Can't calculate inrush current damage to resistor? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: April 30, 2023, 11:37:56 pm
content: Hi,In the "series" of things that cannot be calculated (within reasonable project timescales and cost), the following shows a 1R inrush limiting resistor which receives the shown power pulse....which goes up to 60kW inside a 40us pulse. The resistor is FKN2WSJR-73-1R.Its datasheet gives no possible equation by which the damage can be calculated.We wish to know, supposing this resistor receives one of the shown inrush pulses each day, then for how many days might this resistor last?FKN2WSJR-73-1R datasheethttps://www.yageo.com/upload/media/product/productsearch/datasheet/lr/YAGEO%20FKN_datasheet_2021v0.pdfAll that one can do, is find a pulse rated resistor , and see what its datasheet says....eg, lets take the AC-01 (1W) resistor from Vishayhttps://www.vishay.com/docs/28730/acac-at.pdf...page 6 shows that a 1R,1W resistor could handle 4Ws...so that means 200kW for the 40us....Page 6 also shows that a 1R,3W resistor could handle 8Ws...so that means 400kW for the 40us.Since our FKN series resistor is 2W, then lets take the middle of those two, ie 300kW......well........60kW < 300kW, so if the FKN series is like the AC-0x series, then we are OK........But who knows if it is the same?...it may well not be...so this is an example of something that cannot exactly be calculated in the world of SMPS.We might say, just use the AC-0X series resistor...yes , but its more than 10x the price.

author: coppercone2
author_rank: Super Contributor
timestamp: April 30, 2023, 11:42:37 pm
content: Lol, I honestly figured out this is a treez question in the middle of my reply.You are asking about a 60kW pulse on a 2 watt resistor. I feel like people trying to answer this question without a test in circuit are just bull shit artiststhis is like asking for a prediction of train to coin interactions from the mint to the rail road companyI suspect the specification is put on safety glasses

author: Faringdon
author_rank: Super Contributor
timestamp: May 01, 2023, 12:07:35 am
content: Thanks, and it surprised me that the AC-01 (1W) resistor would be able to manage it...the FKN series is wirewound just the same as AC-01.

author: Nusa
author_rank: Super Contributor
timestamp: May 01, 2023, 12:33:17 am
content: Now tell us how much thermal energy is produced in 40 us, and what the thermal mass of the resistor is. What's your expected thermal load over, say, 1 ms?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2023, 02:34:54 am
content: Looking for pulse ratings on a non-pulse-rated type... beats me. Find one with ratings!For reference, note that the pulse isroughlysimilar to a 1.5/50us surge. At this impedance, you're more likely to find or test surge (per se) at 8/20us rather than 1.5/50, but that's also just about half the duration which isn't likely to be very different thermally, i.e. if it can handle the peak power for a 50 or 100us pulse, or twice the peak power at 8/20us, it's probably fine.Tim

author: mzzj
author_rank: Super Contributor
timestamp: May 01, 2023, 07:17:26 am
content: Test it. You can hit it with similar or harder pulse every 10 seconds, 1 year worth of pulses in one hour and 24 years worth data in a day with a half an hour of actual work.Test at least couple of samples and derate it heavily as it is not manufacturers specified value.Alternatively test at 100kW pulse power with increasing pulse width until you get single-pulse failure. Derate to 1/10 pulse width and it should limit peak temperature to roughly 1/10 of failure temperature and it should last ”pretty long time”

