Isolated standoff about 6/32" - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: February 18, 2023, 02:11:40 pm
content: I'm mounting a a 100W braking resistor to a grounded plate, the bus voltage is about 350Vdchttps://www.digikey.ca/en/products/detail/ohmite/L100J100/258334Not sure I want to use the standard; long bolt, L bracket and nylon washer type mounts considering the plate is grounded (not sure how much they are relying on the thin nylon washers for isolation)So I'm thinking about using some sort of isolated standoff, looking for a source?

author: jpanhalt
author_rank: Super Contributor
timestamp: February 18, 2023, 02:35:02 pm
content: A shoulder washer might alleviate some of your concern. They stay centered on the bolt. Is a nylon bolt out of the question?

author: TimFox
author_rank: Super Contributor
timestamp: February 18, 2023, 02:45:00 pm
content: If you run a long bolt through the tubular resistor, with old-style fiber shoulder washers at each end, nothing metal touches the resistor itself.Nylon can be a problem due to its low service temperature: 250oF = 121oC

author: jpanhalt
author_rank: Super Contributor
timestamp: February 18, 2023, 03:03:39 pm
content: Upon rereading the post, have you considered threaded ceramic standoffs? Use two screws that don't touch. Digikey has them:https://www.digikey.com/en/products/filter/board-spacers-standoffs/582?s=N4IgTCBcDaIMYFMBOBDAtgSzgAgM4BcUA7AEwHsAzCkAXQF8gI got some off eBay awhile back.

author: Jester
author_rank: Frequent Contributor
timestamp: February 18, 2023, 04:13:02 pm
content: Thanks for the suggestions, I started thinking by the time I order something I might as well just make something.I don't expect the resistor to get very hot, because it only gets energized for a second or so while stopping and it's not like I'm constantly starting and stopping.I turned the corners of the nuts to make them self centering. The creepage (tab to nut) looks small but it's about 1/4" per side so that should be adequate. If it has a meltdown I will try something else.Shoulder washer is a good idea, I will include a couple of shoulder washers on my next DK orderNylonhttps://www.digikey.ca/en/products/detail/essentra-components/12SWS2558/9676524Fibrehttps://www.digikey.ca/en/products/detail/keystone-electronics/3069/315845

author: Jester
author_rank: Frequent Contributor
timestamp: February 18, 2023, 04:25:33 pm
content: Quote from: TimFox on February 18, 2023, 02:45:00 pmTim,Temperature range of the fibre washers?

author: TimFox
author_rank: Super Contributor
timestamp: February 18, 2023, 04:36:02 pm
content: Actually, the fiber washer's temperature spec is similar to Nylon's.A good question, but it was harder to find the answer than I thought.I like fiber because it's interface to a rigid ceramic (as in the resistor) is a bit more "forgiving".

author: coppercone2
author_rank: Super Contributor
timestamp: February 18, 2023, 04:58:52 pm
content: I use ceramic shoulder washers, stainless threaded rod, brass nuts and wavey washers for something very similar I built, on a stainless bracket that attach to chassis. And high temp loctite on the thread, if I thought it was going to get hotter I would put lock wire or a castle nutIf you have a slightly imperfect fit, you can use a diamond hand file to ream the resistor a little bit so it sits on the cup washer nicelyThe mounting bracket is a long L shape and a short L shape, that sit to make a |_| shape with the resistor in between, and the whole thing attaches to pcb/chassis with two threaded holes on the bottom bracket (bolts from bottom)I do wonder if fiber washers or wavey washers will better serve the test of time. Often in old equipment (very old) I see the fiber washers deform and the power resistors are loose, but they typically orient the smaller ones so they are upright, with a ceramic standoff from the PCB. With the long one you need to make the holder. Not sure if its the PCB deforming or the fiber washer deforming, but I figure metal + ceramic is better then fiber insulation, so long the washer does not losen up.I very much like the wavey washers BTW, very gradual compression force compared to other methods that seem to tighten up arbirtarily.

author: coppercone2
author_rank: Super Contributor
timestamp: February 18, 2023, 05:18:38 pm
content: might as well post it, not the nicest thing I built but it seems strong enough.. bit of a drilling issue and the plasma cut was a bit rough. There are washers hidden in both sides of the ceramic, there is no contact on the edge of the ceramic, just flats interacting. And the resistor floats in there on one side, then the other side is added and bolted on, so if you remove the left bracket the resistor is still perfectly tightened.

