0.001 ohm 3W resistor at 30A gets too hot; impact on accuracy and alternatives? - Page 1
Forum section: electronics / general

author: Shubham
author_rank: Contributor
timestamp: December 14, 2023, 04:50:10 pm
content: I'm using a Bourns CRE2512-FZ-R001E-3 resistor (0.001 ohm, 3W, +-1%) for current measurement in a circuit with a maximum current of 45A. However, at 30A, the resistor gets excessively hot, despite calculations indicating a dissipation of only 0.9W. I've also tested with a generic 0.001 ohm, 3W resistor, observing the same issue. Is it normal for a 3W resistor to get very hot at 0.9W? Could the temperature increase impact resistance and affect current measurement accuracy? Any recommendations or alternative for high current measurement?Datasheet:https://4donline.ihs.com/images/VipMasterIC/IC/BOUR/BOUR-S-A0013301828/BOUR-S-A0013301828-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0The PCB is made of double-sided FR4 with a board thickness of 1.6mm, standard operating temperature of 140°C, and a copper finish of 35.00µ/1oz. I left polygons on both sides. A screw connector is mounted on the top, and the resistor is on the bottom. While soldering the screw connector and resistor, I applied a dab of solder. However, with a 30A current, it gets extremely hot to the point where I can't touch it for even a second.

author: Ian.M
author_rank: Super Contributor
timestamp: December 14, 2023, 05:16:17 pm
content: That's 0.9W in slightly over 20 mm2of board area, about 40% of the dissipation of the same area of a 1KW electric bar fire element. At 35A, without very good heatsinking that resistor (or any other of a similar package size) would glow red hot (and fail) as the power density would be comparable to a bar fire element! Your problem is getting the heat out of it. If you aren't on an aluminium core PCB (or over a heavy copper power or ground plane*) with very wide heavy copper tracks to the resistor for the current connections, you'll probably have to derate it.On regular double sided FR4, you could try four of the resistors in series/parallel with big 'fans' of copper either side of the middle node to keep that cool, or four 0.004 ohm resistors in series-parallel, spread out some to spread the dissipation. 'Mirror' the power tracks on the back of the board, and heavily via stitch them near the resistor pads to get as much heat as possible out through the copper traces.* See Siwastaja's comments below

author: Siwastaja
author_rank: Super Contributor
timestamp: December 14, 2023, 05:16:53 pm
content: Numbers, not adjectives, what do you mean by "excessively hot"? I regularly use 2512 SMD current sense resistors with very little derating, i.e. 1.5W for a 2W rated part, or 2-2.5W for a 3W rated part, and never had any problems, but OTOH my power layouts are 4-layer and the mid layer GND is good at sucking heat away, I also pay attention to large copper fills for the pads, which further couple to the mid layer through the thin prepreg.Post a picture of your layout + stackup specification, what is your intended maximum ambient temperature, how the PCB is cooled, and what you mean by "excessively hot", and maybe we can help further.

author: tszaboo
author_rank: Super Contributor
timestamp: December 14, 2023, 05:19:58 pm
content: You need to use larger pads. Copper on both sides. 4 layer if possible and connect it together as many vias as practical. Then add even more coper.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 14, 2023, 05:20:54 pm
content: Quote from: Ian.M on December 14, 2023, 05:16:17 pmWhole post is colossally bad advice. Series-parallel configuration is hard to Kelvin sense, and for mere 0.9W, extra ESL and board real estate wasted for no reason whatsoever. For mere 0.9W, even a 1210 sense resistor can be used, in a tight spot, for minimized ESL and switch node ringing caused by that ESL (assuming this is a power converter). Single 2512 should be easy-peasy.

author: Ian.M
author_rank: Super Contributor
timestamp: December 14, 2023, 05:28:35 pm
content: Yes series/parallel is far from ideal andnear-impossibleto kelvin sense, so the resistance (and temperature coefficient) of the solder joints and copper in that intermediate node is a big problem. If you cant get the heat out of a single 2512 resistor because you cant justify heavier copper or more layers for the entire board, the sane option is probably a physically much larger, higher power rating resistor, derated for the lack of adequate copper area and thickness to cool it.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 14, 2023, 05:38:04 pm
content: It is not just the shunt to produce heat, but chances are the traces leading to the shunt will also produce heat.One may consider an additional copper wire on top of the traces leading to the shunt. This may also help removing some heat from the shunt.The SMD parts need large pads and quite some copper to get there nominal power rating.With such high current it may be worth to use more than the normal 1OZ copper, not just for the shunt, but for other parts.For precision use 0.9 W from a nominal 3 W shunt may already be on the high side, especially if the layout is not including the needed copper area.

author: magic
author_rank: Super Contributor
timestamp: December 14, 2023, 05:39:09 pm
content: Just came here to post the same. Too thin PCB traces may easily have more than 1mΩ resistance.

author: Shubham
author_rank: Contributor
timestamp: December 14, 2023, 05:54:56 pm
content: The PCB is made of double-sided FR4 with a board thickness of 1.6mm, standard operating temperature of 140°C, and a copper finish of 35.00µ/1oz. I left polygons on both sides. A screw connector is mounted on the top, and the resistor is on the bottom. While soldering the screw connector and resistor, I applied a dab of solder. However, with a 30A current, it gets extremely hot to the point where I can't touch it for even a second.

author: tszaboo
author_rank: Super Contributor
timestamp: December 14, 2023, 06:02:43 pm
content: Quote from: Shubham on December 14, 2023, 05:54:56 pmThat's not extremely hot, thats somewhere above 80C. Measure it with a thermal camera or thermocouple.And place more copper.

author: Shubham
author_rank: Contributor
timestamp: December 14, 2023, 06:09:52 pm
content: I do not have a thermal camera or thermocouple and I think the temperature should be around 150C and even if it's 80C isn't it too much to affect the measurement? How can I place the more copper on already manufactured PCB's? Is the solder bulk not sufficient?

author: Ian.M
author_rank: Super Contributor
timestamp: December 14, 2023, 06:18:44 pm
content: You are out of luck and will probably have to respin the PCB, but you could try soldering heavy copper sheet fins to the tracks right up against the resistor pads, or better round three edges of the pad. You are aiming to increase the surface area dissipating heat by at least an order of magnitude. The fin surfaces should be sanded matte then painted matte black after soldering, with as thin a layer of paint as possible. This is a horrible bodge and Siwastaja will probably tell you I'm crazy!

