DIY -Wirewound Resistor Coating.

[eclorian]

I need to re-make a wire wound resistor for an old sunbeam mixer. From the research I've done, it seems to use 36 AWG nichrome 60 wire. It's wrapped around a ceramic core. I need to coat the resistor with something, and I have no idea what to use. There was a guy who used to rebuild these, and I have a picture of the coating he used, but he doesn't make them anymore.

It originally had some type off white coating that may have been some type of ceramic or plaster. This was from the factory in the 60s or 70s.

I found something called Cerama-Dip, but the smallest jar is over $100. I also saw a recommendation on a similar thread about using sodium silicate.

Any recommendations would be greatly appreciated. I know they heat up quite a bit, so it needs to be high temp resistant.

Picture of brown coating this guy used to use....

 

[rjenkinsgb]

Gun-gum ? eg. A car exhaust repair paste. That must be able to stand extremely high temperatures.

Holts Gun Gum Paste 200g | Halfords UK

Shop the latest Fix your car exhaust at home with Gun Gum Paste. at Halfords UK

www.halfords.com

Or fire cement, used for patching open fire hearths?

 

[Nigel Goodwin]

Historically they were commonly referred to as 'cement coated', presumably dipping them in a thin cement mix would coat it?.

But of course there's no electrical need for it to be coated anyway, it's only for slight safety reasons, and aesthetics.

 

[eclorian]

Historically they were commonly referred to as 'cement coated', presumably dipping them in a thin cement mix would coat it?.

But of course there's no electrical need for it to be coated anyway, it's only for slight safety reasons, and aesthetics.

I was under the impression that some type of coating would be ideal to avoid oxidation. I have seen references to these specific resistors being used just fine without a coating though, so maybe that is with different materials.

My concern was keeping the loops separated because this is 6ft of wire wrapped around a small core, so the loops will be pretty close together.

 

[eclorian]

Gun-gum ? eg. A car exhaust repair paste. That must be able to stand extremely high temperatures.

Or fire cement, used for patching open fire hearths?

Would the Gun Gum be non-conductive? I've seen it mentioned that non-acidic is important, maybe I would have to test for that.

I think fire cement is sodium silicate, so maybe that is the best way to go, I've seen a few recommendations for that or some version of it.

 

[Nigel Goodwin]

I was under the impression that some type of coating would be ideal to avoid oxidation. I have seen references to these specific resistors being used just fine without a coating though, so maybe that is with different materials.

My concern was keeping the loops separated because this is 6ft of wire wrapped around a small core, so the loops will be pretty close together.

If the wires are that close together I suspect coating it could easily cause them to touch?.

I doubt oxidation would be an issue.

 

[eclorian]

If the wires are that close together I suspect coating it could easily cause them to touch?.

You may be right. I tested this using the original wire that was wrapped around it while waiting for my nichrome spool to arrive, but it would slip at the slightest touch from the smoothness of the ceramic tube. That wire is in pretty bad shape though, with bends and such from unwrapping it, so maybe the new wire will hold better being smooth and straight.

Even some kind of thin ceramic coating would give me piece of mind though, but I'm worried if I pick just anything it could cause oxidation issues or break down with the heat.

 

[Nigel Goodwin]

How hot does it run? - if it's not too hot, how about a thin coating of epoxy resin?.

 

[Pommie]

If you just want to get it working then just use a modern wire wound resistor. If it's a restoration type project then don't you need to use the original method?

Mike.

 

[eclorian]

If you just want to get it working then just use a modern wire wound resistor. If it's a restoration type project then don't you need to use the original method?

Mike.

I have read accounts of several professional restorers trying to do this for a long time with no success. People have tried 25W resistors that blow out. My suspicion is that the ceramic core dissipates heat in a way that a modern resistor cannot, but that is just an idle guess. Maybe the nichrome wire used is a higher gauge than what is typically used for a wire wound resistor.

