vitruitous resistor mounting? - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: November 22, 2022, 07:52:10 am
content: I have a big resistor that fits my needs that I want to mount in the chassis. Something like thishttps://www.digikey.com/en/products/detail/ohmite/L50J100E/823581What is the best way to mount these guys? I see a few different solutions.1) pipe clamp thing2) spring bracket on each side3) bolt that mounts through the resistor.I like option #3 the best, but it seems like you should have ceramic bushings for the bolt ? Kind of like a ceramic insulating washer on each side that fits into the resistor and allows the nut on the threaded rod to tension itSomething like this?https://www.amazon.com/TEMPCO-Ceramic-Terminal-10-32-Threads/dp/B07HM646GV

author: JohanH
author_rank: Frequent Contributor
timestamp: November 22, 2022, 08:20:18 am
content: A bolt could affect inductance (if it matters in your case), maybe non-magnetic material would be preferred.How about rubber mounting the lugs? Like this:https://www.amazon.com/M4-Vibration-Isolator-Studs-Anti-Absorber/dp/B0BCK5Z82H

author: srb1954
author_rank: Super Contributor
timestamp: November 22, 2022, 08:28:05 am
content: Quote from: coppercone2 on November 22, 2022, 07:52:10 amYou need a pair of brackets like this. They allow for thermal expansion of the resistor body and also still allow for the flow of some cooling air through the inside of the resistor.

author: Ian.M
author_rank: Super Contributor
timestamp: November 22, 2022, 08:28:45 am
content: Did you look at the Ohmite mounting hardware?https://www.ohmite.com/mounting-hardware/and select under 270 Series: Part # L50xxxxIts just a metal top hat washer to locate on the ID of the resistor, and a significantly larger mica washer that fits over the center of the hat washer between it and the resistor end to provide adequate creepage distance from the terminals.Noninductive is probably desirable, so sleeve your mounting bolt in an aluminum tube spacer that's a loose fit in the resistor ID, which will add eddy current damping to any inductive effects, then you can use plain steel penny washers and matching washers cut from mica sheet at each end. I'd probably add a couple of Belville washers to the stack-up to provide a more uniform clamping pressure allowing for differential thermal expansion.Alternatively, if the device wont experience severe impact shock loading, brackets similar to srb1954's suggestion can also be found on the same page. The difference is, the ends are bent back to form a springy bit that prevents them rattling in the resistor's ID. You could bend them up yourself out of mild steel strip.

author: coppercone2
author_rank: Super Contributor
timestamp: November 22, 2022, 08:38:41 am
content: I went with ceramic insulator bushings that fit (might need a bit of diamond filing). I will use that if I like it but I kind of wanted them for other applications anyway.Ima put 2 ceramic bushings with a bolt in the middle. What material is good for a bolt that will not seize up? I had stainless hardware that held a power inductor in place get totally rusted (like a whole gram of rust powder came out when I was trying to unscrew the thing, not my work, a commercial product emi related thing), and this gets hotter then a power resistor.Any exotic metal suggestions?It is just for a discharge circuit for stored energy like batteries ETC.

author: coppercone2
author_rank: Super Contributor
timestamp: November 22, 2022, 08:40:52 am
content: I am reading the comments about expansion and maybe its not the best idea, but anyway i still want those bushings.I also through to make something like a spring out of very stainless steel sheet I have that is a spiral shape and goes into both sides of the resistorI read in some document somewhere, I think by ohmite, that they have ceramic hardware for mounting this stuff. What do they mean by that if the thermal expansion is a problem?I mean like imagine a tall watch spring on a post goes into each side of the resistor (wound slightly) so when its release it expands inside of the resistor and makes a snug fit thats somewhat compliant

author: Ian.M
author_rank: Super Contributor
timestamp: November 22, 2022, 08:54:56 am
content: Quote from: coppercone2 on November 22, 2022, 08:38:41 amStainless allthread + brass nuts, or even better bronze nuts.(Stainless on stainless tends to gall and can seize just spinning a nut down a long thread!)Notch the edge of the bushings so the air inside the resistor core can easily exchange with the ambient atmosphere as it heats and cools, so it doesn't become oxygen depleted, as most grades of stainless steel are not corrosion resistant in an anaerobic environment.On the expansion issue, a small stack ofBelleville washerswith every other one flipped so they are edge to edge, under one or both nuts will effectively spring clamp the resistor between your ceramic bushings to allow for differential thermal expansion. Depending on the expected vibration environment, you may also need a pair of nuts at each end so you can lock them against each other to prevent them 'walking' loose.Edit: corrected spelling of 'Belleville'.

