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.
  • #121
When I put the resistors on a piece of metal then but another piece of metal on top there is a big space between the two pieces of metal because of the resistors. How can I make this space small? If I just put liquid nails in the space the knife will be too big. What can I do?

Also is there some kind of rechargeable battery that is smaller than a D cell battery but will work just like one??
 
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  • #122
Why do you want to fill the space? If the resistor makes good contact with one metal surface - the front surface - wouldn't that work?

As for battery size, you may be able to use a rechargeable C...I will check a little later.
 
  • #123
Ivan Seeking said:
Why do you want to fill the space? If the resistor makes good contact with one metal surface - the front surface - wouldn't that work?

Are you saying that I should put the resistor on one piece of metal then cover the resistor (which would be on the back of the metal blade)?? Or should I put the resistor in between two pieces of metal? If I use one piece of metal what should I cover the resistor with?
 
  • #124
hmmm, what do you think? On one hand it seems easier to use only one piece of metal. On the other hand, you need to cover the resistor with something... another piece of metal would make sense. I guess my thinking is this: You really only need one heated surface. If the other surface is metal or something else, would it matter? You might also consider that the resistor might fail and need to be replaced.

This is part of the creative design process and I don't know the best answer. You're the engineer. It sounds like you need to think this through. I would tend to use one heated surface and then attach something on the back for protection [of the resistor and heat sink compound] that can be removed if needed. It does need to be a good cover since you certainly don't want heatsink compound or a resistor on your toast! You need to be able to clean the knife...I'm not sure. Maybe using silicone to glue another piece of metal would work; as I think Cliff suggested earlier.
 
  • #125
Okay, it looks like you can use four rechargeable C cells.
http://data.energizer.com/PDFs/nh35.pdf

According to the data sheet on the battery [above] you can expect a little less than 5 knife-hours per charge if using an operating current of 500 mA = one half amp. These cells have a slightly lower output potential of 1.2 Volts [remember that the other batteries are 1.5 volts], so this will reduce the current and power a bit. Keep in mind also that these numbers may vary a bit between brands.

[1.2 Volts/battery] X [4 batteries] = 4.8 volts
V = I X R so I = V/R = 4.8 volts / 10 ohms = 0.48 Amps [about a half amp]
Power = V X I = 4.8 volts X 0.48 amps = 2.3 Watts of total heat energy

We have reduced the voltage by 20%:
1.5 volts - 1.2 volts = 0.3 volts, and [0.3 volts / 1.5 volts] = 0.2 ---> 20% reduction

But the power is reduced by:
3.6 watts - 2.3 watts = 1.3 watts, and 1.3 / 3.6 = 0.36 --> 36% reduction
 
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  • #126
One more thing: Shay you should pick up a fuse block and a [some] 3/4 - 1 amp fuse to put in your circuit. This will prevent any runaway short circuit problems which could be dangerous. You can probably mount the fuse block on the battery holder.


Edit: You can get this at Radio Shack for a few bucks.
 
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  • #127
okay I'm a little confused but the datasheet for size AA rechargeable batteries says that these would work just as well. In fact the D, C, and AA cells of these type all claim 2500 mAH's to one volt. So, it looks like you can use AA's. You should pay a price of time for size, ie. bigger batteries last longer, but for some reason this is not indicated as being true. In either event, smaller batteries mean less time between charges, normally. Even if the data sheets are in error you should still expect a couple of hours of knife-time between charges.

Edit: I see what's happening ;this should be good information. It turns out that you pay such a high energy storage price for using rechargeable batteries that the performance varies little between sizes. In other words, a rechargeable size D has about the same capacity as a non-rechargeable size AA. Also, since they have about the same internal resistance as the larger batteries, the maximum current is about the same.

Go here http://data.energizer.com/ and select datasheets and compare the listed capacities for each type.
 
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  • #128
I just bought an 8 AA battery holder. How can I make it so it will work with 4 AA batteries?? The 8 AA battery holder works better for a handle than the 4 AA holder because it is longer. Also with the 8 AA holder I will be able to disconnect the blade from the holder. If there is no way I can use 4 AA batteries in the 8 AA holder then how many resistors would I need for 8 AA batteries?? Would I need 2 10 ohm 1 watt resistors?
http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=270-407

Also how do I make an on-off switch??
 
  • #129
It looks to me like you could use only one side. You could just connect to the spring located after the four batteries, in the empty slot, or you could short circuit the spring to the output contact where the other four batteries would be. As long as your four batteries are in series [end to end, i.e + to - to +...] it should work.

