Calculations

Calculations

Increasing Resistance

The wattage rating is how much energy the resistor can absorb. Use the highest calculated wattage rating.

It is more important to have an accurate Ohm rating for your resistor than it is the wattage.

When dealing with resistors and rheostats, there are a few points to keep in mind...

Consider the Wattage rating is a minimum,

and the Ohm rating is a maximum.

One difference when calculating a rheostat in comparison to a resistor, is that with a rheostat you will require a range of resistances that correspond to the temperature of your wire.

At this point, you should know that the current flowing through your wire produces heat, and the amount of current is dependent upon circuit resistance and the voltage of your supply.

As a rule of thumb, I recommend calculating resistances at 400°F and 1000°F.

That way when your rheostat is turned off, it will provide no resistance, and the maximum current will be pulled through your NiChrome Wire (to produce a temperature of 1000°F). But when the rheostat is turned completely on, it will provide maximum resistance, and the lowest current and coolest temperature will be provided.

You can set the high and low temperatures at whatever you choose, just know that you need to choose the appropriate amperage ratings for your gauge NiChrome Wire.

Although, it is best to choose a high temperature and high amperage that corresponds to the highest rated safe current from your power source.

You will need to perform this calculation twice, for the high and low amperage.

With all of these calculations, you are able to determine that with a 18” cutter you need to have the dial set at 33% activation.

But if you want to change to a 6” cutter, the dial will have to be set different. Just repeat steps 2 - 5.

2.) Calculate the Resistance of the NiChrome Wire (6 inches)

Rn = 2.67 ohms/foot * 0.5 feet

Rn = 1.3 ohms

3.) Calculate the Resistance required for 600°F

Circuit Resistance (Rc) = Power Source Voltage / Required Amperage

Rr = 12 / 2

Rr = 6 ohms

4.) Calculate the required set resistance of your rheostat

Set Rheostat Resistance = Circuit Resistance - NiChrome Resistance

Rr = Rc - Rn

Rr = 6 - 1.3

Rr = 4.7 ohms

5.) Calculate how far the rheostat dial needs to be set (just a percentage)

Rr / Rc

4.7 / 6

78% rheostat activation

Rather than try to explain any more, it would probably be best to read through the example below, so that you can see how a rheostat is calculated and how it is applied to a circuit.

They’re really cool..!

Visit the Fixed Voltage Power Section for more information on this topic as many concepts overlap.