author: coppice
author_rank: Super Contributor
timestamp: December 14, 2023, 06:21:07 pm
content: Quote from: Shubham on December 14, 2023, 06:09:52 pmWhat accuracy do you need for your measurement? If you are making some kind of power meter you probably need a shunt with excellent power dissipation to keep it cool enough to maintain the accuracy you require. Look at things like utility energy meters. They use a shunt made of low temp. coeff. material, like manganin, and its a big chunky thing so its temperature stays low enough with maybe a 60A load for its accuracy to remain within a tight spec. If the current measurement didn't need to be so precise, as in things like motor control applications, they could use quite a small shunt.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 14, 2023, 06:51:43 pm
content: Layout as shown would heatsink pretty well into the connector, but there is a flipside to this: if the connector itself is used very close to ratings then it will heat up, too. It seems you have some excess room to make the polygons a bit larger, too: use all the easily available board area for heatsinking. Also remove the thermal relief patterns from those connector through holes.Even without these modifications, I'm pretty certain the resistor does fine, as you would be running it below 1/3 rated power, but you can only be sure by taking a measurement. I know it's hard to measure the resistor's internal temperature, so destructive testing is a good alternative: double the power; it if survives +100% overload for more than a few hours, that's a pretty good sign.Internal heating is impossible to exactly calculate given the limited data on datasheet, but "Derated to zero power at 170degC" hints that this would be the maximum internal temperature, near full rated power at rated ambient temperature, with a (very) good layout. Use the temperature coefficient number to calculate the error. The result is vastly different depending on your conditions: for example, if your current varies only slightly, and you don't need to take measurements immediately after powerup, then you can calibrate out most of the error. On the other hand, if the thing has to read current a millisecond after power-up while the internal resistor temperature is still +20degC, and it then later heats up to +170degC, error between these readings would be up to 150degC * 50ppm/degC = 0.75%.Though, note that this type of resistor isn't in practice very accurate because you can never Kelvin sense it perfectly. For 1% tolerance, 1% extra error for TC, and 1-2% variance on mounting, I use parts like this, uncalibrated, for +/- 5% accuracy; with calibration, you can do much better.You can also buy a better resistor which has a "true" 4-pad layout, smaller TC, and better tolerance, but that's going to cost more.

author: nctnico
author_rank: Super Contributor
timestamp: December 14, 2023, 07:08:27 pm
content: Quote from: Siwastaja on December 14, 2023, 06:51:43 pmI think you are right on the money here. From the pictures it looks like the connector isn't suitable for 30A in real world applications. Personally I derate such pluggable terminal blocks to half the rated current for reliable operation.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 14, 2023, 07:24:50 pm
content: Yeah. It is possible that the difference between a good brand part and a cheap part is not as how hot they operate, but rather how well they survive in high temperatures they create. High power resistors are designed to run hot, and high current connectors use plastics that survive high temperature. There are other problems though than the part itself, for example you don't want to melt the PVC insulation of the wires that are screwed into that connector. You also want to mitigate against small user errors like using wrong wire gauge, wrong screwing torque, wire not inserted as far in the connector as it should, etc. Derating gives more margin for all sorts of errors.So: what is the part number of the connector?

author: Shubham
author_rank: Contributor
timestamp: December 14, 2023, 07:41:45 pm
content: I think you are right, the culprit is the screw connector. I am using ZB950 rated for 30A as per UL and 26A as per IEC. But I wonder for 20A also the trace and the resistor are getting warm but not the screw connector body.here is the datasheet for the screw connector:https://drive.google.com/file/d/1kCU42vpFh89YNWzN2WQoyhoutJVTlmAc/view

author: nctnico
author_rank: Super Contributor
timestamp: December 14, 2023, 08:10:15 pm
content: That is not going to work. Such specifications are determined by loading only 1 pin with 30A in an ideal situation.There are some pointers here:https://www.electronicdesign.com/content/article/21186362/demystify-current-ratings-for-connector-selection

author: Siwastaja
author_rank: Super Contributor
timestamp: December 15, 2023, 08:29:56 am
content: Yeah, that's just a drawing file, usually connectors also have more data in a separate datasheet, might be a part of zillion-page catalog. There it would probably read that pin current rating is for one pin being loaded only, and derating factors depending on number of pins used. This is because in many applications, some of the pins carry less current.Screw connector body doesn't feel warm because it's plastic, and plastic insulates heat, so heat has nowhere to go except the wire screwed to the connector (making it super important that users follow the cross-sectional area advice, and you should specify thick enough wire, not only based of the heating of wire, but also for heatsinking the connection), and the pin.

author: Berni
author_rank: Super Contributor
timestamp: December 15, 2023, 08:36:48 am
content: At these sort of currents EVERYTHYNG is a resistor.The resistor gets hot, the PCB gets hot, vias get hot, connectors get hot, cable terminals get hot, cables themselves get hot.So it is all about over rating things for more current than you need. At these currents this means rather beefy connections all over. Things like the terminals and cables can even function as heatsinks to pull the heat away from the power dissipated by the contact resistances, but the cables have to be thick enough to not become hot themselves.One alternative you can also use for these currents is going to hall effect current sensors. Those can have lower internal resistance, since the part the current passes trough is just a piece of copper.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 15, 2023, 10:35:39 am
content: Quote from: Berni on December 15, 2023, 08:36:48 amYes. Usual beginner mistake is to only think about making everything thick/wide. Next step is realize that making everything short is more effective. Aspect ratio is what matters! If a 100mm long, 20mm wide trace works, then 50mm long, 10mm wide trace works too, except that it works actually better because it can heatsink to something from its ends. Key to success in high power design is quite often minimization of distances,even ifit makes the power density look scary - this is also why I called Ian's comment "bad advice". Extremely small objects also have extremely good ratio between surface area and volume, this is why shrews need to eat all the time in cold conditions, they cool down so easily by just being small. 5x5mm square is excellent in connecting two power components together and can easily carry 30A, even though a "5mm wide track" obviously cannot.And from this perspective, OP's design is fine, it clearly does not have unnecessary power traces (at least on the part shown here; don't know about the design overall); you can't make "tracks to the resistor" any wider because there are no "tracks to the resistor" to begin with, which is kinda optimal.OTOH it leads to a situation where, if you connect components together all of which heat up similarly, they won't help heatsink each other. If you placed the two components much further away, they could heatsink separately, but then you'd need really good conductors between the two, the PCB real estate needed grows quadratic per distance unless you stuff like solder thick copper busbars on it, utilizing the third dimension.I bet the biggest problem is with choosing undersized screw connector; you can mitigate by specifying much larger wire gauge than strictly needed from the wire perspective itself, just make sure people actually follow your recommendation. In other words, make sure the wire doesn't become the third "component that heats up so much it can't act as a heatsink for the others"; given that the resistor heats up, the connector can't push heat into the pad as much as it could otherwise; so heat needs to go to the wire.

