Resistor Sizes Vary Greatly?

[floobydust]

Which temp am I misunderstanding? Yageo hides important details I think.
Vishay's similar part (1/4W body size yet rated 1W like the Yageo FMP100) PR01 includes good data but still a little smoky.
"Applied maximum film temperature, θ_F max.: 250°C" but derate graph ends at 200°C.
Looks like you have to have it raised above the PC board 15 or 20mm? that hot degrades the solder.
Cu or FeCu lead options, Cu does run a bit cooler. Thermal resistance 135K/W.

[Zero999]

Call me cynical, but that middle resistor looks like a bog standard 1/4W 5% carbon film resistor that DigiKey have screwed up the data entry on.

It's difficult to tell from the picture, but it appears to have a matt finish, which means it most likely has a ceramic case. If it were an ordinary carbon film resistor, it would have a more shiny finish, indicative of an epoxy laquer.

[mikerj]

Yageo should be called scam-eo. The only reason they are selling well is noobs believing their fake specs - they don't give thermal resistance. When you withhold specs, same thing as lying.
OP's FMP series a usually 6.3mm 1/4w sized part- is rated 1W?! yet the usual 155°C max. rating.

Anyone care to explain how they get the 1W I'm all ears. The MFR-25 same size but 1/4W rated P₇₀.

It's total bullshit, where is the magical unicorn cooling fan?

Power dissipating devices have both maximum power and maximum temperature limits. Depending on the application you typically hit one of these limits before the other and it's up to you to as the designer to ensure neither limit is exceeded. Even without forced air cooling, the power dissipation limit of a resistor will be highly dependant on it's environment and mounting, something with lots of space for convection and lots of PCB area to dissipate heat is going to be able to work at a higher power than a resistor packed into a tight space with other components close by.

There is no magic here, just a misunderstanding of device specs.

[Gyro]

Call me cynical, but that middle resistor looks like a bog standard 1/4W 5% carbon film resistor that DigiKey have screwed up the data entry on.

It's difficult to tell from the picture, but it appears to have a matt finish, which means it most likely has a ceramic case. If it were an ordinary carbon film resistor, it would have a more shiny finish, indicative of an epoxy laquer.

Yes you're right. Now I look closer it does appear to be a matt cement finish. I still wonder if it's actually worth having a 1W body that small though. Unless the rating is only used for intermittent dissipation peaks, it's going to cause greater thermal wear and tear on the FR4 substrate and solder pads, or the hassle of standing it off the board. Also the need to distance other thermally sensitive components like electrolytics will make it's 'effective' [Ed: body size footprint] larger than its physical one.

[Siwastaja]

It's interesting to think that there are a few people who think world's largest resistor manufacturer publishes deliberately wrong data and scams their customers, while their actual customers, who buy these parts in billions, have nothing bad to say about the parts, nor any difficulty designing them in.

Think about it, and it's actually pretty close to flat earth tin foil stuff.

[coppice]

Which temp am I misunderstanding? Yageo hides important details I think.
Vishay's similar part (1/4W body size yet rated 1W like the Yageo FMP100) PR01 includes good data but still a little smoky.
"Applied maximum film temperature, θ_F max.: 250°C" but derate graph ends at 200°C.
Looks like you have to have it raised above the PC board 15 or 20mm? that hot degrades the solder.
Cu or FeCu lead options, Cu does run a bit cooler. Thermal resistance 135K/W.

Everyone fails to document important details in resistor specs. A huge number of resistors are carrying a pulsed current these days. Just how big can that pulsed current be before you are obeying the overall, say, 1W rating, but causing such intense momentary heating that bad things will happen? I never see that properly defined.

[Siwastaja]

Everyone fails to document important details in resistor specs. A huge number of resistors are carrying a pulsed current these days. Just how big can that pulsed current be before you are obeying the overall, say, 1W rating, but causing such intense momentary heating that bad things will happen? I never see that properly defined.

Never? For example, that very resistor posted by OP, see datasheet pages 5-6: https://www.yageo.com/upload/media/product/productsearch/datasheet/lr/YAGEO%20FMP_datasheet_2023v4.pdf Or do you mean this is somehow improper, if yes, what's wrong with it?

[coppice]

Everyone fails to document important details in resistor specs. A huge number of resistors are carrying a pulsed current these days. Just how big can that pulsed current be before you are obeying the overall, say, 1W rating, but causing such intense momentary heating that bad things will happen? I never see that properly defined.

