Trimming pots provide dimmer control in LEDs

Trimming pots provide dimmer control in LEDs

Trimmers also provide the ability to adjust brightness parameters on the manufacturing line

BY BOB HOLMES
Bourns, Riverside, CA
www.bourns.com

Light-emitting diodes (LEDs) have been gaining popularity in consumer electronics and automotive lighting, and now LED lighting is emerging as a potential replacement for the standard incandescent bulb. The LED lighting market is expected to boom in the next decade, but there are design challenges to overcome.

An example of added steps and design challenges for LED bulbs is the requirement to adjust the brightness and unification of LEDs. A trimming potentiometer, also called a trimmer, is an effective way to satisfy these dimming control needs and provides the ability to adjust brightness parameters on the manufacturing line as well.

Trimmer technology is designed for use in applications where the unit must be calibrated before shipment from the factory. In the design of LED light bulbs, the trimmer is incorporated to adjust brightness and unification of the LEDs. This article will discuss unique design challenges inherent in LED light bulbs. It will also provide an overview of trimmer technology and discuss how to use trimmers for dimming control in LED light bulb electronic designs and in the manufacturing process.

LED design challenges

The LED light bulb, as with most new technologies, faces design challenges before it becomes the universal standard. The fundamental difference between an LED light bulb and an incandescent bulb is that LEDs require electronics on printed-circuit boards in order to work whereas a standalone bulb is simply ready once installed in a fixture.

Cost and size are issues that must be overcome in the design of electronics for LED light bulbs. Size constraints for the electronics become critical for LEDs because the available space is the same as for an incandescent light bulb. Although the actual size of an LED is smaller than an incandescent light bulb, an array of LEDs is typically used in most designs and hence space constraints become a more serious issue for brighter bulbs and larger arrays. The array of LEDs must be controlled so a uniform amount of light shines from each point on the LED light bulb.

Since the voltage ultimately determines brightness, LED bulbs must be designed for a broad voltage range so that each LED can accommodate the desired illumination. A dimmer used in the design of LED bulbs causes the LEDs to operate at the desired brightness or to switch on and off according to a timed pattern that gives the impression of decreased brightness.

Within the electronics of an LED light bulb, a trimmer can be incorporated in the interface to the LEDs and typically includes additional resistors. A trimmer is essentially a passive component that does not consume power. It is important in the design of LED bulbs to limit the voltage to a trimmer’s rated voltage to avoid damaging the device. Current must also be limited through the trimmer to maintain the rated power range for the operating voltage and temperature.

Integrating trimmers in LED designs

Brightness and unification must be adjusted according to the voltage at the dimmer or driver circuit. In Fig. 1 , the trimmer is inserted in a way that total resistance can be adjusted by rotating the trimmer. Resistors R1 and R2 in the schematic below are used in such a way that the sum of their values provides the minimal resistance and voltage drop for the circuit, which accounts for margin.

Trimmer VR1 is inserted with a maximum resistance value that, when added to R1 and R2, provides the most resistance and voltage drop to the circuit. In addition to selecting the resistance value, it is important to consider the required resistance tolerance. Tighter tolerance may require a more expensive trimmer, and the importance of accuracy over cost is critical for some applications.

Fig. 1. Typical trimmer circuit.

The exact placement of the trimmer will determine the appropriate package for the application. Since adjustments may be necessary, the trimmer must be accessible and unobstructed from other components. Some packages allow for only bottom adjust, and mounting options include board edge and surface mount styles.

The available circuit board real estate is another factor in determining the appropriate device for LED light bulb electronics. LED lights are also susceptible to environmental factors such as dust and moisture due to the variety of installation points. To avoid dust and moisture damage, fully sealed trimmers are the best and most robust option for LED lighting applications since oxidation or contamination can result if a sealed trimmer is not used.

Once the LED light bulb electronics have been assembled and the trimmer is mounted on the board properly, the resistance value of the trimmer can be modified easily by turning the rotor that controls the voltage and ultimately the brightness of the array of LEDs. When implemented properly within the design of an LED, a trimmer will provide continuous reliable operation the moment the LED is installed in a light socket.

Next-generation trimmer designs include features such as 100-turn life cycles and some include a rotor stop within the design that allows for adjustments within the circuit. Plastic housing in some trimmer designs ensures a longer life, and makes them compatible with designs using radio frequency (RF) components in close proximity, which is important when wireless interfaces are featured in light bulb controls. Single-turn trimmer designs, available in numerous package types, feature cermet construction and are compatible with surface mount designs, which are perfect for pick and place equipment and automatic adjustment equipment on manufacturing lines.

Easy adjustments ensure reliability

Accuracy, reliability, and sensitivity are the key features that make trimmers ideal for use in LED light bulb electronic designs. Trimmers that feature a sealed package provide reliability even with prolonged exposure to dust or damp conditions.

The convenience and reliability of adjusting the trimmer during the manufacturing process and prior to shipping the LED light bulb offers tremendous cost and time savings potential, ensuring that each unit is precalibrated to its specification and ready for installation. This also helps to guarantee that the LED light bulb has consistent brightness and uniformity over its entire lifetime. Bourns offers two models of sealed trimmers that are especially well suited and proven in LED light bulb electronics. Each model provides specific benefits to the design of LEDs. ■