Understanding some of the often complex factors that go into resistor construction and how various characteristics of resistors are specified is useful in ensuring correct component selection.
The power rating for resistors is generally specified at 25 degrees C (ambient) and it is important to understand when designing circuitry that a derating factor applies as temperature increases. For example, the precision wound series of resistors derate to 75 percent of their rated load at 100 degrees C and 50 percent at 175 degrees C. This can be an important consideration in some industrial applications where equipment housing these resistors may be located in areas (like blast furnaces, for example) that experience high ambient temperatures. If this is the case, a higher wattage resistor may be more appropriate than the initial power calculations suggest. Most resistor data sheets specify the temperature range that the resistors will operate under. For wire wound resistors this is typically -55C to +275 C, but varies depending on the physical construction of the resistor, its coating material and surface area. Ratings are generally specified ‘suspended in free air’ and taking into account the effect of any enclosing structure around the resistor assembly is an additional factor you should consider when determining power rating selection. The linearity of the derating is typically quite linear for wire wound resistors as is shown on the derating graph for the precision wound resistor range.
- Ease of assembly into the finished product
- Long term electrical reliability of the equipment
- Mechanical mounting considerations. For example some resistor assemblies are designed for ‘snap fit’ ferrule mounting
Methods of termination onto the resistor assemblies can include:
- Quick connect terminals employing standard 6.3mm spade lugs
- Flying leads
- Edison base – allowing for ‘screw in’ termination
- Nut and bolt through terminal termination
- Solder terminals
Mounting Of Resistor Assemblies
- Threaded rod mount with lock nuts to ensure rigidity and security of large mass power resistorsMounting ‘feet’ for tubular ceramic wire wound resistors. Plated metal mounting feet inserted into the ceramic former make for ease of mounting.
- Ceramic ‘boat’ style resistors can have a variety of mounting options including standard and custom brackets mounted to the ceramic body of the resistor. The brackets can allow for both horizontal and vertical mounting of the resistor assembly as shown below:
- vertical mount power resistor
Average Temperature Rise Versus Load