Power supply resistor runs at 100C: Is this OK for long-term?

[bostonman]

Out of curiosity, can you run two 4.x resistors in parallel? Then you'll divide the current.

[Alex Nikitin]

Hi Alex,

Just want to say thanks very much for designing the 5350SE.  The Destiny has brought me many hours of joyful music to share.  I'm hopeful this maintenance will increase its longevity. Turns out Creek Audio reverted to THT resistors in their Destiny 2. Would you recommend wire wound or metal oxide for these resistors? I chose wire wound because I couldn't find metal oxide resistors with lower than 5% tolerances, but now I'm learning 5% tolerance is probably fine in this case. Or is either fine?

Best Regards,

Gregory

Hi Gregory,

5% tolerance is OK for these resistors, and either metal film, metal oxide or wirewound should be fine. IIRC, we've used PR02 series in 5350SE. The Destiny has a higher supply voltage however 2W should be sufficient.

Cheers

Alex

[rfengg]

Really absurd, who would design  in a electrical component like a resistor to run at 100C?

[bdunham7]

Really absurd, who would design  in a electrical component like a resistor to run at 100C?

Hmmmm.  Do you mean they should run cooler or hotter?   

[alm]

Really absurd, who would design  in a electrical component like a resistor to run at 100C?

I agree, a power resistor that only gets to 100°C is way over dimensioned. That's how manufacturers are making smaller resistors with higher power ratings: they just increase the maximum temperature. For example (part of) this power resistor series is rated for up to 350°C. Obviously you need to make sure the PCB does not get up to this temperature.

[wraper]

Really absurd, who would design  in a electrical component like a resistor to run at 100C?

How about checking maximum operational temperature for such resistors. If you look at computer hardware, VRM MOSFETs or power stages often run hotter than this. Of course it's suboptimal for reliability of semiconductors, but for resistors like these it's not even hot.

[TimFox]

Really absurd, who would design  in a electrical component like a resistor to run at 100C?

How about checking maximum operational temperature for such resistors. If you look at computer hardware, VRM MOSFETs or power stages often run hotter than this. Of course it's suboptimal for reliability of semiconductors, but for resistors like these it's not even hot.

The data sheet for the resistor used by the original poster is rated up to 275^o C ambient temperature at zero dissipation, which corresponds to the maximum operating temperature of the resistor itself.
This is noticeably higher than the junction temperature rating for power semiconductors, typically 200^o C for silicon.
However, FR4 PCB substrate material is rated up to only 110^o C to stay below the glass-transition temperature of 130^o C.
Higher temperature rating substrates are available.

[wraper]

The data sheet for the resistor used by the original poster is rated up to 275^o C ambient temperature at zero dissipation, which corresponds to the maximum operating temperature of the resistor itself.
This is noticeably higher than the junction temperature rating for power semiconductors, typically 200^o C for silicon.
However, FR4 PCB substrate material is rated up to only 110^o C to stay below the glass-transition temperature of 130^o C.
Higher temperature rating substrates are available.

That's why you elevate them above PCB. 100^oC is fine for any PCB material as long as resistor is sufficiently raised and pads are not super tiny. Junction temp for most of silicon semiconductors is not not higher than 150^oC, for some MOSFET it's 175^oC. SiC MOSFET usually go up to 175^oC, but I've read that they can go to about 200^oC,

However, FR4 PCB substrate material is rated up to only 110^o C to stay below the glass-transition temperature of 130^o C.

It's a very overbroad statement. There are many substrate types which are categorized as FR-4 but have vastly different properties. 170^oC Tg is not rare at all. 130^oC Tg is as low as you can possibly get from what I've seen.

[TimFox]

That was a recommendation from a board house, with a conservative allowance below the nominal Tg.
They also had higher-temperature materials available.

[Siwastaja]

Really absurd, who would design  in a electrical component like a resistor to run at 100C?

Any design engineer worth their job. Why do you think there is anything absurd in it?

Remember that intuition and hearsay often lead you to wrong ideas. Look at facts. Datasheets may be optimistic, but wouldn't it be weird if, for example, 175degC rated components (very usual number in datasheets) could not be ran at 100degC?

[mzzj]

That was a recommendation from a board house, with a conservative allowance below the nominal Tg.
They also had higher-temperature materials available.

Yup. AFAIK Tg rating is more of short-term maximum but not something you want to use as continuous operating temperature.

Suprising amount of  test gear from big names like HP and Fluke runs so hot that pcb gets toasted with time. 
Many of the modern chinese brands today seem to have more robust thermal design.