author: coppercone2
author_rank: Super Contributor
timestamp: November 22, 2022, 09:37:20 am
content: yeah maybe it was anaerobic corrosion because I never saw a stainless bolt that bad, it was colored like a old BBQ part, this was like a 2.5x2 inch inductor with a bolt and washer

author: coppercone2
author_rank: Super Contributor
timestamp: November 23, 2022, 02:32:25 pm
content: ok so if I do the ceramic washer mount method, how should I support the bolt?I thought maybe if the washers fit inside of the thing, then to have two bolted washers on stands on both sides of the resistor, so the resistor is sandwiched in there semi tight, but does not have a bolt running through the entire thingor to put it on a steel base plate that is coupled to the chassis bottom with rubber standoffs? Like I am worried that a impact on the botom of the chassis will transfer to the resistorIf it breaks and falls down nothing bad will happen really, its just to discharge the supercapacitors on a spot welder inside of a thick steel box (12 gauge) that has big vents, and its for learningi think the manufactures solution with the metal springy things is maybe better, but I will still put it together to see how it looks and feels.

author: jonpaul
author_rank: Super Contributor
timestamp: November 24, 2022, 02:45:55 pm
content: Coppercone,R, W rating ?Max V to ground?cooling? application?Jon

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 24, 2022, 03:39:29 pm
content: If you want to use a bolt, then make sure there is some kind of elasticity in the system. The resistor can get quite hot and ceramics are always brittle.I've saved several old resistors in that form. some have a bolt though them, but have plastic (some kind of thermo harder) washers to keep the bolt in the center. One of them has a single metal spring in an "U" shape with dimples that camp into both holes on the end of the resistor.

author: coppercone2
author_rank: Super Contributor
timestamp: November 25, 2022, 06:57:58 am
content: I am not sure if it needs a fan, its 37W Pdisp, 60W rated resistor, 12V to ground. Going to be next to a chassis vent with a chassis that has big vents on both sides

author: jonpaul
author_rank: Super Contributor
timestamp: November 25, 2022, 02:17:01 pm
content: Coppercone, Rebonjourhave worked in power electronics, we used all types of load R, to 20kW.5..10 w axial WW20..250 W Ohmite céramique50..500 W Dale extruded aluminum, heatsunk, NH non inductive type RH inductive.https://www.vishay.com/docs/30201/rhnh.pdfTermination is lugs or threaded boltsThe céramique tubes use metal Z shaped brackets for the resistor body diameterEnd shoulder washers of fiber and long bolts are the alternativehttps://www.ohmite.com/assets/docs/hardware_resistor.pdfOr a few 10W axial WW in series or parallel.bon chanceJon

author: coppercone2
author_rank: Super Contributor
timestamp: November 25, 2022, 06:09:22 pm
content: I am avoiding the chassis mount ones because I think I can separate it enough to prevent it from heating something else when its right next to the vent, and its only to de-energize when powered off via a safety switch.Does the fiber washer allow for expansion? I see you did not mention ceramic hardware (other then the resistor). I never saw one before. Maybe its a better choice then bevel washer + ceramic isolator.

author: coppercone2
author_rank: Super Contributor
timestamp: November 25, 2022, 06:13:03 pm
content: Ian.M, should the bevel washers be a particular thickness?I ask because I have seen the hardware store thickness ones, and I have seen ones (probobly copper?) that were only a little thicker then paper. Flimsy. And I have also seen them rounded with rubber (also called a bevel washer), it said it was a seal + a isolator. Rubber seems like a no-no.

author: Ian.M
author_rank: Super Contributor
timestamp: November 25, 2022, 06:39:34 pm
content: The ones you are after for this are thin spring steel, usually with a blued or black oxide finish. The thick ordinary steel ones are vastly inferior when you need to maintain a near constant clamping force and the ridged conical lock washers, or any with any sort of rubber or sealent pre-applied are totally unsuitable.

