cement for cemented power resistors - Page 1
Forum section: electronics / general

author: NWerner
author_rank: Contributor
timestamp: January 05, 2023, 06:32:16 am
content: Hello,I would like to DIY some low valued aluminium housed power resistor with a power dissipation of approx 500W.Due to some project specific constraints none of commercial available models fits.Does anyone know which cement is usually used? My first foolish experiments yielded some unpleasant suprises (outgassing of water ).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2023, 06:37:32 am
content: And you can't use series-parallel combinations?Engineering a whole resistor, that needs to be stable, reliable, and handle high power, is no small feat. I very strongly recommend pushing back on the project constraints first.Tim

author: SeanB
author_rank: Super Contributor
timestamp: January 05, 2023, 08:56:20 am
content: Yes normally those use a tight packed magnesium oxide powder as heat transfer medium, which is then sealed in by capping the ends in epoxy for low temperature use, or for high temperature by using refractory cement to cap the ends, or by crimping in a nice ceramic plug. 500W is well within heating element area, which is easiest to do if you use the services of a regular heating element supplier, as you can buy off the shelf 500W push in heating elements, or for a little more have them custom made for you, with a standard diameter tube, and desired resistance at temperature. Those are cheap, though if you want to DIY you need to basically get a roll of resistance tape wire, and a sheet of formed mica, which is used to make a former, and to make a set of insulators for the sides, and then the whole lot is clamped between 2 heatsink blocks. Connections are made with high temperature fibreglass insulated wire, and the joints are either spot welded, or are made using stainless steel eyelets, that crimp the wire and the tape together tightly. Last one cost me $100 to have custom made locally to me, using the old 1.5kW element as sample for dimensions.

author: NWerner
author_rank: Contributor
timestamp: January 05, 2023, 03:42:26 pm
content: Thank you for your comments. I was especially unaware of the possibility to use MgO powder.I want to add some specific requirement however as these are probably relevant: The design calls for a total of three resistors:a) 2x approx 50 Ohm each with approx 500W and 3KJ surge capability.b) 1x approx 0.05 Ohm , non-inductive with approx 500W and 3KJ surge capability (using a meandric stainless steel sheet).The total power sum of all resistors will however never exceed 500W. So the idea to use three resistors housed in a common heatsink is tempting. I'm fine with tolerances up to 20%.In present project I use commercially available resistors which results in an absurdely oversized solution. The 0.05 Ohm resistor is especially troublesome as commercial offering has very impractical form factor.Present solution is fine for intended use - but certainly triggered personal curiosity regarding alternatives.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2023, 11:41:33 pm
content: This kind comes to mind,https://www.usresistor.com/index.php/products/disc-resistorsthough with the smallest at 7kJ maybe it's overkill here, and it doesn't look like they suggest heatsinking, not with average power ratings that low.What's wrong with standard wirewound? Those are standard values.The combined rating doesn't get you anything, each element has to be rated whatever it does, period. Maybe you use less overall heatsinking (metal case type), maybe slightly less ventilation than otherwise (silicone/vitreous/etc.).If you need a very compact solution, water-cooled types also exist; you'll have to consult more specialized industrial suppliers to find those, though.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: January 06, 2023, 01:00:06 am
content: Use Stock Dale type NH non inductive or type RH inductive.avail 10w ...500w heatsin extruded Al housingjon