[Siwastaja]

Suprising amount of  test gear from big names like HP and Fluke runs so hot that pcb gets toasted with time. 
Many of the modern chinese brands today seem to have more robust thermal design.

Yeah. Modern design culture of avoiding excess power dissipation (i.e., better efficiency) to begin with helps; for example, switch mode converters instead of linear. Old designs use simple discrete parts in lazy ways, exactly as shown in this case, using a wasteful shunt regulator when a pass regulator would have had better efficiency and not much more complex, because you can buy regulator ICs.

Modern abundance of small and cheap ICs for every possible job helps.

Regarding the FR4, I agree 110degC is a good absolute long term maximum for a design where you don't control the fabrication, i.e., carry the risk of your product being manufactured using any random FR4. But this is all about the projected operating hours. You can go higher if it's not operating 24/7. You can also go higher if you control your supply chain and buy better rated FR4.

[wraper]

Suprising amount of  test gear from big names like HP and Fluke runs so hot that pcb gets toasted with time. 
Many of the modern chinese brands today seem to have more robust thermal design.

Yeah. Modern design culture of avoiding excess power dissipation (i.e., better efficiency) to begin with helps; for example, switch mode converters instead of linear. Old designs use simple discrete parts in lazy ways, exactly as shown in this case, using a wasteful shunt regulator when a pass regulator would have had better efficiency and not much more complex, because you can buy regulator ICs.

Modern abundance of small and cheap ICs for every possible job helps.

Regarding the FR4, I agree 110degC is a good absolute long term maximum for a design where you don't control the fabrication, i.e., carry the risk of your product being manufactured using any random FR4. But this is all about the projected operating hours. You can go higher if it's not operating 24/7. You can also go higher if you control your supply chain and buy better rated FR4.

Not that much power is dissipated compared to the whole circuit. Then input voltage is high, so using linear regulators and buck converters is troublesome. Not to say, it's an amplifier input stage, so power must be clean.

[tooki]

Really absurd, who would design  in a electrical component like a resistor to run at 100C?

I agree, a power resistor that only gets to 100°C is way over dimensioned. That's how manufacturers are making smaller resistors with higher power ratings: they just increase the maximum temperature. For example (part of) this power resistor series is rated for up to 350°C. Obviously you need to make sure the PCB does not get up to this temperature.

What are you talking about? Those resistors aren’t small for their power rating. I’d be willing to bet that their high temperature rating is because there’s some application where a resistor needs to run in a very hot environment. I very much doubt it gets used on a standard PCB.

[alm]

What are you talking about? Those resistors aren’t small for their power rating. I’d be willing to bet that their high temperature rating is because there’s some application where a resistor needs to run in a very hot environment. I very much doubt it gets used on a standard PCB.

I said smaller, not small.Carbon composition resistors rated for 2W were the same size as modern 5W or even 7W resistors. See this 1W carbon composition resistor (rated for 125°C) which has the dimensions of a modern 2W resistor.

[tooki]

What are you talking about? Those resistors aren’t small for their power rating. I’d be willing to bet that their high temperature rating is because there’s some application where a resistor needs to run in a very hot environment. I very much doubt it gets used on a standard PCB.

I said smaller, not small.Carbon composition resistors rated for 2W were the same size as modern 5W or even 7W resistors. See this 1W carbon composition resistor (rated for 125°C) which has the dimensions of a modern 2W resistor.

My point wasn’t that resistors haven’t shrunk over time. My point was that your example is a really bad one because that model of wirewound resistor is not smaller than most other wirewound resistors of the same power rating.

My other point is that a high temperature rating is not necessarily due to shrinking the component size. Yes, in some cases that will be, but in others, the high temperature rating is needed for other reasons. (Wirewound resistors are a particularly bad example for the first, since they’ve always been used for high-power, high-temperature applications.)

[rfengg]

Then they are not doing much of a design, are they?.....open up as many quality gear  as you want that are proven to be reliable and have a look on how many components run at 100C?
Operating temp is indirectly proportional to MTBF whether its a resistor, a toaster or your car engine.
If a 100C component are near other components which have a temp coeff which has not been accounted for in the design, then good luck with the "design". Also in my previous company (medical devices) we had done controlled expts to ascertain  the reliability of soldering at elevated temps......at around 110C, solder becomes malleable and repeated heating and subsequent cooling causes cracks in the solder joints. 
The OP's original query was "Power supply resistor runs at 100C: Is this OK for long-term?"......the short and long answer is NO.