Design details for a butter knife warmer

[Gokul43201]

Okay here's my final word (until shay makes a decision) on the direct knife heating design :

I don't see how you can run an external circuit, have it not interfere with the purpose of a knife, and provide a good thermal short to the front of the blade.

So, the only way I can see to implement this is by making a hollow blade (by bending and grinding stainless sheet metal) and having the heater wire run inside it.

Even then, with the power limitations, the heating time is going to be long.

 

[Gokul43201]

Besides your decision on the above question, Shay, answer these questions :

1. What's your budget ?

2. Do you have someone that can buy things for you online using a credit card ?

3. When's the deadline ?

4. Can you borrow a digital multimeter from anyone ? A neighbor, perhaps ? A small DMM can be bought for less than 10 bucks.

5. Where are you ? In the US, I hope.

6. This is a stretch, but you wouldn't have any experience with and access to a soldering kit, would you (it's not essential, but very handy) ?

And to answer your question about testing the resistor :
a) You need a battery holder. These cost no more than a couple of bucks (or you might find one in your lab). http://www.thomas-distributing.com/battery-holders.htm If buying, get one with wire connections.
b) You must attach extra wire to anyone end of the resistor leads to make it long enough. The attachment can be done by twisting the wires together and taping over, or better still would be using a wire nut (these may also be found in your lab). There are some wire nut connectors here : http://shopping.msn.com/marketplace.aspx?pmpType=1&pcId=10035&catId=1899 for you to see what they look like.
c) Connect the ends to the terminals on the battery holder,
d) Put the battery in.

Before you make any decisions to buy anything, check with us.

 

[Ivan Seeking]

Shay, the one unanswered question is the maximum current that you can safely pull from a D cell battery. We really need this number. I was going to check this myself but for the life of me I can't find any resistors around here that are small enough. If Cliff doesn't chime we will get this one way or the other. The web sites that I checked have not been helpful. It appears that you can pull at least one amp [some sites suggested more like 3 amps]. If we assume that you can safely pull 1/2 amp, then using two D cells in series, we have 3 volts X 0.5 Amps = 1.5 watts. Of course we have to use a 2 watt resistor having a value of R = V/I = 3/.5 = 6 ohms.

Again, I don't know how far we can safely push this number.

 

[enigma]

If you're dead set on a knife with the heating device inside, would you consider a hybrid design?

Have the heating part located in the knife, but the batteries in a holder? Then complete the circuit with contacts of some sort.

I'm concerned that the knife will be too heavy to use (much less be marketable... ) otherwise.

This is a fun thread. :tongue2:

We don't get too many "help me design this, please" threads. Engineering at its best.

 

[Shay10825]

Besides your decision on the above question, Shay, answer these questions :

1. What's your budget ?

2. Do you have someone that can buy things for you online using a credit card ?

3. When's the deadline ?

4. Can you borrow a digital multimeter from anyone ? A neighbor, perhaps ? A small DMM can be bought for less than 10 bucks.

5. Where are you ? In the US, I hope.

6. This is a stretch, but you wouldn't have any experience with and access to a soldering kit, would you (it's not essential, but very handy) ?

1. I don't want to spend over $50.
2. No my mom won't let me
3. I think the report (not the final report) is due Oct. 15 but it could change since we have been through so many hurricanes and I have not been in school for almost a month. I need to as my teacher as soon as I get back.
4. I could just but one from Home Depot for $10. What do I need it for?
5. Florida
6. I have never used one before. I saw one at home depot that I could get for $14 (we are at home depot a lot because of the hurricanes).

I like the idea of a knife warmer but would that be harder to make? I would have to make a toaster like thing but for butter knifes.

"So, the only way I can see to implement this is by making a hollow blade (by bending and grinding stainless sheet metal) and having the heater wire run inside it." - Gokul43201
That's wht I was going to do. I was also thinking about taking two real butter knifes, cut the handle off, put the wire between them and welding them (or soddering them however you would do it) together. I really wanted to use silver because it would heat up faster but I could not find sheets of silver. I think I'm going to have to use stainless steel.

A resistance wire might be better since the heat will run all through the blade. What would be the best thing to get so the blade would get hot the fastest and not blow up the battery. I want to use as little batteries as I can. I think if I'm using D batteries 2 or 3 would probably be the max amount (I think more than that would be too much because D batteries are big).

