Design details for a butter knife warmer

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The discussion revolves around designing a butter knife warmer using nichrome wire or resistors to generate heat. Participants emphasize the importance of understanding electrical principles, such as voltage, current, and resistance, to safely achieve the desired heating effect. They suggest starting with a low-voltage setup, using a resistor to control heat output, and recommend consulting an electrician for safety. The conversation also touches on the need for proper thermal coupling between the heating element and the knife blade to ensure effective heat transfer. Overall, safety precautions and accurate calculations are crucial for successfully building the butter knife warmer.
  • #51
Shay10825 said:
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. :biggrin:

Is there an adult around. You should not be doing this without someone around. Make sure that they know what you're doing.
 
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  • #52
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.
 
  • #53
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.
 
  • #54
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?
 
  • #55
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.
 
  • #56
Gokul43201 said:
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?
 
  • #57
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...

Code: 
                /\ blade
              /    \
            /        \
          /            \
        /                \
      /                    \
    /                        \
  /                            \
 |        Resistor (R)         | 
 |     |------VVVVV------|     |
 |     |        R        |     |
 |     |------VVVVV------|     |
 |     |        R        |     |
 |     |------VVVVV------|     |
 |     |        R        |     |
 |     |------VVVVV------|     |
 |     |        R        |     |
 |     |------VVVVV------|     |
 \     |                 |     /
       |                 |
       |                 |
       -----Battery-------
 
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  • #58
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?
 
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  • #59
No, hang on...let me fix the picture. Gimme a minute here.

Okay, I'm done now.
 
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  • #60
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?
 
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  • #61
Shay10825 said:
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.
 
  • #62
Shay10825 said:
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 ?
 
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  • #63
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.
 
  • #64
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?
 
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  • #65
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.
 
  • #66
Shay10825 said:
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.
 
  • #67
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.
 
  • #68
Gokul43201 said:
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?
 
  • #69
That information would have been on the outer insulation...the one you slit down the middle. Did you throw that away ?
 
  • #70
Shay10825 said:
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 ? :confused:
 
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  • #71
Yeah I threw it away. Yeah I can buy more. Should I still do the battery test with the copper wire I have and if so should I only use one strand or the number of strands that were in the wire?
 
  • #72
Use all the strands. And yes, do the test with what you have. Right now, your current will be small, so the wire is most probably just fine.

How do you connect the wire to the battery terminals and the resistor leads ?
 
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  • #73
Shay10825 said:
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?

Do the basic test before trying aluminum foil.

The foil should not touch any of the metallic parts of the circuit - the resistor leads, the wire, the battery terminal. If the leads and wires are all taped up, it's okay for the foil to cover them too. To get a more realistic idea, I would double up the foil, to make it twice thick.

And use the heat sink paste over the ceramic part of the resistor (the part with the color code), and make sure the foil is making contact with the paste, or even the ceramic itself.
 
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  • #74
I tape the wire to the battery and wrap it around the resistor wires.
 
  • #75
Shay10825 said:
I tape the wire to the battery and wrap it around the resistor wires.

Okay, just to make sure the wire is making good contact to the battery terminals, you might want to press down with your fingers. Make sure you only put your finger over the tape, and not the metal. Not that it will harm you...
 
  • #76
Gokul43201 said:
I'm thinking (since Ivan says that 1 amp is safe)

Keeping in mind that this was for the Energizer e2 and intermittent use only.
 
  • #77
Yay, you're here !

Hope that didn't sound like I was going to point my finger at you if something went wrong ?

Okay, I've independently verified that the Eveready A95 (D-cell) can be used with 1.5 ohms (drawing 1 amp) for 4mins out of every 15 mintes, and will have a life of 15 hours of usage at this current.

So I too would say that 1 amp draw should be okay, as long as you don't run it for more than 4 minutes at a time, with at least 10 minute breaks.
 
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  • #78
I think you're doing a great job! I just wanted to add a word of caution.
 
  • #79
I probably can't experiment and post anything tonight because I have a really important Physics test to study for (what I get on this determines my grade :frown: ), and a ton of Physics homework. I might be able to start back testing tomorrow.
 
