Design details for a butter knife warmer

[Ivan Seeking]

Won't the 1.2 volt battery heat up slower than the 1.5 volt battery?

Yes, as indicated by your power calculations. More power means quicker heating. This is not a great difference - 36% - but it would probably be noticable.

The rechargeable batteries are so much more expensive than the normal 1.5 volt batteries

Yes!

It really gets your attention doesn't it?

 

[Ivan Seeking]

btw, your fuse could be as small as 0.2 amps if your circuit demands 0.12 amps. With a purely resistive circuit like yours you can cut this pretty close without causing any problems; ie without causing the fuses to blow too early. Other types of circuits require greater overhead.

 

[Ivan Seeking]

oh yes...did I forget this? Switches.

You need to decide if you want a momentary switch [must be held on] or one that latches on. How and where do you plan to mount it? It must be rated for at least twice the current used by your circuit, and the rated value must exceed the amp value chosen for your fuse. In other words, you want the fuse to blow before the switch is damaged. There is quite a selection here to choose from.
Search switches at Radio Shack

 

[Ivan Seeking]

Shay's deadline has passed.

 

[Shay10825]

I'm sorry I forgot to tell all of you. I found out on Tuesday that the new dedline is December 8th. At first they wanted me to send in the report to them on November 18. Now they are just saying have the report and project at the science fair which is December 8th.

 

[Shay10825]

I bought all of this today. Is this ok?
http://www.radioshack.com/product.a...11_002_014_003&product_id=271-152&site=search

http://www.radioshack.com/product.a...11_002_015_006&product_id=275-614&site=search

http://www.radioshack.com/product.a...1_004_004_000&product_id=270-1238&site=search

http://www.radioshack.com/product.a...1_004_003_000&product_id=270-1047&site=search

 

[Shay10825]

What do I mount the switch on?

 

[Cliff_J]

I think something like this would work much better as suggested by Ivan above:

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=275-609

The rest looks fine as long as the 100ohm resistors will get warm enough to do the job.

Cliff

 

[Shay10825]

I have a couple questions.

1.) how do I hook up the switch?
2.) how do I hook up the fuses and fuse holder?
3.) What do you think I should do to the back of the blade? I think I night cover the resistor with really thick aluminum foil. Do you think this would work?

I'm going to use the 10 ohm 1 watt resistor because the 100ohm did not get hot enough.

I hope you all had a great Thanksgiving !

 

[Ivan Seeking]

You should use your ohm-meter to check the function of the switch which simply makes and breaks the connection in one of the two wires from your batteries to the resistor. It is standard to switch the positive side [to make and break the connection in the positive wire].

The fuse should be in-line with the switch. It acts just like the switch - making and breaking the positive side of the circuit - but as a failsafe rather than a regular switching device. The fuse should be the first device after the batteries.

As for your choice in resistors, you had better check your calculations. You have a problem.

 

[Moonbear]

Shay, do you have a digital camera? I'd love it if you'd post a picture of your knife when it's completed! I'm sure with so many people contributing ideas to its design, they'd also enjoy seeing the final product.

 

[Math Is Hard]

I agree. Can you have some pictures taken while you're at the science fair demonstrating it? That would be so cool!

 

[Shay10825]

As for your choice in resistors, you had better check your calculations. You have a problem.

I'm using the 10 ohm 1 watt resistor and 4 batteries not 8.
Do you think the aluminum foil would work?

Shay, do you have a digital camera? I'd love it if you'd post a picture of your knife when it's completed! I'm sure with so many people contributing ideas to its design, they'd also enjoy seeing the final product.

I agree. Can you have some pictures taken while you're at the science fair demonstrating it? That would be so cool!

Yes I'll post the pictures .

 

[Shay10825]

I bought a momentary switch. The switch has two metal loops. Should I connect the positive wire comming out of the battery holder to one loop and one end of the fuse holder to the other?

 

[Ivan Seeking]

Oh good I was going to ask about pictures.

As a good practice the fuse should come first in line, then the switch, but you have the idea. The switch just makes the connection to complete the circuit. The "positive current" flows from the positive terminal of the battery, through the fuse, through the switch, then through the resistor and back to the negative battery terminal.

The foil should work okay. Be careful not to create a short circuit by making contact with the resistor's bare leads. Also, make sure that you apply a good layer of heat conductive paste to the body of the resistor, and the points of contact with the blade and foil. Since you have chosen to risk over-driving the resistor you need to pull the heat away from the resistor's body as fast as possible. The foil will help to do this.

Note that 3 watts of heat is just that, 3 watts of heat. In principle it doesn't matter if we get 3 watts from 1, or from 4 resistors. Are you having trouble trying to fit more than one resistor to the blade? How are you securing the resistor?

 

[Shay10825]

would i connect it like this?
http://img47.exs.cx/img47/7294/projcombo.png

Sorry that my picture really sucks

 

[Shay10825]

This blade is driving me crazy! I cut the sheet metal and it's not turning out right. The edge of the blade I cut is jagged and it does not really look like a knife blade. I filed down the edges of it but it still has not turned out right. I don't think it really looks like a blade. The blade is not perfectly flat either. I've cut myself a million times (not purposely). Does anyone have any suggestions on what I can do.

 

[Shay10825]

Would it look bad if the blade was shaped like this?
http://img109.exs.cx/img109/9249/blade.png

 

[Ivan Seeking]

The circuit looks correct. The blade looks fine to me as well.

We know how you feel. Welcome to the world of invention and design. I'll bet you never thought that engineering and science could be painful.