I got goggles, a .5 W, 10 ohm resistor, and heat sink paste today.

Thanks everyone soooo much for helping me with this. I was going to ask my physics and chemistry teachers for help but these hurricanes have put us out of school for so long. It really does not have to be perfect.

PS - I'm a she not a he lol. Shay10825-Shayla

 

[Ivan Seeking]

PS - I'm a she not a he lol. Shay10825-Shayla

Gees I'm sorry. The 10825 sounded masculine to me. :tongue2:

I got goggles, a .5 W, 10 ohm resistor, and heat sink paste today

Great job! You also have some money and access to a metal shop. I think a volt-ohm-amp meter is not only needed here, it is a great investment. I would also check with Radio Shack if you have one near.

The soldering iron is a little tougher. This is your call but I would save the budget and borrow one if at all possible. The amount of soldering needed should be very limited.

Why don't you put on your goggles and connect your resistor to one battery? See how long it takes to get hot. How hot does it get? When you get your volt meter, learn how to measure the voltage, current, and resistance in the circuit. Try to secure the resistor to a small piece of metal. how does this affect the heating? Also, don't forget to look up that RC color code. It is shown on the back of the resistor pack, most likely. If not, Google it.

As for the resistance wire, the nice thing about resistors is that we don't need much length. I never looked at the numbers for the nichrome wire but we can string resistors together in order to spread the heat out more. This also eliminates [mostly] insulation problems.

For example, if we use two, 1/2 watt, 10 ohm resistors connected in parallel, we can safely [without burning up the resistors] produce 1 watt of power across a total of 5 ohms. If we use two, 1/2 watt, 5 ohm resistors connected in series, we can produce 1 watt of power over 10 ohms. So we have a lot of flexibility by playing with the configuration. We could use four or five resistors if needed.

Still need that battery current number. We will be back.

 

[Ivan Seeking]

One thing that I have to add. When I do things like this I usually worry about function first, then esthetics. Also, I think everyone who has commented makes good points here, and I don't know what your best course of action may be, but as for a functional [not pretty] handle, if you opt for a self contained unit, a modified flashlight would hold the batteries and provide a switch. Of course, we don't know if the switch could handle the current because we don't yet know the current. Anyway, just a thought...

Also, you could have the knife connect to a circuit in a separate holder, or you could put the resistors and batteries in a separate holder and use a heavy [massive to retain heat] blade. In other words, use the resistors to make your "toaster like thing" or the TLT.

 

[Ivan Seeking]

Okay, here is some good information on the Energizer bunny. Take the following link to [datasheets], [Energizer e2], then scroll down and click on the name (left column) X95 - for D cells.

http://data.energizer.com/

It looks like we could run up to 1.5 amps but that may be pushing things. After a quick review of the graphs and data, I think one amp looks like the safe limit based on intermittent use. What do you all think?

 

[Cliff_J]

Ivan - I agree with the 1 amp , I'd wayyy overestimated the power of a D cell battery as well. I guess that's why I cuss up a storm when the batteries are dead in my flashlights. Besides, the bulbs the closest I could find to small value resistors in my stash, it'd be a whole lot of 220 ohm resistors in parallel otherwise...so the manf data is definitely close enough for here.

Gokul - Your suggestions are good but possibly a bit advanced for the task. I'd always thought of it as a give a fish vs. teach to fish type thing where the process of figuring out how exceeds the result for learning purposes anyways. And household power sources seem out of place unless a good isolated source of low voltage could be found - maybe an old gaming console?

Shayla - How warm does the knife need to be? The hot water in your house is going to be under 140F in most cases and I'd think that would still be plenty warm. Extremely easy to test by filling a cup and placing the knife in for a minute and then slicing butter. Works good on ice cream scoops for at least one scoop! Then Ivan's suggestions, especially with hooking up the resistor and finding out how hot it gets (all by itself and again with the resistor touching metal) are great next steps. Just be careful not to burn yourself or overheat the thermometer as they can explode too.