author: jonpaul
author_rank: Super Contributor
timestamp: November 25, 2022, 06:44:30 pm
content: The Ohmite hdwhttps://www.ohmite.com/assets/docs/hardware_resistor.pdfThick fibre shoulder washers.We use ceramic standoffs for HVfragile and threads easy tro srip...Jon

author: TimFox
author_rank: Super Contributor
timestamp: November 25, 2022, 10:00:57 pm
content: I used that style decades ago, and always used the Ohmite brackets.Besides the thermal expansion question, it is good to space such resistors away from the chassis for better airflow.(Back in 1969, I learned the hard way how hot such a resistor gets when operated at its rated power.)

author: coppercone2
author_rank: Super Contributor
timestamp: December 02, 2022, 05:28:10 am
content: are wavey washers or belleville washers better for this?I got the feeling that wavey washers were for lower force applications, which makes sense for ceramics?

author: coppercone2
author_rank: Super Contributor
timestamp: December 02, 2022, 03:29:45 pm
content: I bought wavey washers because it said low force applicationsMy plan is to peen the stainless bolt, put a nut, washer and the stainless steel bracket on (which is brazed to a steel plate), braze those together, put a stack of wavey washers between the flat washer on the bolt and the ceramic resistor (with another flat washer between the stack and the resistor), then put a flat washer and a bolt to compress the assembly (should i put wavey washers on both sides?, then put the other bracket and another bolt on that so the assembly is mounted together, then mount that to a steel plate, and mount the steel plate in the chassis with some strain relief in the middle for the wire, and put some high temperature loctite on the compression nut and the nut that holds the second bracket part thats not brazed on.or I can drill a hole through the whole thing and put a locking pin through the last nut (like a castle nut).

author: Ian.M
author_rank: Super Contributor
timestamp: December 02, 2022, 03:37:30 pm
content: Either will work, but you'd have needed several Belville washers stacked edge to edge to lower their spring rate, vs a single wavy washer. The Belville washers would typically be more durable as due to their thinness, wavy washers don't stand up well to corrosion.

author: coppercone2
author_rank: Super Contributor
timestamp: December 02, 2022, 03:39:37 pm
content: oh ok, I just managed to crack one of these resistors by dropping it the other day and i got worried that the standard washer i am able to find is too thick, and those washers were alot thinner.On the plus side I only measured the temp to go to around 135C with a thermal camera for a full capacitor bank discharge (8V). Does not look like I will need more then 8V. If it ends up getting hotter then that then it means something really broke and it needs service.

author: coppercone2
author_rank: Super Contributor
timestamp: December 02, 2022, 03:41:57 pm
content: on the other hand do you have a belleville washer that has a spec ? Like thickness and steel composition?Last time I try to do something with spring I had nothing but problems and nothing worked like I thought it would. The middle bolt is M5 dimension (it fits the ceramic hardware). I ended up getting three diff things and they were all wrong.. mad annoying.btw that wire is tough, the cracked resistor even if it fracture holds together pretty good with just the wire, so if you keep it on a bolt it will survive alot more then those brackets that don't have a center structure. Like you will have pieces of resistor that are still wired together rattling around the chassis. I am not sure but I believe this is how a power-line destruction bomb worked (desert storm 1). I feel alot better with the part on the bolt, because it should keep in place pretty good unless there is a grievous failure. If one of those resistors breaks in half and slips of the spring brackets, you will have a "ball and chain" projectile getting all over the chassis, unspooling and getting longer as time goes on until a short circuit current fuses it into smaller pieces. terrible. Amusing to think about what would happen if a ceramic resistor containing object got some airport and UPS love and tumbling down a staircase or three until it is plugged in for the first time on a 3 phase. Wired2blowhttps://en.wikipedia.org/wiki/Graphite_bomb(but with a conductive wire)

author: coppercone2
author_rank: Super Contributor
timestamp: December 02, 2022, 04:33:59 pm
content: this is what happened after a fall from bench height on wooden floor (2.5feet) (I unwound it a little longer with ease for dramatic effect, it just crumbles away and unwinds)

author: jonpaul
author_rank: Super Contributor
timestamp: December 10, 2022, 12:31:19 am
content: all ceramic parts, like these power resistors or Ferrite cores are brittle and fragile.We had same break in 100W WW dropped perhaps 6"Ferrite cores chip or break at the slightest drop.Jon

author: coppercone2
author_rank: Super Contributor
timestamp: December 10, 2022, 10:15:06 am
content: I think it depends on where it hits, if it lands on the copper legs they will deform but the ceramic will be alright, but if it hits on the ceramic it cracks