Power supply resistor runs at 100C: Is this OK for long-term? - Page 1
Forum section: electronics / beginners

author: gjpmho
author_rank: Contributor
timestamp: March 28, 2023, 02:38:21 am
content: Hello,I have a wirewound 2.2kOhm 3Watt rated resistor dissipating 0.7Watts.https://www.vishay.com/docs/28731/pacserie.pdfI've measured the temperature at 100C using a thermal camera.Is this okay for the long-term?Thanks

author: bdunham7
author_rank: Super Contributor
timestamp: March 28, 2023, 02:55:45 am
content: The resistor will be fine, but you need to worry about the stuff that is around it. 100C isn't too hot in general--it won't melt the solder out or anything--but if the resistor is very close to the board or another component, it may do some damage long-term.

author: TimFox
author_rank: Super Contributor
timestamp: March 28, 2023, 02:56:01 am
content: According to the graph on page 6 of the data sheet, you are allowed to run 70% of the rated power at 100oCambient(air) temperature.Of course, the case temperature of the part will be hotter than the ambient air, so you should be fine at < 25% of rated power.The part derates to 0 dissipation at 275oC.

author: gjpmho
author_rank: Contributor
timestamp: March 28, 2023, 03:07:23 am
content: Ok thanks for confirming! Yes, it is at least an inch from other components. Important reminder that heat radiates in all directions, just like light.

author: jonpaul
author_rank: Super Contributor
timestamp: March 28, 2023, 03:48:01 am
content: thermal camera or temp probe useless for this.Use smallest guage thermocouple on the surface.Jon

author: Gyro
author_rank: Super Contributor
timestamp: March 28, 2023, 09:31:43 am
content: Quote from: jonpaul on March 28, 2023, 03:48:01 amI don't think such one line dismissals, without explanation, are helpful.The emissivity of ceramic resistor packages (unless glazed), PCB materials, plastic sleeving (electrolytics) or epoxy packages of surrounding components are well defined. There's no reason, that I can see, why a thermal camera should be "useless" for this application. In fact it should give a more complete 'picture of what is happening than the added thermal leakage of even a fine thermocouple lead attached to various points. The OP already knows that the resistor body is running at 100'C, he's more interested in the effect on surrounding board and 'loosely' thermally coupled surrounding components.

author: wraper
author_rank: Unknown
timestamp: March 28, 2023, 09:57:59 am
content: Quote from: jonpaul on March 28, 2023, 03:48:01 amThermal camera works extremely well on resistors. Unlike thermocouple for which you need to provide good thermal interface and fix on resistor somehow. Which already changes cooling conditions quite a bit. I've seen too many times clueless people pushing thermocouple against something and thinking they measure actual temperature.

author: bookaboo
author_rank: Frequent Contributor
timestamp: March 28, 2023, 09:59:34 am
content: QuoteNonsense.The emissivity of the surface of the part linked is likely to be 95% so any thermal camera reading is likely to be very accurate. Certainly accurate enough for this question.Had it been an anodised aluminium resistor then you would need to either(a) Find the emissivity of the surface and background temperature, calculate from thator(b) Improve the emissivity of the surface with paint or tape."b" being much easier.Though to answer the OP, the resistor will be fine but it's still not an ideal situation. Likely to speed up the drying of any electrolytic caps nearby. There's likely a better way to do whatever you are doing.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 28, 2023, 10:13:29 am
content: Unless that circuit is designed on purpose as a heater, 100*C on a PCB is rather unusual, no mater if that resistor alone can withstand long term to that temperature or not. A component at 50*C feels hot, at 70*C one can barely keep a finger on it, and at 100*C will blister or even burn the skin.If the design allows, I would pick a bigger resistor, something that will run at 50..70*C.

author: wraper
author_rank: Unknown
timestamp: March 28, 2023, 10:20:16 am
content: Quote from: RoGeorge on March 28, 2023, 10:13:29 amSo did you ever look on power supplies with a thermal camera? Snubber resistor or output resistors for minimal load often heat quite a bit more than that. Not to say something like phone chargers and laptop bricks often heat up more than that internally.