Cliff

 

[Ivan Seeking]

This is also a good time to figure out just how to attach the resistors to metal. This must be done no matter how you proceed otherwise. Again, you want to maximize the amount of surface area [of the body of the resistor] touching metal directly. The the leads cannot touch the metal. Then apply the heat sinking compound to the body of the resistor and the metal. This paste will not cause a short circuit so don't worry about making a bit of a mess.

Edit: Actually, I think that paste could be conductive or non-conductive. The label should indicate this. If not, let me know exactly what you bought and I'll check.

 

[Ivan Seeking]

If you're dead set on a knife with the heating device inside, would you consider a hybrid design?

Have the heating part located in the knife, but the batteries in a holder? Then complete the circuit with contacts of some sort.

I like this idea.

 

[Shay10825]

How exactly would I do something like this? Wuld it be easier than what I'm doing?

 

[Ivan Seeking]

My take is that you would have a wire connecting the knife to the batteries. I think that's what Enigma meant.

Or, you could design the holder with little contacts to make the connection between the knife [resistors] and the batteries when the knife is in the holder. This would rely on the mass of the blade and the resistors retaining enough heat to do the deed. You might see if this is practicle by using a blade and hot water as Cliff suggested. Do some butterological testing.

Or, you could make a TLT and use a regular knife blade [with enough mass for heat, as above].

Keep in mind that we have quite a bit more power to work with than discussed earlier. We might go as high as 5 watts or more - 3 or 4 batteries. Your chosen design could mandate the practical limit for batteries. Earliler you thought two was the limit if contained in the handle.

Hey, I heard schools opened in Florida today.

 

[Shay10825]

I got a volt-ohm-amp meter, and the 10 ohm .5 V resistor yesterday.
Would the wire connectors be too big to fin inside the blade?


I'm going to connect the resistors (the .25 V and the .5 V but not at the same time) to one D cell battery. When I do this could I just tape the ends of the resistor to the ends of the battery? Can I touch ALL parts of the resistor? Can ALL parts of the resistor touch the metal so I can see how hot the blade gets? Can I find out how they get without using a thermometer? Would the resistor and/or battery burn (maybe not burn but feel really hot) before it explodes? If the resistor and/or battery exploded what is the worst thing that could happen? Should I measure the current when I'm testing this out?

I'm sorry I have so many questions.

Yeah I started school today . The person over the science fair at my school is there on even days and today was an odd day so I'm going to ask her tomorrow about the deadline.

 

[Cliff_J]

Yes, you could just tape the ends to the battery.

You could touch all parts of the resistor, but it may get hot.

All the non-metal parts of the resistor can touch the blade. The metal wires should be covered in electrical tape.

If it gets hot enough to melt the butter, that would be a success. So you don't need an exact number. But it would be nice to have a number or comparison (cut as well as knife in hot tap water) for the report wouldn't it?

Yes, the battery will get very warm before exploding. If exploded corrosive stuff is released and might spray out. I'm not sure if it has enough power to self-destruct, but if the battery is hot disconnect it!

You should measure enough to write a good report and answer any questions that fit the hypothesis and what not that you started with. More numbers means more graphs and charts, that usually impresses anyone.

Cliff

 

[Ivan Seeking]

I got a volt-ohm-amp meter, and the 10 ohm .5 V resistor yesterday.

do you mean 0.5 watts?

Would the wire connectors be too big to fin inside the blade?

I don't know how you plan to make your blade.

I'm going to connect the resistors (the .25 V [watt]and the .5 V [watt] but not at the same time) to one D cell battery. When I do this could I just tape the ends of the resistor to the ends of the battery?

That will probably work

Can I touch ALL parts of the resistor?

Yes but, the resistor could get quite hot. Always assume that the resistor will burn. Be careful.

Can ALL parts of the resistor touch the metal so I can see how hot the blade gets?

Not the metal leads. These must touch only the battery; one on each end. You can use black tape to help insulate the leads. If they both touch the same piece of metal [or each other] then you have a short circuit and the battery could overheat. Keep a very close eye on this. Wear your goggles, always. Check the battery frequently to make sure its not getting hot.

Can I find out how [hot] they get without using a thermometer?

That's pretty tough. Getting a feel for things is good for now.