The strange part is, I have seen one guy who claims to insert the Dale 5W resistors into the ceramic tube and then adjust the lowest speed setting to a higher speed. (It's a governor resistor, higher speed = less stress on resistor). My concern here is that perhaps this is not a good long term solution. I don't think there is enough space for a modern wire wound resistor that is rated higher than 25W, and I'm not sure why the 25W heat sink style resistor blew out, and the 5W apparently doesn't as long as a higher motor speed is maintained.

 

[eclorian]

How hot does it run? - if it's not too hot, how about a thin coating of epoxy resin?.

So from my research, I don't know an exact number, but I've read they get pretty toasty. I bought a repair book for it from the guy who does the restorations and in it he mentions a 2-part refractory cement, but does not give any more clue than that.

 

[shortbus=]

in it he mentions a 2-part refractory cement, but does not give any more clue than that.

2-part refractory cement, - Google Search

I'd stay away from the ones used to lay bricks, they have a lot of sand like most mortar mixes have.

You could also try regular old Plaster of Paris. It has a pretty high temperature rating 1200C/2200F.

Plaster mold casting - Wikipedia

en.wikipedia.org

 

[eclorian]

You could also try regular old Plaster of Paris. It has a pretty high temperature rating 1200C/2200F.

Plaster mold casting - Wikipedia

en.wikipedia.org

I actually did a search for the 2 part cement, but everything I could find on a consumer level seemed to be a tub of something, and I wasn't sure if any of those could have issues with acidity or other complications.

I had considered plaster, but I didn't know if that would be a safe bet, once again, my knowledge of material science is elementary at best.

 

[Pommie]

Have you considered trying a 50W (or 100W) resistor? EG, these.

Mike.

 

[eclorian]

Have you considered trying a 50W (or 100W) resistor? EG, these.

Mike.

I did look at some, but I think they were all too big to fit in the small space. The circuit uses a point to govern the speed, I think it's similar to points on an old car. If I'm seeing this right, it seems like it opens and closes at different speeds based on the amount of contact, the resistor is in parallel with this contact point and at high speeds it is almost bypassed. The resistor is less than 40mm in length and only about 15mm or so in width, and seems designed for the small space it fits in.

Here is a video I found of the governor in action:

 

[Tony Stewart]

I might have expected magnet wire over a metal thermal conductor rather than nichrome wire that is higher resistance but can short out adjacent turns.

 

[eclorian]

I might have expected magnet wire over a metal thermal conductor rather than nichrome wire that is higher resistance but can short out adjacent turns.

Based on what I just read about magnet wire, perhaps inductance would be an issue, not to mention it would take a lot of turns to get the same resistance. In a coil like that I'm not sure what power dissipation would look like, but I would think multiple layers of wire would have more issues dissipating heat than a single layer would.

 

[eclorian]

In my head I almost see the nichrome wire heating up like a space heater would, but perhaps that is over the temperature it would actually get to.

 

[throbscottle]

My first thought was water glass, but I see you've mentioned it in your original post. It's cheap and widely available. See wikipedia for it's use in refactory cement: https://en.wikipedia.org/wiki/Sodium_silicate#Refractory_use - just need to find you some powdered minerals and you can make your own "secret recipe"

 

[eclorian]

My first thought was water glass, but I see you've mentioned it in your original post. It's cheap and widely available. See wikipedia for it's use in refactory cement: https://en.wikipedia.org/wiki/Sodium_silicate#Refractory_use - just need to find you some powdered minerals and you can make your own "secret recipe"

I found some water glass on Amazon here:
Lynn Manufacturing High Temperature Adhesive, 2000F Rated, Sodium Silicate, Water Glass, 40% Solution, 4oz Bottle, 5602 https://a.co/d/7STK2Ca

I was also considering this furnace cement:
Rutland Products Black, 32 fl oz Furnace Cement, 32 fl oz, 32 Fl Oz https://a.co/d/2pnTLiv

I was just worried if I bought one and it didn't work, I would be out the ceramic tube I'm recycling.