author: jonpaul
author_rank: Super Contributor
timestamp: December 15, 2023, 11:47:24 am
content: for current sense >10A a real current shunt , hall effect, or CT (AC only) is usual.The the very low R eg 0.1 Ohm, 0.01 Ohm are special types with 4 term, see Isabelhutte, etc.https://www.isabellenhuetteusa.com/precision-power-resistors/I have not seen any 1 mOhm R in use as the voltages sensed are overwhelmed with noise in an SMPS.It may be wiser for the OP to reconsider the design conceptJon

author: Siwastaja
author_rank: Super Contributor
timestamp: December 15, 2023, 12:05:57 pm
content: Quote from: jonpaul on December 15, 2023, 11:47:24 amOP has "real current shunt".QuoteYou have not looked hard enough. I have used 1mOhm current sense resistors in 2512 and 1210 packages numerous times, in SMPS designs, without problems with noise. These parts are sold for this purpose, so I guess others use them, too. The trick is to place the sense amplifier close and, as always, average slow readings, and blank small period of time near switching event from decision making if possible. The sense wires are low impedance, can be made short, and can be routed differential, to the differential-input amplifier, and are usually shielded by a ground layer, so noise doesn't need to be a problem even if full range signal level is just some millivolts.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 15, 2023, 01:50:17 pm
content: As coppice correctly noted above, for accurate/precision current measurements you do not need a resistor, but a full-size current shunt made of a special alloy, such as constantan or manganin. If you are satisfied with the accuracy of measurements with a resistor, then you can try to compensate for heating and temperature drift of the resistance (it is predictable, depending on the current). There is no workaround.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 15, 2023, 02:07:14 pm
content: Quote from: Alex Wolf on December 15, 2023, 01:50:17 pm"Shunt" is just short-hand for "shunt RESISTOR". It is a resistor, not a special component. The resistor as shownisa shunt resistor and obviously made of "special alloy" with moderately low TC, which would be obvious if you looked at the datasheet, the title of which says "current sense resistor", which is the same thing as "shunt resistor". Cheap-ass resistors made of less "special" alloys have worse TC than 50ppm/degC.How much size is "full size"?If you want to say "you need 4 separate terminals for better Kelvin sensing", then say it. If you want to say "50ppm/degC TC is not good enough", then tell us what is. 20? 10?

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 15, 2023, 02:09:07 pm
content: Quote from: Siwastaja on December 15, 2023, 12:05:57 pmNot really. A "real current shunt" for high currents looks like this:The large size is needed for heat dissipation, since there are no materials with zero TCR. And when it comes to currents of more than 10A, this becomes especially relevant.QuoteAn approximate current limit for overload protection and an accurate current measurement for regulation are two different things.Quote from: Siwastaja on December 15, 2023, 02:07:14 pmFirstly, it must be taken into account that this TCR value is always given for a very limited range of operating temperatures. Once you go beyond this range, it can easily become 10 times worse or even change unpredictably. Thus, the larger the heat exchange area for heat removal, the more accurate the final measurement result will be.Secondly, the typical TCR values for constantan and manganin are at least twice as good.Third, just try to calculate the error when the temperature changes by 100 degrees (which can easily happen in a closed SMPS case without forced ventilation). Looks a little unsuitable for accurate current measurements and regulation, doesn't it?It should also be taken into account that the actual parameters of "cheap-ass" resistors can be very far from the stated specifications.And please don't take it so personally, I had no intention of offending you.

author: nctnico
author_rank: Super Contributor
timestamp: December 15, 2023, 03:23:02 pm
content: Quote from: Alex Wolf on December 15, 2023, 01:50:17 pmThat is exactly like the part the OP choose to use though. If you read the datasheet, you'll see it is a Mn/Cu alloy current shunt.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 15, 2023, 03:32:26 pm
content: Quote from: nctnico on December 15, 2023, 03:23:02 pmThere are many different alloys that contain copper and manganese, where the proportion and purity determines the final result. Also, size matters. 😁

author: coppice
author_rank: Super Contributor
timestamp: December 15, 2023, 05:47:15 pm
content: Quote from: Alex Wolf on December 15, 2023, 02:09:07 pmI think you are very confused about the difference between "real" and "accurate". An SMD resistor with poor cooling will give you low accuracy, but many applications of current shunts are fine with low accuracy.

author: nctnico
author_rank: Super Contributor
timestamp: December 15, 2023, 05:57:06 pm
content: Quote from: Alex Wolf on December 15, 2023, 03:32:26 pmNo, cooling matters. The OP choose to run the shunt at 30% of the power rating so there is quite a bit of headroom there provided the shunt has enough ways to cool down. So far the cause of the problem is that there are other heat sources (AKA the connector) near.Another area I'd look into is whether it is a good idea to cover the pads with lots of tin (tin conducts heat almost 6 times less compared to copper; this requires some number crunching / testing to determine what is better or worse) and having so many vias. In the end a via is hollow and this removes surface area which can otherwise be useful for cooling.

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 15, 2023, 05:59:56 pm
content: https://www.vishay.com/docs/53048/pprachp.pdf

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 15, 2023, 07:06:44 pm
content: Quote from: nctnico on December 15, 2023, 05:57:06 pmSize and cooling are practically synonymous when it comes to resistors. But the rated power has nothing to do with the temperature stability of the resistance: it's simply the maximum amount of heat at which the resistor will not burn out or desolder itself. And 1 watt of heat over an area of 19.5 mm2 (SMD 2512) is quite hot.QuoteThis design doesn't look good initially. If the OP really needs high accuracy, and according to the requirements of the project it is necessary to make it with only SMD components, then in his place I would use 5 resistors of 5 mOhm each (placing them on large copper polygons).

author: nctnico
author_rank: Super Contributor
timestamp: December 15, 2023, 07:16:38 pm
content: Quote from: Alex Wolf on December 15, 2023, 07:06:44 pmSMD components aren't rated that way; the assumption is that any SMD component which dissipates a significant amount of heat has sufficient heatsinking through the pads + PCB. A good example are white LEDs. These will burn out soon if the pads they are mounted on aren't large enough to provide adequate cooling.