If you wish to use all eight batteries then you should go through the calculations to determine the resistance needed. We can check your results. How much power do you wish to risk across the resistor.

I really liked the reduced power of 2.3 watts, rather than 3.6 watts, on the 1.0 watt resistor. You don't want the resistor to fail. Remember, we are guessing here...a very, very dangerous thing to do. We are absolutely safe at 1.0 watt per resistor.
 
  • #131
That looks real good! One minor correction is the voltage: Remember the rechargeable batteries are rated at 1.2 volts, instead of 1.5 volts.

If you want more power you can add a second or even a third resistor in parallel. This means that the resistors are not in-line, like the batteries, but they are side by side, so to speak. Note that in a parallel circuit, the resistance in each leg is still 100 ohms. If you use two 100 ohm resistors in parallel, the power is doubled. This is not true with two resistors in series though since the resistance goes up and reduces the current.

Series circuit
+ ---- R1----- R2 ----- -

Parallel circuit
+ ---- R1 ----- -
+ ---- R2 ----- -
 
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  • #132
Could I use the 1.5 volt battery and just use one resistor?
 
  • #133
One more clarification. Note that the current demand on the batteries has dropped significantly as a result of increasing the voltage and resistance. This is why we can start adding resistors in parallel, if needed.
 
  • #134
Shay10825 said:
Could I use the 1.5 volt battery and just use one resistor?

I was only suggesting the other resistors in the event that you want more power. This will depend on how happy you are with the knife's performance. The rechargeable batteries only put out 1.2 volts as a nominal value. If you want to use regular batteries, the 1.5 volt type, then we need to check the datasheets again for AA throw away batteries; just to double check the maximum current. We were basing this all on size AA rechargeable batteries. Do you want me to check the regular AA batteries?
 
  • #135
Ivan Seeking said:
Do you want me to check the regular AA batteries?

Yes could you please :smile:. I want to use the 100 ohm 1 watt resistor and 8 AA batteries.
 
  • #136
Okay hang on...

While you're waiting, I'm just curious, what about the 8 X 0.3 volt drop concerns you? This actually puts us within the 1.0 watt limit per resistor at 0.92 watts.
 
  • #137
Ivan Seeking said:
One more thing: Shay you should pick up a fuse block and a [some] 3/4 - 1 amp fuse to put in your circuit. This will prevent any runaway short circuit problems which could be dangerous. You can probably mount the fuse block on the battery holder.


Edit: You can get this at Radio Shack for a few bucks.


Should I get this?:

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=270-742

What kind of fuse should I get?:
http://www.radioshack.com/category.asp?catalog_name=CTLG&category_name=CTLG_011_004_008_000&Page=1

http://www.radioshack.com/category.asp?catalog_name=CTLG&category_name=CTLG_011_004_003_000&Page=1

http://www.radioshack.com/category.asp?catalog_name=CTLG&category_name=CTLG_011_004_002_000&Page=1

http://www.radioshack.com/category.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F004%5F000%5F000&Page=1&find=fuse(keyword)&site=search
 
  • #138
Ivan Seeking said:
While you're waiting, I'm just curious, what about the 8 X 0.3 volt drop concerns you? This actually puts us within the 1.0 watt limit per resistor at 0.92 watts.

Won't the 1.2 volt battery heat up slower than the 1.5 volt battery? The rechargeable batteries are so much more expensive than the normal 1.5 volt batteries.
 
  • #139
The AA batteries looks good. You should take a look at the graphs on the second page here http://data.energizer.com/PDFs/e91.pdf and try to understand them. This is for a regular AA battery. With 8 batteries and about 0.12 amps, you should expect about ten good hours of knife-time per battery set.
 
  • #140
Shay10825 said:
Should I get this?:

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=270-742

I would tend towards something like this
http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=270-1238
or this
http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=270-739

You want the fuses that go with the fuse holder that you purchase. This may be easier to do at the store. As long as the fuse is rated for at least 12 volts [which all will be] and a little more current than we need, in this case say 0.5 amps, that's all that you need.

Also, keep in mind that you only need one fuse but you want a few spares.
 
  • #141
Shay10825 said:
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![/size]

It really gets your attention doesn't it?
 
  • #142
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.
 
  • #143
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
 
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  • #144
Shay's deadline has passed.
 
  • #145
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.
 
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  • #147
What do I mount the switch on?
 
  • #149
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 :smile: !
 
  • #150
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.
 

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