While LEDs are mostly driven by current sources, there are also several applications where constant voltage drivers are used. In that case current limiting resistors are mandatory. But that raises the question which resistor is the right one to guarantee reliable and accurate control while not compromising efficiency.
 
Not all resistors are the same and growth in markets like high-power LED lighting is serving to highlight the importance of understanding all aspects of an application in order to correctly and safely specify the right type of resistor.
 
Having understood the biasing calculations for some typical lighting scenarios, it quickly becomes apparent that in many applications the necessary ballast resistor is likely to dissipate several Watts of power. This not only dictates the need for a suitable high-power resistor type but may call for a design suitable for mounting on a heat sink, to help carry heat away from the LED rather than contribute to an already challenging design requirement.
 
All these considerations and more will be explored in the context of potential design solutions from specialist resistor manufacturer and supplier, Microhm Electronics, whose product line includes its NTW series of wirewound power resistor for up to 13W dissipation, its PF series of power film resistor with ratings up to 20W and other series that allow for additional heatsinking and surface mount options.
  For LED lighting applications, devices with a nominal rated forward current of 350mA are quite typical but LEDs designed for operation at 700mA, 1A and even 1.5A are becoming increasingly common. So many applications requires a resistor with a rating just over 2W, higher power LEDs may well require resistors rated at 10W or more.
 
Axial-leaded wirewound resistors offer reasonable power handling with low resistance tolerances, excellent low TCR performance and they can operate over a wide temperature range. For example, Microhm Electronics’ NTW series wirewound power resistor offers power ratings up to 13W and a temperature range of -55oC to +350oC. For even higher power dissipation, or where it is important to remove heat more effectively from the ballast resistor, Micrhom Electronics has its NUAL series of aluminum-housed wirewound power resistor with ratings to 50W and above.
 Wirewound resistors are available with non-inductive windings but thin film resistor technology provides an alternative that may suit some applications. Microhm Electronics NLR series offers low-inductance power film resistors in various package housings to support different power ratings e.g. 30W,35W TO-220 and 50W TO-263. For surface mount designs the NLR35E of power SMD film resistor from  Microhm Electronics can handle up to 35W.