author: srb1954
author_rank: Super Contributor
timestamp: March 28, 2023, 10:25:50 am
content: Quote from: gjpmho on March 28, 2023, 03:07:23 amAt the relatively low body temperatures you are seeing most of the heat will be lost through convection and direct conduction through the leads. However, the proportion lost through radiation will increase rapidly at higher resistor temperatures as radiation loss goes up as the 4th power of temperature difference above ambient.Conducted heat will tend to burn the PCB material over a long period and it is important that the resistor body not touch the PCB surface; this is usually accomplished by forming the leads so as to prevent it being pushed down against the PCB. It is sometimes advantageous to have large pads and thick tracks on the PCB to help carry away some of the heat immediately adjacent to resistor leads to reduce any localised burning.

author: Gyro
author_rank: Super Contributor
timestamp: March 28, 2023, 12:24:11 pm
content: For reference, here's a short article from Teledyne (FLIR) on measuring the temperature of low emmisivity surfaces, eg. shiny metal, anodized heatsinks etc. by applying bits of tape, labels, blobs of correcting fluid, paint etc. This allows decent, predictable, temperature measurements with a thermal camera without affecting their temperature.https://www.flir.co.uk/discover/rd-science/use-low-cost-materials-to-increase-target-emissivity/

author: bdunham7
author_rank: Super Contributor
timestamp: March 28, 2023, 01:48:10 pm
content: Quote from: RoGeorge on March 28, 2023, 10:13:29 amDesigning anything practical that involves anything beyond trivial power and insisting on keeping component temperatures below 70C is pretty difficult and IMO wasteful. For many power resistors you'd have to derate them by 80 or 90 percent and you'd end up with an unnecessarily huge and expensive device. Additionally, after going to all that trouble it still dissipates the exact same amount of heat, so if it is an enclosure you haven't accomplished much!

author: Siwastaja
author_rank: Super Contributor
timestamp: March 28, 2023, 03:03:25 pm
content: Quote from: RoGeorge on March 28, 2023, 10:13:29 amPerfectly usual for a resistor. Basically the things to look for:* Is this 100degC measured at worst-case ambient temperature?* Are there any heat sensitive components like 85degC rated electrolytic capacitors close to this resistor?100degC is something that does not affect the reliability of solder joints or PCB materials, so you see this in successful, reliable designs, but it's quite close to something that could become a problem so some care is needed.

author: gjpmho
author_rank: Contributor
timestamp: March 28, 2023, 03:49:43 pm
content: Some back story: There are actually 4 of these 2.2kOhm resistors per channel: 2 for powering the +/-15V DC Servo circuit, 2 in the Amp stage. It turns out these 4 were originally SMD resistors packed in a small area which heated nearby components via the PCB (ICs and electrolytic capacitors) to 80C+! One workaround is to replace the resistors with THTs and mount on a perf board below the lid, which is 3mm thick and vented. Clearance from output device heatsink is 1/2inch. I don't see being a problem vs the heat generated by the output devices. Photos and related schematics attached. Thanks for the responses. I've looked into TO-220 packaged resistors (mounting those to to amp side panels - thick aluminum=heat sink) but those are too expensive, also no luck with finding a way to heatsink smd resistors on seperate boards, any recommendations would be appreciated. Seems like better heat management than original design, at least.

author: MathWizard
author_rank: Super Contributor
timestamp: March 29, 2023, 09:11:29 am
content: Have you made any of the circuit in the sim, and see if changing that R value or alter something in the biasing really makes much difference ? I wouldn't want 100C anything, so I'd use 2-3 resistors, or try change something.IDK the input voltage, but if that R9 and zener are just a regulator, I'd use or make some circuit with a pass element I could put on a heatsink.

