Intro: Hot Wire Cutter Battery Power Supply
The basics of building a hot wire cutter (be it for foam, plastic, ice, or otherwise) are fairly simple. The goal here is to show how to build a precision power controller so that you can heat up just about any hot wire cutter to the exact temperature you need it.
If you want a plugged in power supply, the same principle can be used
To see how I built the cutter that I use as an example in this instructable, please see this post https://www.instructables.com/id/Adjustable-Height-Foam-Block-Cutter/
Step 1: Materials
To build a proper power supply you will need the following material:
1- Multimeter: the important part is the ability to read DC volts, and resistance (ohms also sometimes identified as Ω).
- This is a simpler version of the one I use
- This is the more advanced one that I use; includes contactless live current detection. Good for housework.
2- A battery pack.
2b- The connector for the battery pack. Since I used a powertool battery, that meant fashioning a connector for the battery, but you can just as easily use a small 9v battery in which case you would need a clip on connector with wires coming out. You can also get AA battery pack for projects (usually you can put 4 to get 6v out).
4- Cutting tools at your disposal (I used snips, a utility knife, and wire stripper, because I had them all. However, when I was just starting at building years ago all I had was a pocket knife and long nose pliers; use what you have and grow into your tool collection as you go).
5- Some wiring as needed. I used about 4 feet total. I personally like using extension cord wiring from the hardware store (they come in spool), because it is a threaded core wire rather than a solid core wire, making it much more flexible and less prone to cracking. This is the best quality to price and availability compromise I can find; but if you want the best for flexiblity, then silicone sheethed wiring is the cadilac of wiring. I used 14 gauge wiring to avoid adding unmeasured resistance to the circuit and for alternate higher power uses.
7- 3mm (1/8th) flat head screw driver (for the screw terminals of the DC-DC converter)
7- A good soldering iron. I use a butane type one myself, because larger gauge wiring dissipates the heat too fast. If you don't use connectors (see item 8), you can possibly get away with using aligator wires and screw-terminals and won't need this.
8- If you want to connect the different components with connectors rather than making it a more or less permanant construction, then you will also need connectors. Anything will do really, but for high quality and versatile I recommend Anderson Powerpole Connectors. Genuine connectors are materially better; they can be reused more easily, and clip better.
I get mine from this guy in Canada http://www.ebay.ca/usr/ve3vft?_trksid=p2047675.l25...
Actual nicrhome wire, not guitar wire, and here is why. For a hot wire cutter, you would need to fashion a holder that meets your requirements, as well as some nichrome wire. I mention this because for the precision aspect to work, we need to know what we are working with exactly, and the app mentionned later on needs to know what gauge of nicrome wire you are using specifically. I get mine off amazon when I am being impatient, otherwise, ebay from China if you don't mind waiting 1-2 months and want to save some money.
Step 2: Assembly
Assembly is quite simple. You want to feed your battery terminals in the input of the DC-DC converter, and leads out from the DC-DC converter to your hot wire jig.
If you chose to go with connectors, of course, you want to put connectors on your wires before assembly to make your life easier.
The DC-DC connector has screw-type terminals, so assembly there is quite simple. Open the opening, put the wire, tighten.
Step 3: Calibration / Adjustment
This is where you get to have versatility and precision. For this we will need to use the Jacobs Nichrome wire calculator http://www.jacobs-online.biz/nichrome/NichromeCal...
Before heading there, we will need the following information
-Target temp (normally 600f for foam cutting)
- The resistance of your cutter. Use your multimeter to calculate the resistance of your circuit (everything that goes after the output of the DC-DC converter). By adding the wiring and whatever else in the resistance measurement, you will have more precision. This does matter at this scale, because small cutters will be in the 0.5-1.5 ohm range.
- Gauge of your nichrome wire (very important to have used nichrome for this to work)
Now head on over to the calculator, and select the button for voltage (meaning we will set everything else, and the calculator will give us the voltage we should use).
1- enter the temperature
2- enter the gauge (important to do this before length)
3- length: At the bottom, you can see the "Total Resistance (ohm)" box. As you change the length, look at the resistance until it matches what you had measured earlier. This will be much more accurate than using actual nichrome length (because it factors in tying lenghts lost, connection resistance, wire resistance, etc.)
We now have a proper voltage. Now connect the multimeter to the DC-DC controller's output, and turn the dial until the voltage is at the desired value and that's it.