Never? For example, that very resistor posted by OP, see datasheet pages 5-6: https://www.yageo.com/upload/media/product/productsearch/datasheet/lr/YAGEO%20FMP_datasheet_2023v4.pdf Or do you mean this is somehow improper, if yes, what's wrong with it?

I just looked through the data sheet. That one does seem to properly address the concerns of a designer of modern systems. Well done YAGEO.

[Zero999]

Call me cynical, but that middle resistor looks like a bog standard 1/4W 5% carbon film resistor that DigiKey have screwed up the data entry on.

It's difficult to tell from the picture, but it appears to have a matt finish, which means it most likely has a ceramic case. If it were an ordinary carbon film resistor, it would have a more shiny finish, indicative of an epoxy laquer.

Yes you're right. Now I look closer it does appear to be a matt cement finish. I still wonder if it's actually worth having a 1W body that small though. Unless the rating is only used for intermittent dissipation peaks, it's going to cause greater thermal wear and tear on the FR4 substrate and solder pads, or the hassle of standing it off the board. Also the need to distance other thermally sensitive components like electrolytics will make it's 'effective' [Ed: body size footprint] larger than its physical one.

Who said anything about FR4? If the resistor is on a ceramic PCB, next to other high temperature rated components it will be fine.

[Siwastaja]

Who said anything about FR4? If the resistor is on a ceramic PCB, next to other high temperature rated components it will be fine.

Yeah, or more commonly, even FR4 comes in different temperature grades, you can easily place the resistor further away from the board surface, you can add cutouts for cooling, or the circuit runs at low enough duty cycle that FR4 does not heat up too far.

The first thing transitioning from hobbyist to professional is to stop assuming things and start learning, calculating, modelling. We are not paid for nothing, we are paid for our expertise. $1000 worth of work can go into a design around a $0.05 resistor, that's completely normal.

[Siwastaja]

I just looked through the data sheet. That one does seem to properly address the concerns of a designer of modern systems. Well done YAGEO.

I rarely have hard time finding such basic information, maybe try looking at the datasheet instead of assuming that it's never there?

That being said, omission of important data is not uncommon, and not limited to resistors. Just a decade ago, it was quite normal to omit DC bias characterstics of MLCC capacitors even though it's a key parameter; people had to make assumptions (often wrong) or expensive testing. This is much better now, too.

[Siwastaja]

Also one wrong assumption seems that higher surface temperature of resistor means higher temperature for the surrounding components. This is only the case if you utilize the smaller size of the resistor to fullest extent possible and squeeze everything tightly together "because you can". On the other hand, if you leave the same amount of room you would use for a larger resistor, then the smaller resistor enables you to place it, within that free space, further away from some heat-sensitive component (e.g., electrolytic capacitor); with a larger resistor, you wouldn't have that luxury, it has to be placed where it fits.

[coppice]

Also one wrong assumption seems that higher surface temperature of resistor means higher temperature for the surrounding components. This is only the case if you utilize the smaller size of the resistor to fullest extent possible and squeeze everything tightly together "because you can". On the other hand, if you leave the same amount of room you would use for a larger resistor, then the smaller resistor enables you to place it, within that free space, further away from some heat-sensitive component (e.g., electrolytic capacitor); with a larger resistor, you wouldn't have that luxury, it has to be placed where it fits.

Do you know its standard practice in power products to put a large wet electrolytic next to the highest consuming resistor or transistor in the whole product?

[Gyro]

Who said anything about FR4? If the resistor is on a ceramic PCB, next to other high temperature rated components it will be fine.

Oh please, that's a rather tenuous use case don't you think?  Higher cost ceramic substrates are used for specific purposes and normally use SMDs and /or deposited resistors, they're not usually drilled for the benefit of some random through hole resistor. I'm sure lots of us have seen browned areas on, and the occasional detached pad, on boards due to a hot running component. It's usually the first place to look when tackling a fault.

[Siwastaja]

Do you know its standard practice in power products to put a large wet electrolytic next to the highest consuming resistor or transistor in the whole product?

I know it's a tongue-in-cheek comment, but yeah, confirmation bias amplifies this perception - we of course see those failed products where the hottest resistor or transistor is next to the electrolytic capacitor, maybe one which is already stressed close or beyond ripple current limits, - and then 99 out of 100 products that do not do this go unnoticed!