author: wraper
author_rank: Unknown
timestamp: March 29, 2023, 09:50:08 am
content: Quote from: MathWizard on March 29, 2023, 09:11:29 amThey are on the separate boards so do not heat anything nerby. 100oC for such resistors themselves is quite low. Sure you should avoid running semiconductors this hot long term, but for wire wound resistors it's ridiculous.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 29, 2023, 11:41:46 am
content: Personally i hate everything who heat a lot, say in power amps, psu's etc.. even if it was designed like this ...If i see one heating a lot, i move it away from the pcb if possible and or i change it for an doubled wattage value ...Between the old era and "Now", the parts size(s) where reduced by a lot, you have 1 watt or even 2 watts resistor(s) who are the same size than old 0.5 watts ones lollUnless it will act as a fuse in some casesIn this case i would say parallel some SMT values on top of each other to get a good value but boubled wattage or even tripled wattage

author: wraper
author_rank: Unknown
timestamp: March 29, 2023, 11:59:55 am
content: Quote from: coromonadalix on March 29, 2023, 11:41:46 amIt's totally stupid for SMT. Surface area will barely change and PCB will remain the same. So you stack up resistors which then will run at almost the same temperature as a single resistor.

author: Gyro
author_rank: Super Contributor
timestamp: March 29, 2023, 12:47:47 pm
content: Just looking at the photos in #14 again. That heatsink compound on the power transistors iswayover the top! The idea of the compound is to fillmicroscopicair voids between the mating surfaces, any thicker than that and the thermal resistance goes up, not down - it is not very thermally conductive in bulk, just better than air that it replaces. I think I can see Silpads poking out of the mess, in which case, they are designed to compress and not need any compound at all.The best electrically insulated thermal coupling is achieved using thin Mica washers and the faintest smear of heatsink compound on each side. You should barely be able to see it squeezed out around the sides of the package when tightened down.

author: Zero999
author_rank: Super Contributor
timestamp: March 29, 2023, 01:00:42 pm
content: Shunt regulators draw the same amount of power, irrespective of the load.If you've got 0.7W dissipated by 2k2, then the voltage across the resistor must be:P = V2/RV2= PRV = √(PR)V = √(0.7*2200) = √1540 = 39.2VThe output voltage is 15V, so the input voltage must be 39.2+15 = 54.2V, call it 55V.To reduce the power dissipation, you could use a linear regulator such as the LM7815 & LM7915, with a zener diode in series to drop the input voltage to around 25V.

author: TimFox
author_rank: Super Contributor
timestamp: March 29, 2023, 02:27:57 pm
content: An old trick from Heathkit, to avoid overheating the PCB material near a leaded power resistor:They recommended cutting a small piece of cardboard from the original shipping box, about 1/4" thick.When inserting the bent leads into the PCB, place the cardboard under the resistor, spacing it away from the board.Remove the cardboard after soldering the pads, to avoid fire.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 29, 2023, 09:04:07 pm
content: Hmm, thank you for posting the schematics. I've designed the originalCreek 5350SE, from which the "Destiny" is obviously derived (I've left the company in 2002). In the original design these 2K2 resistors were 2W through hole, well lifted off the board.CheersAlex

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 29, 2023, 10:18:54 pm
content: Quote from: TimFox on March 29, 2023, 02:27:57 pmThat is exactly how it should be done, add some spacing between the hot component and the board itself.Keystone electronics sells or used to sell some spacers to insert on the leads. There are also tools which will form a “kink” on the lead and achieve the spacing.Even using this technique, in the past I repaired several power amp boards, charred by the component. Having said this, they were either paper phenolic or CEM-1 boards.

author: gjpmho
author_rank: Contributor
timestamp: March 30, 2023, 02:25:18 am
content: Quote from: Alex Nikitin on March 29, 2023, 09:04:07 pmHi 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

author: bostonman
author_rank: Super Contributor
timestamp: March 30, 2023, 02:31:59 am
content: Out of curiosity, can you run two 4.x resistors in parallel? Then you'll divide the current.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 31, 2023, 08:34:33 pm
content: Quote from: gjpmho on March 30, 2023, 02:25:18 amHi Gregory,5% tolerance is OK for these resistors, and either metal film, metal oxide or wirewound should be fine. IIRC, we've usedPR02 seriesin 5350SE. The Destiny has a higher supply voltage however 2W should be sufficient.CheersAlex

author: rfengg
author_rank: Regular Contributor
timestamp: March 31, 2023, 09:15:48 pm
content: Really absurd, who would design in a electrical component like a resistor to run at 100C?