author: DavidKo
author_rank: Frequent Contributor
timestamp: February 20, 2023, 11:29:24 am
content: Such a resistors are used at high temperature to increase the cooling effect by flowing air (the reason for 350°C max. temperature).You should be careful with the wire connection - use the screws or welding. Wire insulation should withstand the maximum expected temperature.Standard fixation is done through metal brackets due to the temperature of the resistor itself - spring washers to be used due to the screw prolongation from the heat. If you do not expect vibrations you can fix the resistor through the connection wires.

author: DavidKo
author_rank: Frequent Contributor
timestamp: February 20, 2023, 11:41:31 am
content: By the way. The informations you need are in datasheet.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 20, 2023, 12:28:36 pm
content: If you look at the bottom of the L100J100 Digikey page, you see the recommended mounting hardware. Which is of a completely ridiculous price, so I would never ever buy it just for that reason alone. I simply refuse to pay USD21 for a bracket that costs less then USD 50ct to make.A quick check on Ali suggests cheaper prices, but the difference is smaller then I expected. But it does include the "mounting hardware" and shipping. (If you buy ten, then shipping is EUR4.3 per resistor).I really don't like the thick bolt and plastic standoffs in your picture. These resistors are designed to become very hot, and if this happens quickly, the ceramic expands quicker then the bolt and this stuff is very brittle and may break. With a very thin center bolt, then the ceramic is probably strong enough to just stretch the bolt a bit, or the thin brackets at the end bend a bit. The plastic standoffs will also melt at the normal working temperature of these resistors. There must be some room for thermal expansion as ceramics are very brittle.As for isolation...The plastic standoffs won't add very much if the steel bolt shorts the two ends of the resistor. But as these resistors are designed to be mounted with metal brackets, I assume the terminals are far enough from the end to just use metal brackets without extra precaution.The Ali model I linked to appears to have a slightly different construction. It appears to have separate ceramic washers at the end, but I'm not entirely sure.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 20, 2023, 12:38:15 pm
content: Quote from: DavidKo on February 20, 2023, 11:29:24 amI'm not sure whether the isolation has to withstand the fully rated temperature.The steel lugs have a bit of a standoff from the resistor, and therefore will be a bit cooler. If you use relatively thick copper wire (good heat conductor) then it will wick away more heat from the connection. As far as I know the terminals are designed to be soldered to, even though the solder would melt below the overall working temperature of these resistors.

author: coppercone2
author_rank: Super Contributor
timestamp: February 20, 2023, 12:42:48 pm
content: jon paul has written advice in another thread of mine about mounting this resistors (I guess his way is more industry standard). ALso, someone in that thread mentioned that you can put a stack of cut washers to act as a expansion spring.For my setup, I tested how much extra I can tighten it before I start to feel heavy resistance to turning from the wavy washers, and I think its enough. My system is limited by the super capacitors, and I have discharge tested it, so it does not get above ~95c, I mostly just wanted to built it so the super capacitor mount is similar in construction to the resistor mount.The fiber washer does seem like a poor substitute for a spring, but I see just fiber washer + ceramics used in Keithley equipment. And some HP equipment just has these resistors that are bolted to chassis with no expansion hardware. It all depends on the maximum temp you expect I guess, but I would not be suprized if reliability was lower.

author: coppercone2
author_rank: Super Contributor
timestamp: February 20, 2023, 12:44:37 pm
content: Quote from: Doctorandus_P on February 20, 2023, 12:38:15 pmI have a case of these resistors that i removed from stuff and the trash, some of them have wires that are spot welded on, and some of them have wires that were soldered on. Some resistors appear to have steel leads, that are not solderable (that is, you can only solder to the wire that was spot welded (or it could be a braze I guess, copper steel spot weld not sure if possible) or crimped to the resistor, like some of them have rolled crimps that are indented IIRC (like the type of crimp you see inside of a old spraqe capacitor). And I never saw a bolted one (at least from anything I removed). But I never service power electronics.You need thermal imager testing

author: Jester
author_rank: Frequent Contributor
timestamp: February 23, 2023, 05:40:04 pm
content: Lots of good comments.The nylon blocks are threaded. I'm going to make one of the conical nuts into a conical spacer by drilling the threads out and then putting a spring between the block and nylon block on that side that should resolve any expansion issues.In practice in my application the resistor only gets warm not hot, it only dissipates power for about 1-2 seconds tops.