  • #80
I just tried connecting the resistor to the battery and it does not get hot. I wrapped the copper wire around the resistor wire then taped it to the end of the battery. I did this for both sides of the resistor and battery. Why is it not getting hot? Could it have something to do with the way I'm wrapping the copper around the resistor?
 
  • #81
Gokul43201 said:
And use the heat sink paste over the ceramic part of the resistor (the part with the color code), and make sure the foil is making contact with the paste, or even the ceramic itself.

I did not use the heat sink paste over the ceramic part of the resistor. Could this be the reason why the resistor does not get hot?
 
  • #82
First, which resistor are you using - the 1/4 watt or 1/2 watt resistor? Ten ohms right?

The heat sink paste only serves to transfer the heat from the resistor to the metal.

Do you understand how to measure voltage and current?

You should measure about 150 milliamps of current flowing with the circuit made Check that you have current flow. Be patient. We are starting slowly but things will pick up quickly once we're all set in just a few more steps.
 
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  • #83
Did you find out what the deadline is for your report and project?

How did you do on your physics test? :biggrin:
 
  • #84
Ivan Seeking said:
First, which resistor are you using - the 1/4 watt or 1/2 watt resistor? Ten ohms right?

Do you understand how to measure voltage and current?
Both the .25 and .5 watt to see which works best.

Do I understand how to measure voltage and current with the multimeter? No. What should the wires and needles be touching?

Ivan Seeking said:
Did you find out what the deadline is for your report and project?

How did you do on your physics test?

Yeah the report and the project is due November 18.

Not so good :cry:. I got a 72 :frown: . But it does not determine my grade anymore because the school board extended the quarter. So we will have 1 or 2 more tests.
 
  • #85
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  • #86
Ivan Seeking said:
Do you have a Radio Shack nearby?
Yeah there is a radio shack nearby. why?
 
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  • #87
They have some really nice, very short books that introduce the basics with pictures for clarity. They are small pamphlets - thin paperbacks and hand written! You will usually find them in the electronic's sections with the resistors and transistors. It would be worth a stop to take a look and see what they have. In fact, there is a thin, full sized (8.5" X 11") book, green I think, that covers much of the basics... In fact I just checked and I have my 1983 version. It is called "Getting Started in Electronics"; Cat no. 62-5003. It is a handy, practical reference.
 
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  • #88
Shay, with just one 10-ohm resistor, your not generating enough power to get it hot. You will need either a higher power, lower resistance resistor, or a bunch (say 5 to 7) of .5 watt, 10-ohm resistors to generate enough heat.

You may also not be making proper contact between wires. The right way to maximize contact (short of soldering or using wire nuts) is to hold the wires side-by-side and twist them together, several times. Then tape over this, tightly.

To check if your circuit is okay, test it with the multimeter. First connect the probes of the meter to the socets corresponding to voltage measurement. The black (negative) lead goes into the ground socket, and the red (positive) lead goes into the socket labeled V (not the one labeled I, for current).

Now the dial on the meter has different settings. Find the positions for DC voltage measurement, and set it to the value that is just above the expected voltage of 1.5V (so, if there's a 5V or 10V position, use that). Check the voltage across the battery and across the resistor. They should be nearly the same. Similarly, by changing the dial position to read resistance (ohms), you can check the resistance across one or more resistors. The current can be found using Ohm's Law.
 
  • #89
When I'm titling this invention for the science fair does it need to be a question?
 
  • #90
Did the rules for the fair indicate this?
 
  • #91
Yeah never mind I found it. It has to be something catchy. Any ideas :smile: ?
 
  • #92
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
 
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  • #93
The Better Butter Cutter (BBC) :biggrin:
 
  • #95
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.
 
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  • #96
Ivan Seeking said:
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
 
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  • #97
Ivan Seeking said:
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. :wink:

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.
 
  • #98
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? :biggrin:
 
  • #99
Ivan Seeking said:
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 ?
 
  • #100
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?
 
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