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 15, 2023, 07:27:49 pm
content: Quote from: Shubham on December 14, 2023, 04:50:10 pmNumbers, not sensations/guessing. 50ºC will already burn your skin.Following the stupidly small derating curve: 2W @70ºC, 1W@~130ºC.

author: TimFox
author_rank: Super Contributor
timestamp: December 15, 2023, 07:30:20 pm
content: Quote from: Alex Wolf on December 15, 2023, 07:06:44 pmFor critical current sensing, you need a "four-terminal" connection.The data sheet for this resistor shows a proper PCB layout for the part that has specific "sensing" traces.This doesn't work well with multiple resistors in parallel.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 15, 2023, 07:41:12 pm
content: Quote from: nctnico on December 15, 2023, 07:16:38 pmAll components are rated that way. Absolute maximum ratings indicate a range of survival, not a range of performance within guaranteed parameters. Calculating the correct operating temperature range is the task of the development engineer.Quote from: TimFox on December 15, 2023, 07:30:20 pmI would say that separate traces for the sense are needed by default, especially when we are talking about such high currents as the OP has, since copper has a relatively high TCR, which will greatly affect the measurements. But what do you mean by doesn't work well with parallel resistors? In my experience, everything works just fine if you place the resistors literally parallel, next to each other on the PCB.

author: nctnico
author_rank: Super Contributor
timestamp: December 15, 2023, 07:47:07 pm
content: Quote from: Alex Wolf on December 15, 2023, 07:41:12 pmYou are mis-understanding. What I'm saying is that the area used to dissipate the heat is much larger. Like many components that dissipate heat, their performance rating is based on adequate heatsinking. So your 19.5mm^2 number just doesn't apply. I agree it is up to the designer to make sure the temperature stays within a range where the component performs as required.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 15, 2023, 07:47:11 pm
content: Quote from: Alex Wolf on December 15, 2023, 02:09:07 pm<images of resistors rated to maybe 5-50W>Quite obviously, resistors designed to dissipate larger amount of power would be larger. This part in question is rated to only 3W, which is why it can be smaller. High-power or low-efficiency applications are not any more "real" than low-power or high-efficiency applications.Accuracy indeed improves with derating. This advice equally holds for small and large resistors. Accuracy also improves by increasing burden voltage (in which case dissipation is again increased enough to require a bigger part), but you can't always afford that - efficiency is often important.Main reasons to use a small SMD part is lack of space (no room for a larger one), cost (they are cheaper, especially if the PCB is otherwise 100% SMD load), and low inductance (can be used within a switching loop). Main reason to use small resistance value is minimizing efficiency loss or minimize the unintended voltage drop seen by the DUT. If small power dissipation is the goal in itself, then small resistor does just fine, and using an excessively large one because it gives you a feeling of "Real Thing" is just cargo cult engineering.I have had good results with resistors like posted by OP, modestly derated (by 30%), on a 4-layer PCB, calibrated to 0.5% (of full range) accuracy on measuring instruments, without complaints of them being out-of-spec.Nearly all designs where I have needed current sense resistor have shared some common features:* Accuracy in order of +/- 1%* Efficiency is of paramount importance (e.g., switchers in battery-powered devices)* Small size is important* Current sense resistor is in motor control or DC/DC switching loopSMD resistor as shown by the OP is the only sensible choice in these kind of applications, which would be again obvious if you finally took the time to look at the datasheet which also lists typical applications. On the other hand, if you are building a measurement instrument, where accuracy much better than 1% is desired, where efficiency and burden voltage is unimportant, power dissipation does not matter, device can be built as big as you want (e.g., in a rack), and measured current is low-bandwidth DC, then you would most likely use the large shunt resistors, with separate Kelvin terminals, as you have shown.PS. I don't think you realized it, but the "TANKII alloy" images you posted are not that accurate; they are low-cost stuff difficult to very accurately Kelvin sense, just like the SMD versions we talked about. All you get by this design, compared to SMD, is double-triple the power rating, especially on a 2- or 1-layer PCB - and more inductance. These are used in cheap and mid-grade multimeters with 1% current measurement accuracy or so. Nothing special.

author: TimFox
author_rank: Super Contributor
timestamp: December 15, 2023, 07:55:24 pm
content: Quote from: Alex Wolf on December 15, 2023, 07:41:12 pmIf you place rectangular resistors parallel to each other, with individual current-sensing traces to each resistor, there will be uncontrolled currents through those current-sensing traces due to imbalance in the total resistance (resistor plus current traces) between the individual units.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 15, 2023, 07:59:35 pm
content: Quote from: TimFox on December 15, 2023, 07:55:24 pmExactly, and the current distribution is hard to perfectly control even within the footprint of a single resistor, it depends how the part gets aligned during pick&place and how it moves during reflow. Even the placement of the Kelvin sensing traces is important. Even a single SMD resistor with optimized layout is still a compromise. There are numerous appnotes about this.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 15, 2023, 08:22:50 pm
content: Quote from: TimFox on December 15, 2023, 07:55:24 pmSuch a stack of resistors should be considered as one resistor; accordingly, only one pair of current-sensing traces is needed to their common contact pads. In this case, no problems will arise.Quote from: Siwastaja on December 15, 2023, 07:47:11 pmBut in this case it doesn't, otherwise this topic would not exist. I thought that we were here trying to help the OP solve a specific problem, and not have a competition about who has the biggest balls in engineering.QuoteYou just couldn't stop yourself from nitpicking about it, right?That was just a random image from Google, the first one that came across, to roughly show what they look like. Take apart any of your handheld multimeters and you will see something similar. And there is a reason why there will be such "real" current shunt.P.S.: This is exactly the alternative that the OP asked for.

