Design details for a butter knife warmer

[Shay10825]

Yeah never mind I found it. It has to be something catchy. Any ideas ?

 

[Cliff_J]

You might borrow from the old "...building a better mousetrap..."

Maybe two words or word fragments like many other products so maybe a combo of thermo and knife or slicer or something

Or visit places that sponsor inventions like Hammacher Schlemmer, they have tons of gadgets.
http://www.hammacher.com/
http://www.hammacher.com/sfi/sfimain.asp

Hope all is progressing well.

Cliff

 

[Shay10825]

The Better Butter Cutter (BBC)

 

[Cliff_J]

Hey I own a "Butter Butler", its handy for the few times I use butter. My dad has a coffee mug with a adapter to plug in the cigarette lighter to keep the coffee warm while driving, also handy. Who knows what people might like?

http://www.krbctv.com/Does It Work/DIW Archive/diw_archive_cooking_butter butler.htm

 

[Ivan Seeking]

Shay,
I wanted to try this before saying too much. Once you know how to use your meter you are safe to go ahead as follows: Using your 1/2 watt resistor and two D-Cell batteries connected in series [end to end like in a flashlight], go ahead and apply 3 volts across the resistor for ten or fifteen seconds. The resistor will get hot. If you leave the circuit connected long enough, eventually the resistor should burn up.

Potential [V]= 1.5 volts + 1.5 volts = 3 volts
Resistance [R]= 10 ohms
Current = 3 Volts / 10 ohms = 0.3 Amps
Power [P]= VI = 3 volts X 0.3 Amps = 0.9 watts

Since the resistor is rated for 0.5 watts and you are running it at 0.9 watts, you are running the resistor beyond its design limit. This is why it starts to get too hot.

Next, use 4 D-Cells, a 1.0 watt -10 ohm - resistor [available at Radio Shack], a piece of metal about the size of a knife blade, and your heat sinking compound.

By running the batteries in series we get:

1.5 volts X 4 = 6 volts
I = 6/10 = 0.6 amps
P = 6 volts X 0.6 amps = 3.6 watts

Again, I used a 1.0 watt resistor, so I was running the resistor at 360% of the design limit.

I did this using a fairly heavy piece of brass sheet metal - about 1/2" X 2" in size. First, a good layer of heat sinking compound was applied to the metal. Then a toothpaste sized blob was applied and the resistor was secured to the metal using only the cohesiveness of the compound. Pull the resistor's leads back so they don't short out on the metal and wiggle the resistor around a little to ensure a good bond. Also make sure that it's buried in the heat sink blob. This will pull heat away from the resistor and transfer it to the metal; thus preventing the resistor from burning up.

After applying power I that found that an area of about 1/2" X 1/2" got hot very quickly - in less than 30 seconds. Afer five minutes the entire piece of brass was very warm, with half of it too hot to hold. The batteries did not get hot and the resistor was hot but it seemed fine. So it appears that we can safely heat sink the resistors and overdrive them. This seems to work pretty well.

 

[Arctic Fox]

You may not have seen the original thread in GD where some similar suggestions were made. Shay10825 is hooked on a heated blade for now.

Hooked, huh?

No chance in a heated wire on a cheese cutter?

http://www.twinsupply.com/fdick_cutlery/81055-00.jpg

 

[Gokul43201]

After applying power I that found that an area of about 1/2" X 1/2" got hot very quickly - in less than 30 seconds. Afer five minutes the entire piece of brass was very warm, with half of it too hot to hold. The batteries did not get hot and the resistor was hot but it seemed fine. So it appears that we can safely heat sink the resistors and overdrive them. This seems to work pretty well.

There, now if you can only convince Ivan to ship over his stuff to you.

I'm not sure how long the resistors will last if you overload them repeatedly. You could consider using a couple or more resistors in parallel, to divide the power.

 

[Ivan Seeking]

The advantage is the significantly reduced mass and volume of the resistor. To a point she should be okay but I agree that this is unconventional.

Is this just our project now or is Shay coming back?

 

[Gokul43201]

The advantage is the significantly reduced mass and volume of the resistor.

In my approximation, the mass of the blade dominates the mass of the resistor.

Shay's got a couple weeks to go and seems to have fallen prey to the powerful yet illusory bliss afforded by procrastination. Haven't we all bitten on that, sometime or the other ?