But sometimes this placement is partially unavoidable, after all the very purpose of a capacitor is to offer low-impedance power supply, in particular low-inductance, which requires minimization of distance, especially to a switching transistor. On the other hand, just moving it a few millimeters could be enough to cut the destructive heating. And this is exactly where smaller power resistors help, even if they have higher surface temperature; you have better control where that heat is generated.

[tggzzz]

Which temp am I misunderstanding? Yageo hides important details I think.
Vishay's similar part (1/4W body size yet rated 1W like the Yageo FMP100) PR01 includes good data but still a little smoky.
"Applied maximum film temperature, θ_F max.: 250°C" but derate graph ends at 200°C.
Looks like you have to have it raised above the PC board 15 or 20mm? that hot degrades the solder.
Cu or FeCu lead options, Cu does run a bit cooler. Thermal resistance 135K/W.

Everyone fails to document important details in resistor specs. A huge number of resistors are carrying a pulsed current these days. Just how big can that pulsed current be before you are obeying the overall, say, 1W rating, but causing such intense momentary heating that bad things will happen? I never see that properly defined.

The max pulse power vs time rating is sometimes documented.

I like to point out to people that my maximum heart rate is anywhere from 220bpm to <1µbpm, depending on the averaging period chosen.

[schmitt trigger]

Not only that, but many times is less expensive to have four 1/2 W resistors replacing a 2 W resistor.

[Wil_Bloodworth]

Why would you want to make that huge mess (with 4 resistors instead of one) for saving a few pennies?

- Wil

[Gyro]

A few pennies saving in BOM cost is huge in a high volume product.

[Wil_Bloodworth]

And I would venture to say that ZERO people on this forum are doing anything in volume!  Let's not kid ourselves and be real for a minute!

- Wil

[Gyro]

Hey, I've had consumer products in production that shipped 60k units/month. There are a lot of professionals on here. $0.10 saving adds up to a lot when you multiply by all the mark-ups before the customer gets his hands on it!

[coppice]

And I would venture to say that ZERO people on this forum are doing anything in volume!  Let's not kid ourselves and be real for a minute!

- Wil

For quite a few of us run rates below a million a year are considered low volume.

[Siwastaja]

Why would you want to make that huge mess (with 4 resistors instead of one) for saving a few pennies?

Why do you think it's a "mess"? There is some elegance in using larger number of parts, when possible; that way, heat is spread over a larger area. Resistors are very good candidates for both series and parallel connections because they share voltage/current well. Sometimes you also can reuse a part which already exists in BOM. But of course it isn't automagically always the right solution, rarely such thing exist in electronic design.

And I would venture to say that ZERO people on this forum are doing anything in volume!  Let's not kid ourselves and be real for a minute!

Why would you think so? While hobbyists are well regarded here, this also is the #1 English language professional electronic design forum on the planet. Every professional I have worked with at least knows this place even if they do not post (or admit posting) themselves. I'm 100% certain there are a few contributors here who design stuff that sells in tens if not hundreds of thousands a year, but quite obviously, lead designer of Apple or Asus would not be here.

Nevertheless, many design principles are similar even when dealing with one-offs or small batches. Personally, I apply the principle of cost reduction even in small scale customized stuff whenever it does not interfere with other constraints. Surprisingly often the customer still wants that a PCB does not cost more than say 50€ to manufacture (each), even when the NRE costs dominate, and the product which originally was meant to be an experimental batch of 50 is now explored for the possibility to be manufactured in thousands. Well at thousands a few cents of a single BOM line doesn't mean much, but when applied to the whole thinking pattern it makes a difference.

[tggzzz]

And I would venture to say that ZERO people on this forum are doing anything in volume!  Let's not kid ourselves and be real for a minute!

Prepare to be surprised

Some of the people here are very competent; others less so. A very few notoriously less so.

By and large the more competent people get pleasure from nudging the less experienced people in the right direction. It doesn't always work, but that's merely a microcosm of real life.

[wraper]

And I would venture to say that ZERO people on this forum are doing anything in volume!  Let's not kid ourselves and be real for a minute!

- Wil

Do not kid yourself. There are many senior industry professionals including millionaires on this forum. I wouldn't call myself a large producer but some of the products that I designed and produced are used even by multi billion dollar organizations AFAIK.