MFR0623 Axial resistors are only 1/4W rated - Page 1
Forum section: electronics / beginners

author: Faringdon
author_rank: Super Contributor
timestamp: October 31, 2023, 10:21:38 pm
content: 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?MFR0623https://www.digikey.co.uk/en/products/detail/viking-tech/MFR0623FTEV1651/12141860And strangely, the SFR25 axial resistor, is the same size as the MFR0623, and yet is rated at 400mW.SFR25 resistorhttps://www.vishay.com/docs/28722/sfr16s25.pdfSomething just doesnt add up...do you know what it is?

author: dmills
author_rank: Super Contributor
timestamp: November 01, 2023, 09:53:55 am
content: 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.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 01, 2023, 11:42:31 am
content: 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.

author: ConKbot
author_rank: Super Contributor
timestamp: November 01, 2023, 12:22:34 pm
content: 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.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 01, 2023, 12:38:46 pm
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: November 01, 2023, 01:00:51 pm
content: 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.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 01, 2023, 01:11:55 pm
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: November 01, 2023, 01:55:38 pm
content: Quote from: Gyro on November 01, 2023, 01:00:51 pmSeconded... except that there will probably be (if there isn't already) a very near duplicate post somewhere.Today'sexamples arehttps://www.eevblog.com/forum/projects/probing-noisy-signals-to-the-scope/msg5145303/#msg5145303https://www.eevblog.com/forum/beginners/wound-torroids-not-useable-as-scope-lead-common-mode-chokes/msg5145261/#msg5145261For background, seehttps://www.eevblog.com/forum/renewable-energy/mains-zero-cross-detector-pulse-issue/msg5127057/#msg5127057

author: Faringdon
author_rank: Super Contributor
timestamp: November 01, 2023, 03:14:07 pm
content: 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".

author: dmills
author_rank: Super Contributor
timestamp: November 01, 2023, 04:47:18 pm
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: November 01, 2023, 05:03:00 pm
content: Quote from: Faringdon on November 01, 2023, 03:14:07 pm... 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.

author: ebastler
author_rank: Super Contributor
timestamp: November 01, 2023, 05:56:57 pm
content: Quote from: Faringdon on November 01, 2023, 03:14:07 pmYou 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.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 01, 2023, 07:37:40 pm
content: 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.

author: sparkydog
author_rank: Regular Contributor
timestamp: November 01, 2023, 09:49:02 pm
content: Quote from: Faringdon on November 01, 2023, 03:14:07 pmLR1F39K; 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...

author: Siwastaja
author_rank: Super Contributor
timestamp: November 02, 2023, 11:43:44 am
content: Quote from: sparkydog on November 01, 2023, 09:49:02 pmYeah, 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.

author: The Chump
author_rank: Contributor
timestamp: November 02, 2023, 11:10:55 pm
content: https://www.vishay.com/docs/28729/pr010203.pdfjob done

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 05, 2023, 05:52:47 pm
content: 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

author: rsjsouza
author_rank: Super Contributor
timestamp: November 05, 2023, 06:00:12 pm
content: Quote from: Wil_Bloodworth on November 05, 2023, 05:52:47 pmCompletely different insulation resistance voltages; the Yageo is rated for up to 600V while the Vishay is rated for up to 10kV.

author: floobydust
author_rank: Super Contributor
timestamp: November 05, 2023, 06:07:04 pm
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: November 05, 2023, 06:20:51 pm
content: Quote from: floobydust on November 05, 2023, 06:07:04 pmOh, 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 and operating closer to a limit.It has always been necessary to read and understand the specs in sufficient detail that you understand whatisn'tbeing guaranteed. That's much like legal laws - and some lawyers make a good living from exploiting whatisn'tcovered. Yes, there are similarities between the mental processes in experienced engineers and experienced lawyers e.g. electrolytic capacitors used to be -10% +200%, but now they are +-10%. e.g. MLCC capacitors with a DC bias

author: floobydust
author_rank: Super Contributor
timestamp: November 05, 2023, 06:24:57 pm
content: 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.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 05, 2023, 07:12:11 pm
content: 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 isdifficult, it requires both understanding and hard work. New resistors are not any more difficult to design in than the old ones.

author: ebastler
author_rank: Super Contributor
timestamp: November 05, 2023, 07:38:42 pm
content: Quote from: floobydust on November 05, 2023, 06:07:04 pmCould 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?