 

[Shay10825]

Ok I'm back. I'm sorry. I've just been so busy with a ton of homework and tests these last couple weeks. It's been crazy at my school. The 9-weeks just ended so I'm working all weekend on this project. Ivan Seeking I'm going to try what you said right now.

Sould I use a 20 gauge wire?

 

[Shay10825]

Would this work?
http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=270-396

or do the batteries have to be on top of each other?

 

[Shay10825]

When I connected the .5 watt 10 ohm to the resistor it seemed hot when I touched it. It was weird (or maybe I'm just weird and it's suppose to feel like this). Are the lines on the resistor the only thing that's suppose to get hot? It did not get the metal hot. Is this because I did not put and heat sink on the metal? Is it suppose to be like this?

 

[Ivan Seeking]

20 gauge is perfect.

If you look at the connections on the battery case that you linked, you will see that they are wired in series - as if end to end, or on top of each other. So this should work fine. This means that you will have about 6 volts to work with = 4 X 1.5 volts per battery.

On your last question I'm a little confused. The entire body of the resistor will get hot. The lines are your color code - which you are supposed to know by now.

How many batteries are you using? Two? Then yes, the resistor should get hot. Again, this is producing 0.9 watts of heat in a 0.5 watt resistor. In principle, the resistor should eventually burn up or fail from excessive heat.

When you talk about the metal, do you mean the leads on the resistor or did you get a separate piece of metal? The metal leads on the resistor cannot touch your metal knife blade; or any other conductor for that matter. And you will need to use the heat sink compound to transfer the heat from the resistor's body to the knife blade. This compound will not short out the resistor leads; so it can touch the leads.

Make sure that you learn how to measure the voltage and current as you do this. check that the values measured agree with what you calculate.

 

[Shay10825]

What would happen if I used a 22 gague wire?

 

[Shay10825]

Which one is better?

http://www.radioshack.com/product.a...name=CTLG_011_010_006_000&product_id=278-1388

http://www.radioshack.com/product.a...name=CTLG_011_010_003_000&product_id=278-1222

 

[Cliff_J]

Home Depot.

They have speaker wire for a few cents a foot, one dollar would get more than you need.

Glad to hear its getting warm now, all that's needed is some compound to get the heat to the blade and then test to see how well it works and tweak from there.

Cliff

 

[Shay10825]

Does it matter if I use a 2-conductor Speaker Wire or a Hookup Wire? What happens if it is a 22 gague wire?

 

[Ivan Seeking]

22 gauge is fine but don't go any smaller. If the wire gets too small it could start to overheat and melt the insulation. The exact ampacity of the wire depends on the insulation used, but 22 gauge should be fine. I think the lowest rating for this size wire is a little over 2 amps.

 

[karmicthreat]

Man now I'm hooked on butter cutter technology. Instead of using a resistive circuit to heat the knife why not use a LC circuit resonating inductively with the metal in the blade to heat it. You would get nearly instant heat in the blade I'm not sure on the efficentcy though. This is the same method that inductive ranges and the metals industry use.

Also since you could get nearly instant heating on the blade, divide the handle into two electrically insulated halves. have a circuit measure the capacitance between the two hallves, when someone is holding it there will be a fairly reasonable range of change and the knife can activate. This way you won't wear down the battery of the knife by having to wait for it to get up to temp. Or having to dump a bunch of extra power into the knife to get it up to temp fast enough.

This sounds nearly as weird as the electric spoon I helped design. Had a temp sensor in the laddle and charged itself inductively when in its cradle.

 

[Shay10825]

I'm so happy ! The whole piece of metal got hot in a very short amount of time. I used aluminum. The problem is getting the resistor between the two pieces of metal and connecting the two pieces of metal together. A guy at Home Depot told me to use some stuff called liquid nails. Would this work?
http://www.homedepot.com/prel80/HDUS/EN_US/diy_main/pg_diy.jsp?CNTTYPE=PROD_META&CNTKEY=misc%2fsearchResults.jsp&BV_SessionID=@@@@1241869940.1099859104@@@@&BV_EngineID=ccdkadcmmlfdfeecgelceffdfgidgjj.0&MID=9876