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2023, 09:10:32 am
content: Quote from: mzzj on May 01, 2023, 07:17:26 amRemember to repeat the test regularly, to find out in case the manufacturer decides to change their process in a way which affects this undocumented characteristic; and have a product recall process in place, ready to fire in case faulty parts are received, used and shipped, and later tested and discovered. Properly, this is a statistically sampled parts receiving test. (Since the test is potentially wearing/destructive, a per-component test might be ill advised; lot sampling is also indicated as a monitoring process to save expense/labor.)...I don't expect that the OP's employer / end customer(s) would follow such a process to the letter, though.Tim

author: David Hess
author_rank: Super Contributor
timestamp: May 01, 2023, 04:18:48 pm
content: Quote from: T3sl4co1l on May 01, 2023, 02:34:54 amWithout better data, this is what I would do.Ignoring other considerations, the thermal mass involved is smaller as the pulse width decreases because there is no time for the heat to propagate. Since it is a linear distance to volume ratio, I would derate based on the 3rd power of the ratio of pulse width. I suspect there is also a factor of 2 in there.Other considerations include things like high voltage breakdown. Some resistors are physically constructed to support much higher surge voltages without breakdown or developing hot spots.

author: Faringdon
author_rank: Super Contributor
timestamp: May 01, 2023, 04:52:01 pm
content: Thanks, i see your point. Sounds like good reasoning. Ayk, the problem is that the FKN series of wirewound resistors doesnt give any pulse ratings whatsoever, it just says "2W"....so its difficult to know what to de-rate from. The AC-0x series gives pulse ratings, but is ten times the price, and surely, is "just another wirewound resistor"?As you know, those "1.2/50" pulses are surge pulses in the standards....and the product isnt meant to have any abaility to withstand those in multiple numbers of occurrences.....whereas i am speaking of regular everyday inrush current, which the product is meant to be able to handle with insignificant cumulative damage.

author: TimFox
author_rank: Super Contributor
timestamp: May 01, 2023, 04:57:37 pm
content: If you apply a large stress to a component that is not rated for that type of stress, then you must take responsibility for using that component, including doing your own testing.There is a reason why components with pulse energy ratings are more expensive than components only rated for steady-state power--someone has to be paid to do the testing and design.

author: David Hess
author_rank: Super Contributor
timestamp: May 01, 2023, 05:30:39 pm
content: Quote from: David Hess on May 01, 2023, 04:18:48 pmThinking about some more, if the resistance element is planar like a deposited film, then most of the thermal mass is normal (perpendicular) to the planar film, so then the volume of material involved is a length into a constant area so linear relationship instead of a cube rule. That is the volume increases to the linear distance instead of the cube. So the cube rule I gave is pessimistic and the linear rule is optimistic. Unfortunately that leaves a lot of range for the answer but it is better than nothing. I would not consider it inaccurate since we have such poor data on how the resistor is really constructed. Inspection of some datasheets that show power versus pulse width should reveal exactly what the relationship is for a given type of construction.The power semiconductor guys do this all of the time for determining pulse ratings but they can get temperature measurements of the junction during testing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2023, 05:52:33 pm
content: Quote from: David Hess on May 01, 2023, 05:30:39 pmMind the heat diffuses as sqrt(t), so the power goes down and energy goes up as sqrt(t) generally. It can be shallower, and I think often is for wirewound parts (closer to t^(1/3)?), probably some effect of heat spreading out in all directions but also more slowly as it goes from relatively conductive metal through poorly conductive enamel or resin, into ceramic or metal (and, give or take if the substrate is steatite, alumina, aluminum, etc.).Constant-energy (power inverse with time) parts can happen, I think where the element is uniformly heated (at ~us time scales, and elements of this size, skin effect won't be applicable; it will for much larger (100s/1000s W, low/fractional ohms?) parts), and where the heat transfer is dominant-pole (like wire in air, no enamel or core to sink heat into). Conversely, constant-power (energy proportional to time) parts can happen, say because the material heated is proportional to the time scale; I don't think skin effect has the right exponent for this to happen, but I could contrive a situation, say some kind of ceramic PTC subject to skin effect so that the outer skin heats up first, and heat travels inward like a wave until the whole thing is heated uniformly. Oh, that example would be sub-constant actually, because the amount of material being heated decreases with time (the effective conductor radius shrinks as its skin heats up and carries less current), heh.Tim