author: nctnico
author_rank: Super Contributor
timestamp: December 15, 2023, 08:46:28 pm
content: Quote from: Alex Wolf on December 15, 2023, 08:22:50 pmI'd like to see an equivalent circuit sketch + accuracy formulas of such a setup including the copper resistance (on which temperature has a large influence).

author: TimFox
author_rank: Super Contributor
timestamp: December 15, 2023, 10:14:25 pm
content: The tempco of copper is very important to the discussion of low-resistance current shunts.The usual value for copper is 3.93x10-3/ K, and it doesn't take much copper to add up to a good fraction of 0.001\$\Omega\$.The current sensing must be done at a point where the copper leads and the SMT terminations (typically silver alloys) do not contribute much to the low variation of the resistance element itself.Good resistance alloys are below 10-4/ K

author: coppice
author_rank: Super Contributor
timestamp: December 15, 2023, 10:17:47 pm
content: Quote from: TimFox on December 15, 2023, 10:14:25 pmThat's why people making SMD shunts are now publishing suggested PCB layouts to keep 100% of the copper out of the sensed path, generally in the vein ofhttps://www.analog.com/en/analog-dialogue/articles/optimize-high-current-sensing-accuracy.html

author: ahbushnell
author_rank: Frequent Contributor
timestamp: December 15, 2023, 10:34:06 pm
content: QuoteOn the data sheet they show how to do four terminal which I have used in the past. Run two kelvin leads from under the resistor from the center of the pads.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 16, 2023, 05:56:16 am
content: Another example of a “real” metrology grade current shunt:

author: trobbins
author_rank: Frequent Contributor
timestamp: December 16, 2023, 07:00:07 am
content: One aspect of that original pcb layout is that the Golten connector datasheet has no thermal characteristics to relate its 30/26A terminal rating. Some connectors qualify that aspect with respect to the wire size/type used, given that the wire connection provides the bulk of the heat flow path. I don't think the OP tested with different gauge wiring as a way of characterizing the resistor temperature rise, especially as the Golten datasheet indicates up to 4.0mm2 wire capability.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 16, 2023, 07:34:07 am
content: Quote from: Alex Wolf on December 16, 2023, 05:56:16 am"real metrology grade"says 1% on topPlease, can you just go away now, we are dying out of laughter. I think you have totally lost all credibility by now.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 16, 2023, 07:43:42 am
content: Quote from: Siwastaja on December 16, 2023, 07:34:07 amOh come on man, what's wrong with you? Can you just stop acting so childish, trying to prove to everyone that you are absolutely right and the superiority of your design, which is objectively wrong (against industry standards)? I’m not even talking to you, but you continue to try to start a flame with arguments to the individual. Just stop it, it's not your personal blog about how cool you are in engineering.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 16, 2023, 08:22:41 am
content: Quote from: Shubham on December 14, 2023, 04:50:10 pmYes, this is absolutely normalfor a resistor in an SMD 2512 package, which is usually rated at 1 W. Increasing the power rating in this case was possible because the resistor is made from a small piece of metal alloy, and unlike thin film, it can withstand higher temperatures before failing. But a higher temperature in this case means a higher temperature drift of the resistance, since each degree of temperature increase above the ambient temperature is a multiplier to the temperature coefficient of resistance (see the datasheet, Kelvin is equal in size to degrees Celsius).The main question is: what kind of accuracy do you need in measuring current?In percentage, please, exact value.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 16, 2023, 09:40:50 am
content: Quote from: nctnico on December 15, 2023, 08:46:28 pmI'm sorry, but what you're asking for is extremely time-expensive in-depth research. Furthermore, high level of accuracy is clearly not required for a project with such low-precision components.If a high level of accuracy is required, this method is not suitable. So I can only offer a rough sketch of the PCB layout of what it should look like. The equivalent circuit looks very straight forward, just like the layout looks. The error is negligible at the given level of accuracy.Edit: The resistors must be hand-selected and matched, that's obvious.

author: magic
author_rank: Super Contributor
timestamp: December 16, 2023, 09:59:49 am
content: Awesome, the outermost resistor that you are sensing has a significant fraction of mΩ more in series with it than the central resistors and the exact value of "significant fraction of mΩ" has a significant thermal coefficient too.I doubt it would work well.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 16, 2023, 10:27:47 am
content: Quote from: magic on December 16, 2023, 09:59:49 amThe question is how significant this fraction will be, taking into account the fact that the operating temperature will be significantly lower, and along with it TCR becomes less significant.Of course, you can route the current-sense traces from the central resistor if you need higher accuracy, as ahbushnell noted before.