author: floobydust
author_rank: Super Contributor
timestamp: November 05, 2023, 08:00:57 pm
content: 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.

author: ebastler
author_rank: Super Contributor
timestamp: November 05, 2023, 08:33:07 pm
content: Quote from: floobydust on November 05, 2023, 08:00:57 pmSo 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.QuoteThey 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.

author: johansen
author_rank: Super Contributor
timestamp: November 06, 2023, 03:58:51 am
content: I have to say i successfully guessed the OP was farringdon from seeing nothing more than this on my phone :"MFR0623 axial resistors are only 1/4W rated.However, they are 6mm x 2.3mm in dimension,"

author: tggzzz
author_rank: Super Contributor
timestamp: November 06, 2023, 08:54:28 am
content: Quote from: johansen on November 06, 2023, 03:58:51 amAccurate, and a good snark

author: Xena E
author_rank: Frequent Contributor
timestamp: November 06, 2023, 09:10:56 am
content: To Faringdon:Snip the leads close to the body of the TH resistor and blob the SMD part in the gap, use HMP solder, and the jobs a goodun' lover boy.☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆To the Forum:The real issue here is that the manufacturers are going to rate their components according to any maxima that they are capable of, given the materials of construction: if nothing else it's good marketing.The problem is that it doesn't help the designer who doesn't fully read the spec., sheets; it's a bit of a schoolboy error to assume for instance that power ratings in any particular resistors mean they are suitable for any duty provided that they meet V•I calculations.In the conjunction that you go to the manufacturers data looking for suitability for a particular application, a temperature rise figure in free air is the nearest thing that would be of any use, and specifying maximum dissipation at an arbitrary ambient temperature may be a good quick look up, but wouldn't make good headline figures for them.Floobydust's comments are valid, and I've sometimes shared that frustration with the 'banner ratings', though spec., sheets generally aren't BS however, the relevant information is usually down to the user to extrapolate, or they Gotcha! We've all seen burned up badly designed boards.I will second the creation of spreadsheets for resistor ratings; something we also do for use in higher power scenarios, it's only a minor PITA.OT:The real problem that I have connected with this, particularly with SMD, is when the bean counters start questioning why, (for example), a design may have 10 parallel 10ohm 1W resistors when 1 single 1 ohm one will 'do'. I now refuse to repeat myself to them, but do have a previously enjoyed list of indexed design decisions on the system, and their reasons for existing. I just answer the official queries with bullet point references, (it saves typing, or even cutting and pasting as I do it from a crib sheet thats bluetacked behind my desk), also each time tell them its outside their remit to question engineering decisions: with little knowledge being a dangerous commodity.** anyone who has this particular time suck to deal with, can use this idea with free licenceit let's the bean counters know that you consider their existence being as pointless to you as indicators on a BMW.RegardsXena.

author: tggzzz
author_rank: Super Contributor
timestamp: November 06, 2023, 09:31:26 am
content: Quote from: Xena E on November 06, 2023, 09:10:56 amYou aren't finking of anyone here, are you? Shurely shome mishtake!My pet amusement is those fleabay/aliexpress impossibly small switching PSUs claiming 6V-100V -> 5V 3A. The phrase "getting burned by" isliterallyrelevantGood luck mounting them or attaching a heatsink!EDIT: changed tag from [ i ] to [ img ], doh!QuoteWhat is the sound of two birds being killed with one well-aimed stone

author: ebastler
author_rank: Super Contributor
timestamp: November 06, 2023, 09:51:43 am
content: Quote from: Xena E on November 06, 2023, 09:10:56 amWhat kind of shadow-boxing discussion are we having here?The very first post by Faringdon did link to the relevant Vishay datasheet.And that datasheet does, of course, show hotspot temperature as a function of power dissipation, derating as a function of environmental temperature etc.. And it does point out the (rather high) maximum film temperature which they are basing their specs on, right on the first page in a prominent place.The insinuations that (a) manufacturers are not telling the truth, and/or (b) engineers are too dumb to check the datasheets, are just not correct.