Would the resistor and/or battery burn (maybe not burn but feel really hot) before it explodes?

the resistor would just fizzle and start to smoke. The battery is the thing to watch. You should be fine as long as you don't create a short circuit or use the wrong configuration of resistors. The battery will get hot if there is a problem, but a short circuit could cause the battery to get hot really quickly. If you see or smell smoke, pull the wire loose.

If the resistor and/or battery exploded what is the worst thing that could happen? Should I measure the current when I'm testing this out?

Again, the batteries are the thing to watch. Monitoring the current is a real good idea. In fact, what we should do is to get a fuse in the circuit. You should have an extra fuse with your meter. What is the value? Do you have anyone amp fuses around; maybe automotive fuses? I'm sorry I didn't think of this earlier. I think you could still try some quick tests as long as you carefully avoid short circuits.

I'm sorry I have so many questions.

At this point I would be disappointed and worried if you didn't.

Is there an adult around. You should not be doing this without someone around. Make sure that they know what you're doing.

 

[Shay10825]

How long do I have to wait for the resistor to get hot? I'm using the .25 W 10 ohm resistor and a D cell battery. I connected it but it did not get hot.

 

[Gokul43201]

About touching the resistor : My calculations suggest that the resistor won't get hot for at least a few tens of seconds...perhaps longer. I suggest you touch the body of the resistor once every 5 seconds or so, with the tip of one finger. After this test, I would try again (giving the battery a little time to cool down) with the two resistors connected in parallel.

After this, you could try a test with the resistors enclosed in a piece of aluminum foil - which simulates the blade - and see how hot the foil gets. You could even have a piece of butter sitting on the foil.

Remember, you don't want the foil to touch any of the metal wiring or the battery terminals. So, you can do one of two things : (i) cover all the wiring in tape, or (ii) wrap it all in a sheet of paper (don't do multiple wraps; two at most) and then wrap this in foil. Using the heat sink paste will be useful.

 

[Shay10825]

So I let the metal touch the whole resistor that's covered in tape (the wires too) or should I only let the metal touch the part of the resistor with the different color lines?

 

[Gokul43201]

I'm thinking (since Ivan says that 1 amp is safe) that you probably want a 2 ohm, 1 watt (higher that 1 watt, if possible) resistor.

That will draw about 0.75 amps and dissipate a healthy 1.1 watts. Much better than the measly 0.2 watts your getting with a single resistor. With your two existing resistors, in parallel, you will get about 0.4 watts.

 

[Shay10825]

I'm thinking (since Ivan says that 1 amp is safe) that you probably want a 2 ohm, 1 watt (higher that 1 watt, if possible) resistor.

That will draw about 0.75 amps and dissipate a healthy 1.1 watts. Much better than the measly 0.2 watts your getting with a single resistor. With your two existing resistors, in parallel, you will get about 0.4 watts.

The resistor at Radio Shack with the lowest resistance (in ohms) is a 10 ohm resistor. They also only have .25 and .5 watt resistors. How can I use a 10 ohm resistor with .25 or .5 ohms?

 

[Gokul43201]

Here's an idea : Use 5 (or even 7) of your 10 ohm, 0.5 watt (0.25 watt resistors would work too) resistors in parallel. This achieves :

1) Your effective resistance is now 10/5 = 2 ohms, which gives you better than 1 watt without exceeding the power capacity of any of the individual resistors,

2) It provides a means of having a line (distributed) source of heat instead of a point source. See the drawing below. I'm fixing it now...gimme a couple minutes...

                /\ blade
              /    \
            /        \
          /            \
        /                \
      /                    \
    /                        \
  /                            \
 |        Resistor (R)         | 
 |     |------VVVVV------|     |
 |     |        R        |     |
 |     |------VVVVV------|     |
 |     |        R        |     |
 |     |------VVVVV------|     |
 |     |        R        |     |
 |     |------VVVVV------|     |
 |     |        R        |     |
 |     |------VVVVV------|     |
 \     |                 |     /
       |                 |
       |                 |
       -----Battery-------

 

[Shay10825]

So the resistors would be connected together and then connected to only one end of the battery?

How do you know "resistance is now 10/5 = 2 ohms"? Is it a rule that if you have resistors parallel then the resistance is the resistance of each 1 divided by the number of resistors you have?

 

[Gokul43201]

No, hang on...let me fix the picture. Gimme a minute here.

Okay, I'm done now.