author: coppice
author_rank: Super Contributor
timestamp: December 16, 2023, 08:33:38 pm
content: Quote from: Alex Wolf on December 16, 2023, 07:43:42 amOh come on man, what's wrong with you? Can you just stop acting so childish, trying to prove to everyone that you are absolutely right and the superiority of your design, which is objectively wrong (against industry standards)? Its pointless talking with you. You don't listen.The labelling on that Bourn's 4-port resistor seems to indicate its 500 micro-ohms. The OP is using 1 milli-ohm. At 1 milli-ohm a large SMD resistor with good layout can match the performance of these 4 terminal resistors down to 0.1%. At 100 micro-ohms this become hard, and a well designed 4 port resistor has big advantages. There is, obviously, a sliding scale in between. A well laid out SMD resistor works well enough, that now people like Analog Devices (and me - I came to the same conclusions as AD, doing the same research for the same reasons, but they published their app. note first) have published information on the most successful layouts for high volume manufacture, resistor makers widely endorse those ideas. There is no magic in spending extra for a 4 port resistor when a little care in design can match it with smaller cheaper parts.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 17, 2023, 08:24:19 am
content: Quote from: coppice on December 16, 2023, 08:33:38 pmOh great, another one with a high ego and low qualifications. I hear you perfectly well, I just ignored you in order to avoid a useless argument, so as not to hurt your feelings and provoke a conflict. But it seems that you are looking for conflict as a way of self-affirmation, and perceive my kindness as weakness, as an easy target for your attacks. Therefore, I simply have no other choice but to expose you to your own mistakes in order to shame and stop you.First of all, you both have no idea what a "real" current shunt is by industry standards. No, it's not just a low resistance resistor. Let me enlighten you: this is a very specially designed resistor specifically and exclusively for use in measurement circuits and instruments (a metrology grade "resistor" = a “real” current shunt). Its main difference is greatly increased power dissipation to keep temperature rise low and minimize the significance of TCR. You are mixing together the concepts of current limiting for overload protection, where high accuracy is not needed, and current measurement for regulation, where high accuracy is needed.You also have no idea that TCR is a curve that is relatively straight only in a narrow range that ends much earlier than the operating temperature of your standard 2512 resistors at 1 W or more. The manufacturer doesn't even bother to give this information to you for regular resistors because they were never supposed to be used for accurate current measurement. The manufacturer is counting on you to be skilled enough in these matters to know this on your own and understand this from the list of applications.Is your ego big enough to argue with the manufacturer?And are you really going to pretend to be a fool who doesn’t know the definition of the word “example”? From the photo I provided, you can clearly see the markings, which you can use to find the datasheet and select exactly the resistance value you need. As for the value of resistance: in itself, it has nothing to do with the accuracy of measurements, unless you bring into the context the operating current and power dissipation, that is, the operating temperature. Moreover, a CRE2512 resistor will never match in performance to a proper current shunt like the CSS4J-4026 series. Why, you ask? See above: due to the ability to dissipate much more heat and operate at much lower temperatures, reducing the significance of TCR to a minimum. And no layout will help change this, because the hottest point is in the center of the resistor, and the thermal conductivity of its tiny leads is too low to remove heat quickly enough for any accurate measurements. It's funny that Siwastaja correctly noted earlier that for accurate measurements you need to derate the power rating (by about 10 times – note from me), but then he ignored himself and continued to argue with argumentum ad hominem.If you don’t believe me, then teardown any large-scale device that is designed to accurately measure current, at least your multimeter, and you will see confirmation of my words right there (and you will never ever find there your CRE2512-resistor-like-design, if it’s not a Wun Hung Lo product). The OP expressed concern about the accuracy of the measurements, so it can be assumed that he needs it. Then he asked to name possible alternatives for this. So, this is exactly the alternative according to industry standards, that he needs. Why do you so mad about this? Maybe because you made the same mistake as the OP earlier in your designs, and now you don’t want to admit it even to yourself? It's none of my business and I really didn't care until you started dragging me into it.Finally, regarding the 4 terminals. First of all, this hints to you that you have the proper current shunt for really accurate measurements. Secondly, even Siwastaja himself encountered and admitted a problem with two-terminal resistors, which he mentioned above:Quote from: Siwastaja on December 15, 2023, 07:59:35 pmAnd guess why? Simple: because it's the wrong design for accurate current measurements. PCB assembly machines were never intended to work with such high precision, unlike CNCs, which make proper 4-terminal current shunts that you can't screw up with a misaligned installation.Any sufficiently advanced technology is indistinguishable from magic, especially if you don’t know, how it works.

author: magic
author_rank: Super Contributor
timestamp: December 17, 2023, 09:44:43 am
content: LMAO

author: langwadt
author_rank: Super Contributor
timestamp: December 17, 2023, 11:16:06 am
content: Quote from: magic on December 16, 2023, 09:59:49 amyeh, could atleast turn things 90 degrees so each resistor has the same length of trace in series

author: nali
author_rank: Frequent Contributor
timestamp: December 17, 2023, 11:33:10 am
content: Going back to the OP - He's made some efforts with polygons and vias to thermally connect top & bottombut then uses a footprint with thermal reliefto mount the actual sense resistor. No wonder it's getting hot!@Alex Wolflarge components only help with heat dissipation if there's i)space ii)budget and iii)a path for that heat to go, none of which we really know within the context of this thread.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 17, 2023, 12:15:36 pm
content: Quote from: nali on December 17, 2023, 11:33:10 amIt’s funny that after three pages, no one noticed it before, including me. Well noted.QuoteYes this is true. Moreover, we don’t even know what kind of precision the OP needs. I simply made educated guesses and offered a reliable and proven option that will definitely work for high accuracy, always works. And then this whole pointless argument began. Of course, depending on the specific requirements and limitations of the project, one or the other method will be optimal.One addition: when it comes to larger pure metal resistors made of copper alloys with high thermal conductivity, (all other things being equal) they themselves are better at dissipating heat to the air, as well as better distributing heat throughout their bigger volume, reducing the final operating temperature.

author: langwadt
author_rank: Super Contributor
timestamp: December 17, 2023, 12:21:38 pm
content: Quote from: Alex Wolf on December 17, 2023, 12:15:36 pmlook at the picture of how it is soldered, the thermal reliefs doesn't matter

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 17, 2023, 12:29:51 pm
content: Quote from: langwadt on December 17, 2023, 12:21:38 pmYes, this is also true, with one caveat: it's definitely not the root cause of getting hot (the root cause is too much power for its size), but it definitely contributes significantly to the increase of operating temperature. The fact is that the thermal conductivity of copper is almost an order of magnitude higher than the thermal conductivity of typical solder.

author: langwadt
author_rank: Super Contributor
timestamp: December 17, 2023, 12:37:27 pm
content: Quote from: Alex Wolf on December 17, 2023, 12:29:51 pmthe solder is orders of magnitude thicker than the copper

author: trobbins
author_rank: Frequent Contributor
timestamp: December 17, 2023, 12:44:39 pm
content: Geez, no one seems to want to follow the heat flow path.I'd estimate that >90% flows through the terminal legs and into the terminal blocks, especially if 4mm2 cable was being terminated at the terminals. What the OP hasn't elaborated on was what wire was used during qualitative testing.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 17, 2023, 12:52:18 pm
content: Quote from: langwadt on December 17, 2023, 12:37:27 pmWhich leads us to the conclusion that if the contact pads were solid, the total heat removal would be twice as good.

author: langwadt
author_rank: Super Contributor
timestamp: December 17, 2023, 01:06:25 pm
content: Quote from: Alex Wolf on December 17, 2023, 12:52:18 pmdo you really believe that?

author: magic
author_rank: Super Contributor
timestamp: December 17, 2023, 01:21:02 pm
content: Of course it wouldn't; tin is ~6x less conductive than copper and easily 30x thicker than 35μ in the vicinity of the pin, so ~17% goes through copper. And OP's pattern still left maybe 33% of copper in place, so only 12% is lost.The connector and its long pins are the bottleneck anyway.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 17, 2023, 01:21:10 pm
content: Quote from: langwadt on December 17, 2023, 01:06:25 pmNot really. This is a very rough approximation, more logic than mathematics. But yes, definitely, the larger the terms that make up the final value of thermal conductivity (to which both copper and solder contribute, and copper is approximately an order of magnitude greater), the higher the value of the total thermal conductivity. The exact value depends on the exact thickness of the copper and solder. But I think you already know that.Edit: Clarified the wording.

