Design details for a butter knife warmer

[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 ?

 

[Shay10825]

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?

 

[Gokul43201]

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 ?

 

[Gokul43201]

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.

 

[Shay10825]

I tape the wire to the battery and wrap it around the resistor wires.

 

[Gokul43201]

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...

 

[Ivan Seeking]

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.

 

[Gokul43201]

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.

 

[Ivan Seeking]

I think you're doing a great job! I just wanted to add a word of caution.

 

[Shay10825]

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 ), and a ton of Physics homework. I might be able to start back testing tomorrow.

 

[Shay10825]

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?

 

[Shay10825]

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?

 

[Ivan Seeking]

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.

 

[Ivan Seeking]

Did you find out what the deadline is for your report and project?

How did you do on your physics test?

 

[Shay10825]

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?

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 . I got a 72 . But it does not determine my grade anymore because the school board extended the quarter. So we will have 1 or 2 more tests.

 

[Ivan Seeking]

Read this
http://www.ehow.com/how_16767_voltmeter.html
the section about measuring with a voltmeter here
http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/Basic/Basic3v.html#Measuring
and pages 1,2 and 6, here
http://hibp.ecse.rpi.edu/~connor/education/IEE/labs/Lab_1_Current_Voltage_and_Resistance.pdf

There should also be some instructions with the meter. Make sure that you read those as well.

Do you have a Radio Shack nearby?

Not so good . I got a 72

...better next time.

 

[Shay10825]

Do you have a Radio Shack nearby?

Yeah there is a radio shack nearby. why?

 

[Ivan Seeking]

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.

 

[Gokul43201]

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.

 

[Shay10825]

When I'm titling this invention for the science fair does it need to be a question?

 

[Ivan Seeking]

Did the rules for the fair indicate this?