 

[Gokul43201]

This blade is driving me crazy! I cut the sheet metal and it's not turning out right. The edge of the blade I cut is jagged and it does not really look like a knife blade. I filed down the edges of it but it still has not turned out right. I don't think it really looks like a blade. The blade is not perfectly flat either. I've cut myself a million times (not purposely). Does anyone have any suggestions on what I can do.

What did you use to cut it ? Ideally, you could cut it with a shears (if it's less than1/16"). Any machine shop/metals supplier will do that for you.

What metal are you using, and how thick is it ?

 

[Shay10825]

I cut it with shears. It is aluminum sheet metal and it's 1 mm thick.
I don't know what I'm going to do about the blade. I don't like how it looks with the foil on the back covering the resistors. If I put another piece of sheet metal what would I glue it together with. I was going to use liquid nails but it is so messy. When I put the glue on the metal the two pieces of metal did not want to stick together. It just became a big mess because the glue got all over the resistor and heat sink. So I don't want to use glue. What else could I use if I want to stick the two pieces of metal together? Could I weld it?

I also went to look for something to cover the battery holder. I wanted something like this:
http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=270-1803
but I could not find anything that the battery holder I have would fit in(the one shown is way too tall). So I was just walking around Home Depot thinking about what I could do and I passed the plumbing isle. So I bought a pipe that the battery holder can fit in. I was also in the craft store today and I got some wood. Which one do you think would look better covering the battery holder? Should I use the wood and make a little box, or should I use the white plastic pipe?

 

[Ivan Seeking]

First, you had better not set yourself up by using an under-rated resistor and then preventing easy access for needed replacements. You need to plan for a resistor failure.

What about using a piece of copper tubing instead of a blade? This might be considered a spreading but not slicing device. Maybe you could attach a cutter to the tube?

Otherwise, I'm thinking.

 

[Ivan Seeking]

You could use one piece of metal and fold it over - to fit the resistor and wires but still approximate a blade. This would leave only one long side with edges. One edge could be folded around the opposite edge and sealed.

 

[Ivan Seeking]

You could put the resistor in a small copper tube [maybe 1/8" to 1/4" diameter] after attaching the tube to the top of the blade. You could braze or solder using lead free alloy such as silver solder.

 

[Shay10825]

One edge could be folded around the opposite edge and sealed.

This is a really good idea . I was folding one piece of metal but I did not think of folding it around the opposite edge also. This might work. But what should I seal it with?

 

[Shay10825]

You could put the resistor in a small copper tube [maybe 1/8" to 1/4" diameter] after attaching the tube to the top of the blade. You could braze or solder using lead free alloy such as silver solder.

I like this idea also .

 

[Ivan Seeking]

Just make sure that you don't braze or solder with the resistor in place. Those must be inserted afterwards or they will be damaged.

On another note and as a reminder, four resistors producing 3 watts of power at a relatively low temperature produce the same amount of heat energy as one resistor producing 3 watts of power at a relative high temperature. As long as you have a good thermal bond between the resistors and the metal [by using the heat sink compound], and if in an otherwise closed system like a closed metal tube, the heat produced in the metal tube is the same. It should take a little longer to get warm since we have more resistors to heat up [also, as long as the blade isn't too massive or too large], the difference in the heating shouldn't be very great.

In this case, power is the measure of energy lost to heat in the resistor per second. So, since all of the power is going to heat, the watts of power produced is what matters, not how we produce it.

 

[Ivan Seeking]

You might even pound, press, or somehow crimp a large, thin wall copper tube into a blade.

 

[Moonbear]

If you're not brazen enough to braze or solder, the glue/adhesives section in Home Depot ought to have some plumbing epoxy type materials. There are a bunch of these for different applications. Find one that will bond metals and is okay to get wet (though, don't get that blue marine adhesive they sell...I've tried that and it peels even when it shouldn't). The two-part epoxys usually come in a tube with two plungers that mixes them as they come out. Use in a well-ventilated area and be careful not to glue your fingers to the knife. They are usually pretty thick, so won't run all over the place out of control.

Where on the blade are you attaching the resistors? Near the top?

I think diagrams help to be sure everyone is talking about the same thing. This is a quick sketch of what I think you guys are talking about for the blade and cover for resistors, but I might be misunderstanding. See if this helps or is actually what you are all trying to accomplish.

 

[Moonbear]

Just two more day's until Shay's science fair. Good Luck Shay! Knock 'em dead! And don't forget the pictures.

 

[Ivan Seeking]

Just two more day's until Shay's science fair. Good Luck Shay! Knock 'em dead! And don't forget the pictures.

Hopefully, no news is good news.

 

[Shay10825]

Hi everyone ! The science fair was sooooooo much fun! The judges really liked my project. I had 8 judges for some reason (we were only suppose to have 4). I had more judges than anyone there ( I don't know if this is a good thing or not). They said the title made them wonder what it was .

Guess what?

I won 1st place !

You want to know why? I was the only person in the inventions category .

But it was a lot of fun. I really enjoyed it.

I'll post the pictures as soon as I get them developed.

 

[Ivan Seeking]

That's great Shay! We're all very proud of you.

I can't wait for the pics.


Oh yes, IMO there is nothing better than creating your own category.

 

[Math Is Hard]

WOOOOOHOOOO!
HOORAY FOR SHAY!
Gosh I wish there was some way to make these smilies bigger 'cause I am so happy!

 

[Gokul43201]

Congratulations Shay !