author: coppice
author_rank: Super Contributor
timestamp: December 17, 2023, 05:52:36 pm
content: Quote from: Alex Wolf on December 17, 2023, 08:24:19 amOne resistor is considered suitable for power supplies and motor control. Low resistance values in those products basically means current measurement.One resistor is considered good for current measurement. That will be applications like power supplies and motor control.What's you point?

author: nctnico
author_rank: Super Contributor
timestamp: December 17, 2023, 06:07:00 pm
content: Quote from: trobbins on December 17, 2023, 12:44:39 pmThis has been discussed at length and the conclusion is that the terminal blocks the OP has used are not suitable at all and are very likely the cause of heating the resistor instead of cooling it.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 17, 2023, 06:43:14 pm
content: Quote from: coppice on December 17, 2023, 05:52:36 pmDo you really not understand the difference, especially after I've already pointed it out twice, or is this just an attempt to suck another argument out of thin air to continue that pointless argument?Quote from: Alex Wolf on December 17, 2023, 08:24:19 amTypical power supply design usually doesn't include any accurate current sensing requirements, since there is a good margin between the overload protection level and the power supply's destruction current (and usually this margin is an order of magnitude greater than the accuracy error).There are bench power supplies that actually require accurate current measurement to accurately regulate the current, but this is aspecial casethat requires a resistor of a higher grade (which is usually called just a current shunt, since this is its the one and only special purpose).Yes, you can use a higher grade resistor for less demanding tasks, but not vice versaifyou care about accuracy. And if you don't really care, our argument becomes even more pointless. 😐

author: coppice
author_rank: Super Contributor
timestamp: December 17, 2023, 07:10:01 pm
content: Quote from: Alex Wolf on December 17, 2023, 06:43:14 pmYou don't really argue. You're preferred move seems to be the ad hominem attack, from a position of complete ignorance of the other party's capabilities.Do you not understand what a typical in system current sensing solution looks like? You write like someone in a metrology lab, rather than someone who designs equipment. The electronics connected to the shunt usually needs calibrating if high accuracy is needed, so the shunt might as well be calibrated too. What you need from the shunt is long term predictability, over the operating conditions the equipment will face. Why do shunts get so hot? Because people want to use a big resistance to simplify the electronics. How does it simplify the electronics? I two predominant ways. It helps with a limited CMRR and with the SNR. A lot of shunts now feed into a sigma-delta converter, which can be really good for the CMRR issue. A well designed, but cheap, switched cap front end sigma-delta converter usually has a worst case CMRR of 150dB or more. So, those are really helping to keep down the heat from the shunt. How much the SNR issue limits you depends on the application. If you need a very fast response, like cycle by cycle measurement in a switched mode system, you probably can't do much to smooth out the noise. In most of the applications where you need high accuracy you can do some smoothing. So, one good move is to look carefully at the electronics, and try to get the value of that shunt down to the minimum possible value. I see people with heat problems from 2 or 4 milli-ohm shunts where I could get them down to maybe 200 micro-ohms at no additional cost, and make their heat issue go away.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: December 17, 2023, 07:36:52 pm
content: Quote from: coppice on December 17, 2023, 07:10:01 pmOh really? If you scroll up the topic, you can clearly see that I did not attack anyone, I was only defending myself from all the nonsense and personal attacks that were thrown at me, including from you, which I ignored and tried to nullify using humor until they cross the red line of arrogant rudeness. And your favorite type of attack is to repeat after me my well-founded accusations against the attackers, without supporting them with any justification. I've lost all interest in talking to you, it's hopeless in every sense. One piece of good advice in the end: if you don't have the capability to make an accurate measuring circuit (no matter for what reasons), you should not present your solution as an accurate measuring circuit, much less try to argue with those who know for sure that it is inaccurate (not even close to the worse meter on the market). Usually, people are annoyed by lies, especially blatant lies. Good luck, you'll need it.

author: trobbins
author_rank: Frequent Contributor
timestamp: December 17, 2023, 09:58:56 pm
content: Quote from: nctnico on December 17, 2023, 06:07:00 pmIs that the conclusion based on the OP's qualitative 'gets excessively hot', and 'with a 30A current, it gets extremely hot to the point where I can't touch it for even a second', and 'But I wonder for 20A also the trace and the resistor are getting warm but not the screw connector body.' Considering those vague observations of temperature rise, and no indication of what wire was terminating at the connector to act as imho the dominant heatsink, then I can't see how any conclusion could be made for a current up to the rating of the terminal block. Sadly the OP has used a terminal block only rated for 30A, in an application that may require up to 45A, but again the OP has not provided any detail as to the time profile of current, as 30A, and even the 45A, could relate to short duration peaks - who knows!

author: tszaboo
author_rank: Super Contributor
timestamp: December 19, 2023, 05:38:49 pm
content: Quote from: Alex Wolf on December 17, 2023, 07:36:52 pmYou are not wrong, those 4 terminal shunts are better than the usual 2512 and others.But OP seems to not even own a thermocouple, so I question a little bit the attempt to make a serious measurement equipment.I've used and designed in a lot of shunts during my career. Usually the selection is based on requirements, and we didn't see those, other than "it shall not get hot to the touch".

author: temperance
author_rank: Super Contributor
timestamp: December 22, 2023, 02:33:01 pm
content: @ Alew WolfQuoteAn other factor to consider is that the CTE of standard 2512 ceramic resistors doesn't really match with the CTE of FR4 material. Repeated thermal cycling will cause too much stress on the solder joints.

author: shapirus
author_rank: Super Contributor
timestamp: December 22, 2023, 02:52:21 pm
content: Let me derail this thread a little bit, especially considering that the OP apparently no longer cares :).Is the average power dissipation the only parameter that we care about when we choose the size of a current sense resistor?To be more specific, let's pick a 0.1 Ohm 0603 SMD resistor that will dissipate around 25 mW of power on average, but in very short (tens of microseconds) repetitive pulses with peak current at the end of the pulse reaching, say, 50 A. Will the high current per se cause any damage to it, or the only damaging factor is heat?