author: Xena E
author_rank: Frequent Contributor
timestamp: November 06, 2023, 12:05:15 pm
content: Quote from: ebastler on November 06, 2023, 09:51:43 amNo shadow boxing...No assertion that the manufacturers are pedalling bullshit... (reputable ones like Vishay anyhow).Only labouring the point that designers need to carefully read the specs and translate them to their applications rather than go by the banner specification. It's a PITA but has to be done.I deduce that someone is either:A friend of Faringdon.Or:Drives a BMW?

author: ebastler
author_rank: Super Contributor
timestamp: November 06, 2023, 12:17:33 pm
content: Quote from: Xena E on November 06, 2023, 12:05:15 pmNot really, and no.But while Faringdon has often posted questions which were apparently off the cuff, without doing much research of his own before -- I wanted to do him justice in the present case, where hedidlink to a relevant datasheet right away.And in a way, the subsequent discussion validates Faringdon's initial assumption: Besides real differences in resistor design (and specifically the acceptable operating temperature), there also are varying degrees of bullishness among manufacturers. Hence specs cannot alway be compared at face value, but only after looking into the spec details -- if the manufacturer provides them.

author: Xena E
author_rank: Frequent Contributor
timestamp: November 06, 2023, 12:22:42 pm
content: Quote from: ebastler on November 06, 2023, 12:17:33 pmExactly my point concisely put, thank you.

author: Xena E
author_rank: Frequent Contributor
timestamp: November 06, 2023, 12:36:01 pm
content: Quote from: tggzzz on November 06, 2023, 09:31:26 amYour deduction is correctYour comments on the ebay crappo is also amusing as well as accurate... I've had that shit held up as an example to me before, with comments such as if thay can do it why can't we?We can do it! But do "we" want to ruin "our" reputation?And to be clear I actually like proper BMW cars (RWD with at least a straight six), just not some of the cocks who drive them.

author: tggzzz
author_rank: Super Contributor
timestamp: November 06, 2023, 01:07:55 pm
content: Quote from: ebastler on November 06, 2023, 09:51:43 amThe evidence is against you, in this thread at least!

author: floobydust
author_rank: Super Contributor
timestamp: November 06, 2023, 06:29:04 pm
content: Trusting the resistor manufacturer's data I think is bad idea. Worse now with these super/ultra mini parts.Did you see the picturePost #16YageoFMP100"1W"? It's the size of typical 1/4W part lol.This is what I mean for the race to the bottom- for size, cost requiring some deception because one manufacturer does it and others follow."Ultra miniature size" gives the usual derate curve above 70°C - yet Yageo gives NOTHING about temperature rise. Same for YageoMFR Series.VishaySFR25rated high 200K/W (in unknown test jig) so 80°C rise at rated 0.4W so what is the point of this part being good for 155°C if it's not going to run that hot? Why does it need the high temp materials then?And how is the surface temperature test performed? Free air? On a PCB? Big copper traces/pads?To what standard jig are they measuring this? IEC 60115-1 4.14 is not mentioned. It's important the test is realistic, practical as to how EE's will be mounting them, and that the temperature rise spec is real. Right now, EE's are clueless, naive, blindly trusting about the billboard power rating and it's just another marketing trap.Vishay is good enough to provide temp rise numbers but we don't know the test conditions, if they are practical or magical values.

author: ebastler
author_rank: Super Contributor
timestamp: November 06, 2023, 07:01:00 pm
content: Quote from: floobydust on November 06, 2023, 06:29:04 pmAre you now complaining thatthey are not running them hot enough?Come on, you must have come across the difference between "absolute maximum ratings" and "recommended operating conditions" in other datasheets before. Would you prefer if they had specified it such that it would reach 155°C when run at the specified power at room temperature?

author: floobydust
author_rank: Super Contributor
timestamp: November 06, 2023, 07:32:19 pm
content: Well, how hot is too hot - for a manufacturer to shrink a component's size and hide the fact it will burn your finger?Also pointing out the illogic in the datasheet's numbers or the part's claims = marketing bullshit.Rated resistor power is P70or in a 70°C ambient, SFR25 is "0.4W". As a max. spec it appears to be achieved operating at 155°C, for the billboard power rating.Temperature rise 200K/W says it's gonna rise 80°C above ambient... but derate the part above 70°C ambient. So the numbers appear off.Effectively, this part is _W.Can we figure this out, for the beginners and noobs to avoid the trap.