 

[Shay10825]

Would I use one long wire to connect all of this or 2 different wires? I took a speaker wire and cut off the plastic covering it. I'm using the copper wire inside. Would I only use one strand or double them up to make it thicker?

 

[Gokul43201]

How do you know "resistance is now 10/5 = 2 ohms"? Is it a rule that if you have resistors parallel then the resistance is the resistance of each 1 divided by the number of resistors you have?

Yes, the effective resistance of a bunch of parallel resistors is R/n where R is the value of each resistance and n is the number of resistors.

The formula for the effective resistance of resistors in parallel is :

$$\frac {1}{R_{eff}} = \frac{1}{R1} + \frac{1}{R2} + \frac{1}{R3} + ...$$

If R1=R2=R3=..., let's call them all simply R, then of course :

$$\frac {1}{R_{eff}} = \frac{1}{R} + \frac{1}{R} + \frac{1}{R} + ... = \frac {n}{R}$$

Inverting this, gives you the required result.

 

[Gokul43201]

Would I use one long wire to connect all of this or 2 or 3 different wires? I took a speaker wire and cut off the plastic covering it. I'm using the copper wire inside. Would I only use one strand or double them up to make it thicker?

I'll answer this question in a bit. But first ...how did the test go ? Did the resistor get hot at all ?

Now about your question : You could simply use 2 long wires, one on each side. However, to carry about 1 amp of current you need about 22 gauge wire or thicker (20 gauge would be safer). See this table, and look at the first and last columns. http://www.powerstream.com/Wire_Size.htm

Now if you look at your original wire, it will say on the insulation, what the gauge is (something like 18 AWG or 20 AWG - "AWG" stands for American Wire Gauge). If that number is smaller than 22 AWG, then you are good. Remember, the smaller the gauge, the thicker the wire.

What gauge does it say it is ?

 

[Gokul43201]

One more thing : If you are going to use more of this wire, it is better to remove the insulation only where you are making connections (like at the ends). Don't remove all of the insulation from the wire.

Ask, if you don't know how to do this. There's a couple of correct ways to do this.

 

[Shay10825]

I got the wire form Home Depot so I don't have what it came in (they cut how much you wanted off for you) so I don't know if it is a 20 gauge or not. The wire is about 2 mm thick (with the insulation) so it would be about a 12 gauge? Should I get one that is 20 gauge tomorrow just to be safe?

What is the correct way to remove the insulation?

 

[Gokul43201]

How much wire did you buy ? You didn't remove the insulation off all of it, did you ?

Look carefully at the insulation again...the gauge will be printed on it, in tiny letters...probably along with manufacturer and safety information.

 

[Gokul43201]

What is the correct way to remove the insulation?

I'm guessing you don't have a wire stripper. I'm also guessing the wire is not solid - there's several thin strands of copper wire bunched together, right ?

Use a razor blade or utility knfe/box-cutters. Be very careful. Carefully cut away about 1/2 inch to 1" off of the ends, making sure you don't cut the copper wires themselves. Apply gentle pressure with the blade as you rotate the wire.

 

[Shay10825]

I bought only 1 ft of it (it was only .17 cents) . No. The wire was not just a single wire it had two insulated wires connected (parallel) to each other so I cut down the middle of it to have on wire. Then I took the insulation off one (out of the two now) of the wires. There are red dots on it but it does not look like it spells out anything. It did not have any manufacturer and safety information. I could just but more but in a pack that says the gauge.

 

[Shay10825]

there's several thin strands of copper wire bunched together, right ?

Yeah there is. I took pliers and ripped the insulation off.

Would it be better if I got something that was just one wire?

 

[Gokul43201]

That information would have been on the outer insulation...the one you slit down the middle. Did you throw that away ?

 

[Gokul43201]

Yeah there is. I took pliers and ripped the insulation off.

Would it be better if I got something that was just one wire?

The plier technique is not the best way. Follow the razor blade approach. After cutting around the insulation, you should be able to simply pull it off.

The solid wire will be a lot less flexible than multi-wire. Just be careful not to cut through the wires.

The 1 ft of wire you have may be enough...let's wait and see. You will use a bunch of it for testing, but you can always buy more, if you need to, right ?

Psssst, Any luck with the battery test ?