I also bought wire connectors. They are the kind where you put the wires inside it (it is a tube) and you use pliers to push the middle of the tube in. These will have to be inside the blade. Is there any other thing that I could use to connect the wires because if this is in the blade the blade will be too thick.
http://www.homedepot.com/prel80/HDUS/EN_US/diy_main/pg_diy.jsp?CNTTYPE=PROD_META&CNTKEY=misc%2fsearchResults.jsp&BV_SessionID=@@@@1241869940.1099859104@@@@&BV_EngineID=ccdkadcmmlfdfeecgelceffdfgidgjj.0&MID=9876

Could I use heat shrink tubing to cover the blades of the resistor? I would put the blade inside this tube and put a fire in the tube to heat it so it will shrink around the blade? Can I do this? Or will the fire hurt the resistor?
http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=278-1627

 

[Cliff_J]

Glad to hear its getting very warm now. Of course you could use the multimeter and other resistors and battery combinations to get different heating...wink wink...

Liquid nails would work ok (it comes in different styles for adhering different materials) but just plain 100% pure clear silicone in a squeeze tube would work ok too. I saw some liquid nails brand silicone in a squeeze tube a Walmart just recently for like $3 and you don't need a caulking gun to apply. (its probably at Home Depot too) Clear looks cool. Oh, and work in a well ventilated area and maybe open a window, all this stuff stinks and you shouldn't breathe more vapor than you need.

Instead of wire connectors, you could likely just twist the wire and the resistor lead together really well. Normally solder would then be used to keep the twist together and improve the connection but if you twisted tight and then used heat-shrink tubing that should work well enough.

The heat-shrink is sometimes tougher to get to shrink evenly than one would think beforehand. For a very short period of time the fire wouldn't affect the resistor but it wouldn't take too long to damage it and heatshrink can cause that. If you could shrink the heatshrink outside the knife before assembly that would ease the process tremendously.

Good old electrical tape works as well (but doesn't look at cool) and if you buy any color other than black it doesn't leave the same gooey adhesive residue when you remove it (if you need to). You can get 3M brand colored electrical tape at Home Depot for like $3-4 a roll.

Oh and twisting wires and taping the connection is a bad practice for any kind of long-term application, but for this that shouldn't be an issue at all...

Cliff

 

[Shay10825]

When I cut the sheet metal the edges are sharp. How can I make it so they are not sharp?

 

[Gokul43201]

You can de-burr the edges of the sheet metal using a belt/wheel grinder, but if you don't have access to either, just use a file to blunt the edges. If you don't have a file, sand-paper will work.

How thick is your sheet metal ?

 

[Shay10825]

How thick is your sheet metal ?

It's 1 mm thick

 

[BoulderHead]

Has anyone discussed heating only a slender wire and using it to slice through the hard butter?

Of course, for the really frozen hard butter more drastic steps are demanded;
http://history1900s.about.com/library/graphics/viet48.jpg
...it does toast, also.

 

[Ivan Seeking]

Shay, please note the comments made by Cliff and Gokul. We are running the resistor [10 ohm, 1 watt, using four batteries in series] at 360% of the design limit of the resistor; or 3.6 watts. We get some forgiveness [I think quite a bit, maybe even enough] by heat sinking the resistor to the knife blade, but how much forgiveness we get is a total guess. There is a danger that the resistor will fail due to excessive heat and after some number of cycles. In principle, if we had no heat sinking you would need to use four of these resistors - two resistors in series, and these in parallel with another two in series. This would produce the same amount of heat energy but over four resistors instead of one - putting us within the resistor's rated value of one watt each.

Normally one would never design a circuit or device without knowing exactly how long each part should last. One never designs a device to exceed the rated values of the parts used. The only way to know how long the one resistor configuration will last is to run some tests and make it fail. Otherwise you are taking a pretty serious risk of having a failure at an inopportune moment. You may want to consider using four resistors instead, but this is a judgment call on your part.

 

[Ivan Seeking]

How hard will it be to replace the resistor if it fails? You need to consider your butter knife maintenance needs as well.

 

[Gokul43201]

Boulder just loves those fire-spitting toys, doesn't he ?

 

[Ivan Seeking]

Boulder just loves those fire-spitting toys, doesn't he ?

I think dinner at Boulderhead's house could be quite dangerous.

 

[Gokul43201]

Tiny detail : a hair dryer is ideal for heat shrink tubing...no need to use Boulder's flamethrower.

Boulder, there are problems with having an uninsulated exposed circuit element...but with a (5-10 mil) coating, that would probably work very well.

I think this is a good idea. The only problem now would be getting the right material for the wire, and buying it without using plastic.