Ceramic Resistor

[ivanzu]

Hello,I'm trying to build a Ceramic resistor so I came on this forum to ask few questions since I don't have any experience when it comes to ceramics or pottery making.

 

Basically what I'm trying to do is to build a resistor housed inside of ceramic case so it can withstand higher temperatures produced by electric current,so here are the questions:

 

1. I have few pounds of natural blue clay taken from a river bed, is there a difference between clay depending on its color other than metals they can contain?  

 

2. At what temperature should I bake the clay?

From what I read most of the people say 1000-1400C but I don't know if its the temperature clay turns into porcelain or bisque, I need it to be bisque because I will most likely never be able to reach higher temperature then 1000C in a homemade "kiln" and bisque clay is good enough for my requirements.

 

3.Here is the basic idea of my kiln.The drawing stupid I know  but I think you'll get the idea.

The resistor will be suspended on a wire so it doesn't get in contact with the flame.

 

Resistor dimensions will be 50mmx20mmx20mm would it take long to fire an object of such dimensions?

 

 

Here is how the resistor normally looks when produced in a factory:

[Guest JBaymore]

Are you sure you can't find what you are looking for commercially?  What R and A are you looking for?  And at what breakdown voltage?  

 

Can't you heat sink the exterior of an existing one?  How about active air or liquid cooling?

 

best,

 

..................john  (ham radio op)

[ivanzu]

I can find a such resistor but the cost of shipping isn't very economical because I wouldn't need to order any other components other than a single resistor. I already have all the components required to build a resistor just need some information, in case I figure out that I wouldn't be able to build it without the clay shattering and being too brittle I was thinking of just making the case from lime mortar. 

 

5R

1A

 

So its fairly small current.

[Tyler Miller]

The ceramic in a resistor is a kind of porcelain engineered to have very specific properties.

[ivanzu]

That is true but in my case I just need the ceramic to act as a heat sink since its excellent heat conductor.

[Tyler Miller]

Actually, it's a poor heat conductor and excellent insulator. An aluminum heat sink, like John says above, would be a better option.

[Callie Beller Diesel]

Hey! Welcome to the forums.

In answer to your questions:

1) clay is essentially sediment formed by erosion. It's composition, and therefore properties, vary widely depending on geography, the composition of the parent rock and how far the clay is from said parent rock. The blue colour for instance could be from iron, or it could be from organic matter. Firing a sample of your native clay in a porcelain cup to the temperature you plan to fire to will tell you which. The extremely simplified explanation of the difference between porcelain and earthenware is that earthenware matures in the kiln when the iron in the clay body fuses. Porcelain has very little iron, and actually forms glass at maturity. Stoneware does a bit of both. (No one throw rocks at me, I know there are holes in this.)

 

The geographic differences in clay are part of the reason one of the first questions we will ask you is "where are you in the world?"

 

2)1000 degrees C is bisque temperature.

 

3) Clay doesn't really need to be protected from flame: it does need to be protected from uneven heat. You could try suspending it from Kanthal wire which will survive those temperatures, but I am assuming since you are building your own kiln and using a native clay that Kanthal might be a bit out of your way to acquire. Be reassured the wire is unnecessary. It probably won't take very long at all to fire something that tiny. Given a heating rate of 150 degrees/hour, the firing would take 6 1/2 hours (ish), plus cooling time. You will need to ease the clay through the temperature range of 575 C, as the silica goes through a structural change at this point.

 

Given the amount of testing and experimentation that you'd need to do to get this to work, you still may find it more cost and time effective to order your single part.

[ivanzu]

Actually, it's a poor heat conductor and excellent insulator. An aluminum heat sink, like John says above, would be a better option.

Yes you are right, but I can't use an aluminium heat sink since it would have to be in contact with the resistive wire and both of the materials are conductors which could be dangerous if some part of the circuit was to come in contact with the heat sink.

 

@Diesel Clay Thanks for the clarification! I do have Kanthal wire since its used for building heating elements.

[Guest JBaymore]

Given the amount of testing and experimentation that you'd need to do to get this to work, you still may find it more cost and time effective to order your single part.

