of thermodynamics.

So obviously (and as you state yourself a bit later) those resistors are specified to run hotter, and must be able to withstand that higher temperature. And I don't think the manufacturers make any claims to the contrary.

Do any of us actually design resistors to operate at say 150°C? It's totally useless to make a board-mounted resistor run so hot, as to damage FR-4.
The Vishay SFR series are the worst in their supermini size and grandiose power ratings. Do the math, calculate their actual operating temp. at rated power. Look at the graphs very carefully.

They state the maximum film temperature right on the first page of the datasheet, in a rather prominent place in the "ratings" table. They also give you charts showing hot spot temperature as a function of power dissipation, and of course derating as a function of environmental temperature.

Sure, if you want to push the resistor power spec to the limit it will run hot, and you may not want to use standard FR4, or use a forced airflow. But I don't have any complaints about Vishay and their datasheet.