Resistors and Resistor WireResistors are used to limit the amount of current that flows through a circuit. In electrical circuits, they are also used to divide voltage and dissipate power. Resistors found in electronic devices are commonly made from carbon, carbon film, metal film and metal oxide. Wire-wound resistors employ metal wire wrapped around a core material, often ceramic, fiberglass or plastic. While limiting current flow, resistors produce heat. In electronic equipment, this is considered an undesirable side effect. However, if heat is the desired result, resistors provide an excellent passive heat source. Resistors can be fixed or variable in the amount of resistance available. This means that the amount of heat can also be varied. Using a resistance wire allows the user to create the amount of heat desired using different wire materials and sizes. |
Types of Resistance Wire
Nichrome is often used to manufacture resistor wire. Nichrome is an alloy of chromium and nickel, with iron sometimes included. Different materials and percentages of materials allow for the creation of different resistivity. This basic alloy is resistant to corrosion and has a high melting point, approximately 1400°C (2552°F). The high temperature tolerance makes it an excellent choice for heating applications.
Nichrome 60 is an alloy of 60% nickel, 16% chromium and 24% iron. Nichrome 60 has a melting point of 1350°C (2462°F) and a maximum operating temperature of 900°C (1652°F). A chart is available to determine the resistivity at specific temperatures.
Nickel-iron (NiFe) alloys have a high temperature coefficient with low resistivity. Resistor wires with these alloys create heating elements that will reduce the required amount of power as the temperature increases.
Kanthal wire is also an excellent choice for a resistor wire in heat producing applications. Kanthal A-1 is often used for furnace applications.
Kanthal alloys offer a more consistent temperature over the element and maintain closer tolerances. The wire is lighter in weight when compared to NiFe or NiCr alloys. It is also less prone to corrosion.
Selecting Wire
In order to select the correct resistor wire for a project, the resistivity of the wire must be considered. The thermal ability, including melting point, is critical. Alloys need to be chosen for corrosion resistance and weight. In applications that require precision temperatures the tolerances for the wire must also be factored in the decision.
Common Calculations
Current, or amperage, flows through a device or resistor. Voltage flows across a device, or resistor. Resistance is measured in Ohms. Ohms Law provides simple methods for determining voltage, current and resistance:
V = IR
Voltage (V) = Current (I) times Resistance (R)
To find current, Ohms Law is rearranged:
I = V/R
Current = voltage divided by resistance.
To find resistance:
R = V/I
Resistance = voltage divided by current.
For example, if you measure a voltage of 12 volts with a current of 40 milliamps the resistance is 300 ohms.
The formula used for the physical resistance of a wire is Resistance = rL/A. To determine the amount of resistance provided by a piece of wire you need to know three factors.
1) The length of the wire, indicated by the symbol L.
2) The cross-sectional area, or diameter, of the wire, indicated by the symbol A.
3) The constant resistivity of the wire's material, indicated by the symbol r.
This means that the larger the wire is in diameter, the less resistance it will provide.
Power or heat dissipation can also be determined through formulas.
P = I2R - the amount of current squared times the resistance.
P = IV - the amount of current times the voltage.
P = V2/R - the voltage squared divided by the resistance.
Additional Useful Formulas
The amount of power or heat energy released over time is determined from the integral of the power over time.
P = f t2/t1 v(t)I(t)dt
Resistivity at 20°C (68°F)
W mm2/m (W /cmf)
Determining Resistor Wire Size and Type
Question: If I know the voltage and wattage desired, how do I calculate/find out what size and type of wire is wire appropriate for making a resistor or heating element?
Answer: When you know the voltage of a system and the power, or wattage, that you want to achieve, you can use the standard power formula to determine the resistance the will be required.
P = V2/R
Rearrange this formula to determine resistance: R = V2/P
A rough calculation for creating a 1000-watt heating element on a 250-volt supply shows you will need 62.5 Ohms of resistance.
250 volts squared = 62500 volts. 62500 volts divided by 1000 watts = 62.5 Ohms.
Complete calculations require considering the length of the wire and the resistivity. The chart below provides a quick reference for wire type and gauge size calculations. For example, to heat Nichrome 32 AWG wire to 400°F, 0.68 amps is required. We can help you with the calculations to determine the type and gauge of resistance wire you need.
Gauge (AWG) | Nichrome 60 1150°C (210°F) |
Kanthal A1 1400°C (2550°F) |
16 | * | |
18 | * | |
20 | * | |
22 | * | |
24 | * | |
26 | * | |
27 | * | |
28 | * | |
29 | * | |
30 | * | |
32 | * | * |
33 | * | |
34 | * | |
36 |