 

5 Ohms, one Amp?

 

This thought by Diesel above really "cuts to the chase". 

 

You are likely going to spend a lot of time and wasted materials and fuel on finally getting this to work OK.  And until you have tested the resistor under load for an extended time before using it ........ you'll never know if, in the piece of equipment, they are going to fail and screw up other stuff.

 

Also... some "ceramic" resistors are actually not totally fired in the shape you see.  Some are actually ceramic based materials that are bound in a high temperature "glue".  Think sort of like sand and epoxy.

 

Are you in Antarctica or something where saving the shipping charges would be worth all the work?

 

best,

 

......................john

[ivanzu]

Let's say that one resistor costs 0.2$ since I can't order one resistor I would have to order them in a bundle of 5. So yes it's very cheap and price for resistors ends up at 1$ but shipping costs are 50$ because its under minimum order quantity so it ends up as waste of money and fairly expensive.

Since it looks like a fairly impossible task I'll probably end up making the case from lime mortar and then just stress test it and see how it performs. 

 

Thanks for the help,Cheers! 

[Bob Coyle]

here is a link on eBay http://www.ebay.com/itm/10pcs-Ceramic-Cement-Power-Resistor-10W-5-ohm-/390289107109?hash=item5adf0b28a5:g:41EAAOSwpDdU9-af

 

It lists 10 5ohm resistors for less than $7.00 US. Yes 10 is more than you need but unless your time means nothing to you, it just ain't worth the effort to roll your own. These resistors may fail, even though they are ceramic. I doubt is the manufacturer had your application in mind. The resistance element is usually cemented into the ceramic shell and the cement may not like 1000C.

 

I have fabricated ceramic components for electronic parts, but never an enclosed resistor. It should be doable though. Just wind the wire to get the resistance you need, form a hollow tube of clay and insert the coil( make sure the wraps have room to expand and don't touch each other.) crimp the ends of the tube around the  wires coming out and allow the clay to dry thoroughly, then slowly fire it up to 1000C. If it doesn't blow, measure the resistance again and if it is in the ball park, give it a try. Your local clay may work. You never know till you try.

[MatthewV]

Just so you know: diamonds and alumina are two materials with a high thermal conductivity but low electrical conductivity. All (or 99.9%) of heating elements are packed with alumina powder.

 

You can get alumina powder by opening up the casing of any heating element. Or ordering it.

[High Bridge Pottery]

So is a ceramic resistor to insulate the component or to 'uninsultate' it. I am confused. Surely it is to keep the heat of the resistor in and not going to do what you want it to?

[PeterH]

but shipping costs are 50$ because its under minimum order quantity so it ends up as waste of money and fairly expensive.

 

A search for resistor 5ohm 5w on www.ebay.com finds things like:

10PCS 5W Watt 5R 5OHM 5% Ceramic Cement Power Resistor Free Shipping AK ... for $2.52

http://www.ebay.com/itm/10PCS-5W-Watt-5R-5OHM-5-Ceramic-Cement-Power-Resistor-Free-Shipping-AK-/281783044009?hash=item419b9407a9:g:aocAAMXQfvlSegLK

... so looking at small-order suppliers to your region might be worthwhile.

 

From: https://en.wikipedia.org/wiki/Resistor

Resistors required to dissipate substantial amounts of power, particularly used in power supplies, power conversion circuits, and power amplifiers, are generally referred to as power resistors; this designation is loosely applied to resistors with power ratings of 1 watt or greater. Power resistors are physically larger and may not use the preferred values, color codes, and external packages described below.

If the average power dissipated by a resistor is more than its power rating, damage to the resistor may occur, permanently altering its resistance; this is distinct from the reversible change in resistance due to its temperature coefficient when it warms. Excessive power dissipation may raise the temperature of the resistor to a point where it can burn the circuit board or adjacent components, or even cause a fire. There are flameproof resistors that fail (open circuit) before they overheat dangerously.

Since poor air circulation, high altitude, or high operating temperatures may occur, resistors may be specified with higher rated dissipation than will be experienced in service.