author: bdunham7
author_rank: Super Contributor
timestamp: March 31, 2023, 09:28:04 pm
content: Quote from: rfengg on March 31, 2023, 09:15:48 pmHmmmm. Do you mean they should run cooler or hotter?

author: alm
author_rank: Super Contributor
timestamp: March 31, 2023, 09:31:49 pm
content: Quote from: rfengg on March 31, 2023, 09:15:48 pmI 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.

author: wraper
author_rank: Unknown
timestamp: March 31, 2023, 09:34:20 pm
content: Quote from: rfengg on March 31, 2023, 09:15:48 pmHow 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.

author: TimFox
author_rank: Super Contributor
timestamp: March 31, 2023, 11:00:37 pm
content: Quote from: wraper on March 31, 2023, 09:34:20 pmThe data sheet for the resistor used by the original poster is rated up to 275oCambienttemperature at zero dissipation, which corresponds to the maximum operating temperature of the resistor itself.This is noticeably higher than thejunctiontemperature rating for power semiconductors, typically 200oC for silicon.However, FR4 PCB substrate material is rated up to only 110oC to stay below the glass-transition temperature of 130oC.Higher temperature rating substrates are available.

author: wraper
author_rank: Unknown
timestamp: March 31, 2023, 11:28:55 pm
content: Quote from: TimFox on March 31, 2023, 11:00:37 pmThat's why you elevate them above PCB. 100oC 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 150oC, for some MOSFET it's 175oC. SiC MOSFET usually go up to 175oC, but I've read that they can go to about 200oC,QuoteIt's a very overbroad statement. There are many substrate types which are categorized as FR-4 but have vastly different properties. 170oC Tg is not rare at all. 130oC Tg is as low as you can possibly get from what I've seen.

author: TimFox
author_rank: Super Contributor
timestamp: April 01, 2023, 03:24:51 am
content: That was a recommendation from a board house, with a conservative allowance below the nominal Tg.They also had higher-temperature materials available.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 01, 2023, 05:33:21 am
content: Quote from: rfengg on March 31, 2023, 09:15:48 pmAny 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?

author: mzzj
author_rank: Super Contributor
timestamp: April 01, 2023, 06:24:41 am
content: Quote from: TimFox on April 01, 2023, 03:24:51 amYup. 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.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 01, 2023, 06:45:29 am
content: Quote from: mzzj on April 01, 2023, 06:24:41 amYeah. 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.

author: wraper
author_rank: Unknown
timestamp: April 01, 2023, 09:12:27 am
content: Quote from: Siwastaja on April 01, 2023, 06:45:29 amNot 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.

author: tooki
author_rank: Super Contributor
timestamp: April 01, 2023, 09:34:02 am
content: Quote from: alm on March 31, 2023, 09:31:49 pmWhat 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.

author: alm
author_rank: Super Contributor
timestamp: April 01, 2023, 11:07:59 am
content: Quote from: tooki on April 01, 2023, 09:34:02 amI said smaller, not small.Carbon composition resistors rated for 2W were the same size as modern 5W or even 7W resistors. See this1W carbon composition resistor(rated for 125°C) which has the dimensions of amodern 2W resistor.

author: tooki
author_rank: Super Contributor
timestamp: April 01, 2023, 11:38:53 am
content: Quote from: alm on April 01, 2023, 11:07:59 amMy 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 isnotsmaller 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, insomecases 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.)

author: rfengg
author_rank: Regular Contributor
timestamp: April 05, 2023, 01:39:58 pm
content: 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.