author: coppice
author_rank: Super Contributor
timestamp: December 22, 2023, 03:35:13 pm
content: Quote from: shapirus on December 22, 2023, 02:52:21 pmWhether its short sharps shocks of heat, or average long term heat its pretty much always heat that is the damaging factor. I commented recently, on another thread, that this is something resistor makers don't specify very well, and someone responded with a data sheet from Yageo,https://www.yageo.com/upload/media/product/productsearch/datasheet/lr/YAGEO%20FMP_datasheet_2023v4.pdf, that does provide proper derating curves for one of their resistor product lines when subjected to this type of thermal shock. I guess the market is responding to most things now being switched mode, and very bursty.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 22, 2023, 03:35:59 pm
content: i do think they'll damage to some extend, you over-spec them even for a few millisecs ....

author: shapirus
author_rank: Super Contributor
timestamp: December 22, 2023, 03:44:03 pm
content: Quote from: coromonadalix on December 22, 2023, 03:35:59 pmI feel intuitively that it might be the case, but what the mechanism would be?

author: shapirus
author_rank: Super Contributor
timestamp: December 22, 2023, 03:49:45 pm
content: Quote from: coppice on December 22, 2023, 03:35:13 pmYeah those "Pulse diagrams" are useful: they can be used to extrapolate the values, more or less, to other resistor sizes, I guess.

author: magic
author_rank: Super Contributor
timestamp: December 22, 2023, 04:36:09 pm
content: Quote from: shapirus on December 22, 2023, 03:44:03 pmOne issue is well known and sometimes characterized in datasheets of semiconductor devices (transistors, diodes). Even if the average temperature of the part stays within ratings, the peaks that occur during load bursts may exceed these limits. Amplitude of those cyclic thermal variations is impossible to predict using only information available in typical resistor datasheets.Another thing is, I don't know, maybe some mechanical problems due to repetitive thermal expansion. Total speculation, but possible in theory.

author: shapirus
author_rank: Super Contributor
timestamp: December 22, 2023, 05:33:42 pm
content: Quote from: magic on December 22, 2023, 04:36:09 pmThis assumes that temparature of the resistor follows the power pulses, which, given the thermal mass of even such small device as a 0603 resistor, doesn't sound very realistic to me: while the power pulses are huge, the momentary temperature values shouldn't be much different from the average value.On the other hand, resistor's thermal conductivity is not infinite, thus some momentary overheating in very narrow local spots may very well be taking place, leading to the degradation of the conductive (resistive?) material inside the resistor. Aside from thermal processes, I wonder if there's anything on the electromagnetic side of things that may play a role here.p.s. I can actually test this (well, to be specific, whether the resistor survives and if its resitance changes after a certain duration of such testing). I have everything required almost ready. Will report if/when I have any results.

author: magic
author_rank: Super Contributor
timestamp: December 22, 2023, 06:33:01 pm
content: Most of your 0603 resistor is a dummy brick, the actual "resistance" where heat generation occurs is found in a thin layer of metal or metal oxide.

author: shapirus
author_rank: Super Contributor
timestamp: December 22, 2023, 08:39:19 pm
content: Quote from: magic on December 22, 2023, 06:33:01 pmYes exactly. And then it's a question of heat capacity of that thin layer, its thermal conductivity and thermal conductivity between it and the rest of the resistor.Now, I already have some results. My signal generator (JDS2800) is somewhat limited and doesn't allow to adjust frequency and pulse width separately; it is only possible to set frequency and duty cycle, the minimum value for which is 0.1%. This signal generator drives, in the end, a MOSFET switch that turns on when there is an incoming pulse and allows current to flow via whatever (in this case the resistor under test) is connected to a 2-pin terminal block.Now, the limitation of the signal generator means that the lowest possible average power can be P = 10-3* Pp, where Ppis peak power. At 50 A, for example, this means (50A)2* 0.1Ohm * 10-3= 0.25W, which is 2.5 times the typical nominal power of an 0603 resistor, so I can't properly test it for what I wanted :).I still tested it at 50 A, 200 Hz, pulse width ~5 usec, rise time ~1.5 usec. It died in ~2 minutes, failing open. No magic smoke escaped.I replaced the resistor and continued testing.1) 15 minutes at 30A pulses (0.09W, again not counting the non-zero rise time, so probably ~15-20% less in reality): all good, no change in resistance. Well, maybe 0.2 mOhm less, if my TR1035 isn't lying, and I have no reason to assume that it does -- but I may have simply placed the test leads differently.2) 15 minutes at 35A pulses, ~0.12W calculated, probably ~0.1W actual. Or I think even more: 4 usec at 35A + 1 usec rise time. No changes in the observed voltage drop waveform so far, the resistor became warm. I'll let it run until I get bored and then measure its final resistance and call this mode of operation safe enough :).3) I think I need to build a pulse generator with independent controls for pulse width and frequency....update: having taken a closer look after 35 minutes, I see that the pulsed current has increased to ~36A. TCR in action, I guess.

author: magic
author_rank: Super Contributor
timestamp: December 22, 2023, 10:00:36 pm
content: Another potential possibility: electromigration. Happens at sufficiently high current density per conductor cross section, I don't know if resistors are small enough to be meaningfully affected by it. Such damage would accumulate over long term.

author: shapirus
author_rank: Super Contributor
timestamp: December 22, 2023, 10:19:22 pm
content: This is interesting. After two hours of being subjected to ~35A pulsed current the resistor's resistance increased from 100 mOhm to 116 mOhm (and it's probably this that made the voltage drop increase over time rather than the effect of TCR).It's not what I expected. Very interesting. It'd be very interesting to test it at an even higher current, but well below the max nominal average wattage. I guess I'll do it if/when I build some simple pulse generator.

author: tszaboo
author_rank: Super Contributor
timestamp: December 23, 2023, 05:48:26 pm
content: Quote from: shapirus on December 22, 2023, 02:52:21 pmYou select the resistance to have a meaningful voltage at full scale. For example 100mV is a good goal. You select the size of the resistor based on the dissipation. And you select the type/material of the resistor based on the desired accuracy, for example PPM/K or long term stability. Going below 100mV is possible, but then you exchange heating issues with amplification and thermocouple issues. 100mV is a good compromise.