author: ebastler
author_rank: Super Contributor
timestamp: November 06, 2023, 07:41:58 pm
content: Quote from: floobydust on November 06, 2023, 07:32:19 pmNow you got me confused. 70°C ambient plus 80°C above ambient (at 0.4W rated power) equals 150°C operating temperature. Is your concern that it is 150°C vs. 155°C maximum permissible film temperature?

author: Siwastaja
author_rank: Super Contributor
timestamp: November 07, 2023, 07:07:06 am
content: Quote from: floobydust on November 05, 2023, 08:00:57 pmWhat bullshit you are spewing.Show me the law of thermodynamics that says a certain size of resistor can dissipate certain amount of power. Obviously it doesn't exist.The following can be different:* Maximum allowable temperature for internal materials. A simple example: polyethylene manages maybe 100degC without melting, but glass melts at 1700degC.* Evenness of thermal dissipation. If the resistor hotspots somewhere, said maximum material temperature is locally exceeded even at lower average power density.* Thermal conduction from the internal element to the surface* Surface emissivity of the resistorNew resistors with increased power density are just better. And as a designer, your job is to make sure your surrounding parts can take the power, too. Nothing's changed; if you assumed with old resistors "oh, the resistor can take it, so my PCB can, too", you ended up with charred FR4. Many such examples from 1980's.QuoteTrusting manufacturer data has served me pretty well for two decades, but before you can trust something, you have to understand what it means. It's a well known fact that using component datasheets requires some care and expertise, but that is not limited to resistors (or "new" resistors). For example, MOSFET datasheets are among the least understood IMHO.But quite frankly, if 70+80=150 < 155 makes you confused, I don't know what to say. Maybe you should be doing something else? In my opinion, resistors front-page-rated to70degCambientis quite fair. Many MOSFET front page specs are atjunctiontemperature forced to25degC!

author: Siwastaja
author_rank: Super Contributor
timestamp: November 07, 2023, 07:16:58 am
content: Quote from: tggzzz on November 06, 2023, 09:31:26 amYeah. And actually their power density (conversion power, not dissipated power) isn't impossibly high at all; if they were modern-day high-efficiency designs, something like 5V 3A would be easy-peasy in that form factor (I remember doing 5V 8A in half that size or so). But they are not modern; they aren't even synchronous (so diode loss at 5V is massive, and that small poor diode definitely can't handle 3A), so they simply dissipate too much for given output. Solution: do not heatsink, fix them to dissipate less!The best kind of thermal design is minimizing dissipation to begin with, i.e. aiming for maximum possible efficiency, and because thermal design is difficult to some people (as evidenced in this thread) and heatsinks (plus mounting materials) expensive, total cost is usually lower even if you have to spend a bit more on silicon.And this is what can be seen especially in power electronics designs (PSUs, amplifiers, motor drives etc.). 1980's stuff, linear or switched, had those large TO3 transistors, those "good old" toasty power resistor which toasted the FR4 or even melted the solder joints. Compared to that, even that Chinese module doesn't look too bad, and if you go from a $0.50 design to a $1 design, it can run pretty cool without heatsinking at all beyond the PCB itself. I'm pretty happy with the current times.

author: floobydust
author_rank: Super Contributor
timestamp: November 07, 2023, 08:02:29 pm
content: Quote from: Siwastaja on November 07, 2023, 07:07:06 amIt's not bullshit - surface area and heat radiation as well as convective losses are related. I'm not saying it's a limiting factor - just that a smaller part will run hotter.It's not always OK, even if the resistor's materials are good for high temps that still ends up transferred to the PCB and nobody likes that to get discoloured or burnt. Do you know when standoffs are required? Were they part of the thermal resistance test? You don't know it all and acting like you do is annoying.Insults and criticism aside, the datasheet thermal resistance number is from an unknown configuration.Is that mounted to a PC board, open air, standoffs, forced air etc? No standard jig is mentioned. Maybe it goes back decades, who knows.Because the SFR25H is the same size yet achieves a lower value, I suspect from better (copper) conduction heat losses to the jig, the resistor is mounted to "something". It also has a different coating which might radiate better.Yet you get pissy about me not trusting that number. Who really blindly trusts any manufacturer nowadays? Many specs are stretched.So I've asked Vishay for details, since people here don't know anything about it. IEC 60115 I don't have access to 4.14 if that is the test method.