Calculations

Increasing Resistance

 
I have a 12v 10Ah Battery, and I only want a 18 inch straight wire cutter at 600°F.
You may also consider that you have a 12v 3a Solid state DC Power Supply

Because you know that you want a straight wire cutter, you’ll be dealing with 24 or 26 gauge NiChrome Wire. For this example, we’ll use 26 gauge, which requires around 2 amps to heat to 600°F
1.) Determine the required resistance of your circuit.
You may choose to use Jacobs Online Calculator, or proceed manually.
See the Battery Section for a walk through on the calculator.
Total Required Resistance (Rt) = Power Source Voltage / Required Amperage
Rt = 12 / 2
Rr = 6 ohms


2.) Calculate the Resistance of the NiChrome Wire
Rn = 2.67 ohms/foot * 1.5 feet
Rn = 4 ohms


3.) Calculate the Resistance for the theoretical Resistor
Resistor Resistance (Rr) = Total Resistance - NiChrome Resistance
Rr = 6 - 4
Rr = 2 ohms

4.) Calculate the Resistor Wattage Rating (Wr) for the theoretical Resistor
Resistor Wattage Rating (Wr) = Required Amperage^2  x  Resistor Resistance
Wr = I^2  *  Rr
PR = (2)^2  *  (2)
PR = 8 Watts


:: Resistor Rating ::
8 Watts  &  2 Ohmshttp://www.jacobs-online.biz/nichrome/NichromeCalc.html__FixedVoltageCalc.htmlshapeimage_3_link_0shapeimage_3_link_1

The wattage rating is how much energy the resistor can absorb and dissipate.

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

Resistors

Resistors are solid state components that have a fixed internal resistance that cannot be altered  which is listed as their resistance rating. The amount of circuit amperes they can handle is listed as their wattage rating.

While resistors are useful, I prefer rheostats because you are able to control the resistance of your circuit with the turn of a dial. With this ability you gain control of the circuit’s current flow (and  temperature) when the voltage from the power supply cannot be altered. Visit the Fixed Voltage Power Section for more information on this topic as many concepts overlap.

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.

EXAMPLE
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General Discussion

Calculating and determining a resistor is not all that difficult, but it does require a bit more in depth understanding of resistance wire circuits and their required heating current. If you have not yet visited the Circuit Theory Page, go there before proceeding further.

Resistor Color Code

In days past, small electrical components had colored bands printed on them to designate their internal resistance. But discoloration from overheating, dirt accumulation and those that were color blind made it quite difficult to ensure proper reading. Advances in printing technology made printing the values on the small components more practical and some resistor manufacturers have abandoned the electronic color code.

The Electronic Color Code is used to indicate the values of electrical components including resistors, capacitors, inductors and others. More information available through wikipedia as Electronic Color Code.

Each color band on the resistor has a specific meaning depending on its location within the code sequence.

Brown, Yellow, Red, Grey

14 x 10  ohms

2

1400  ohms

or

Black, Green, Brown, Grey

05 x 10  ohms

1

50  ohms

or

Grey, Blue, Orange, Red

86 x 10  ohms

3

86k  ohms

or

+/- 0.05%

+/- 0.05%

+/- 0.05%

+/- 0.05%

+/- 2.0%

+/- 2.0%

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