Vacuum Former

Step 3 The Electrical Equations

Now that oven housing is done its time to move on to the part that makes it an oven, the heating coils. In the world of vacuum forming machines there are a lot different ways to create the heat, but the simple fact is that you just need a heat source that can effectively heat your entire plastic sheet. I found some Nichrome wire heating elements from a surplus store but there are many other options. You could use electric grill elements, a toaster oven, your stove, a heat gun, and I've even seen a gas heater used. Nichrome wire is the same kind of wire thats in a toaster and works off the fact that as current passes through the wire it heats up. I'm going to run through how I determined how to wire the oven so that it could be run off a normal household 120V outlet. This is specifically how I did it but these same equations and be applied to any other oven like this one.

This first equation we need to look at is commonly known as Ohm's law:

V=IR or I=V/R or R=V/I

In this equation V= voltage, I=current, R=resistance. The units for voltage is Volts, for current its Amps, and for resistance its Ohms. For this oven I want to use 120V, so that gets me one part of this equation. Now its time to measure the resistance of the nichrome wire. This can be done with a common multimeter and for each of my heating elements the resistance is about 74 Ohms. The resistance of nichrome varies depending on the diameter of the wire and the length of the wire your using. If your building your own oven with a roll of nichrome wire this where you can make changes to suit your situation.
So lets calculate the current (I), from these two values:

V=120V R=74 ohms

I=V/R I=120/74 I=1.62 A

So if I was to hook up one heating element to the wall, the current through it would be 1.62 Amps. But I want to hook up 8 heating elements not just one. There are two ways to wire the elements, in series or parallel. Each method has its benefits but we are concerned with the resistance of the entire oven. In this way we will consider each element as a single resistor. If I hooked the elements up in series they would all be wired end to end and the total resistance of the system would be the sum of the resistance's of each element. This would mean there would be 592 Ohms of resistance so the current would be:

V=120V R=592 ohms

I=V/R I=120/592 I=0.20 A

You would find that this is not enough amperage heat the wire to the desired temperature. This chart at infraredheater.com shows some common wire gauges and the amperage needed to heat them to a certain temperatures http://www.heatersplus.com/nichrome.htm To get more amps through the wire you need to hook up the elements parallel. This equation shows how to solve for the equivalent resistance in a parallel circuit:

1/Req = 1/R1 + 1/R2 + 1/R3 + ...

In this equation Req is the equivalent resistance, or the overall resistance of a parallel circuit, and R1, R2, R3 etc. are the resistances of each individual resistor. In this case I have eight individual resistors, or heating elements. So using this equation and the individual resistance of 74 Ohms, the equivalent resistance is:

1/Req = 1/74 + 1/74 + 1/74 + 1/74 + 1/74 + 1/74 + 1/74 + 1/74

Req = 9.25 ohms

So the current for a parallel circuit would be:

V=120V R=9.25 ohms

I=V/R I=120/9.25 I=12.9 A

Now this is more like the amperage we need to heat the coils up. I could wire the oven in parallel and call it a day but I would like some more control over the temperature and amperage. To do this I decided to use a regular lighting dimmer switch. A dimmer switch is basically a variable resistor, also called a potentiometer. By changing the resistance of the circuit I can control the amperage flowing through it and therefore control the temperature. I got the dimmer switches from Home Depot and they are rated at 600 Watts. A watt is a unit of power so I need some way of relating this to the voltage, amperage, and resistance. This equation does it:

P=VI or P=(I*I)R or P=(V*V)/R

To find the amperage:

P=600 W V=120 V

P=VI 600=120 * I I=5 A

So one of the dimmer switches can handle up to 5 amps at 120 V. The current calculated earlier for the parallel circuit was 12.9 amps, this is to much for a single dimmer switch. So I decided used four dimmer switches, each controlling two heating elements. You can see how it was wired in the pdf.

If you didn't understand some of this stuff that all right, because here is a calculator that can do it for you http://www.imperial-armor.com/ovencalculations.html This does not work in the same way I figured out my oven though. It asks for your final oven requirements and then tells you what you need. I started with what I had and figured out a way to make it work. I think this is way that most people would do it. The calculator also assumes that you are building a purpose built oven with coiled nichrome wire. If you decided to go that route here is a link that can really help with finding supplies, http://www.infraredheaters.com/