LEDs Address Drawbacks of Halogen MR16 Fixtures

The MR16 lighting form factor has proved popular because it offers several advantages over other lamps with equivalent power ratings. The lights are compact (although a bulky transformer is required to step down mains voltage), allow better beam control, and offer a brighter, whiter light than traditional incandescent lamps.

One big downside is that the units typically use halogen bulbs that get very hot. This means that lights embedded in ceilings, for example, must be cleared of roof insulation to minimize the risk of fire. And while halogen lamps outperform incandescent bulbs, they lack the efficacy and longevity of other lighting technologies.

Replacing a halogen bulb with an LED light source maintains the advantages of an MR16 fixture while addressing its performance weaknesses. While more expensive than halogen, these advantages – particularly LEDs’ long life, which reduces maintenance costs due to extended periods between replacements – are sufficiently compelling for commercial premises such as shops and restaurants to consider renewal programs. Further price falls will see the units become attractive to domestic consumers.

With an estimated 20 billion MR16 fixtures in service, commercial, and domestic renewal programs represents a lucrative opportunity for engineers developing solid-state lighting.

This article looks at halogen MR16 fixtures and then explores the luminaire, LED module, and driver technologies available to supersede them.

Benefits and drawbacks of MR16 fixtures

The multifaceted reflector, measuring 16 x 1/8 in. (51 mm) in diameter, first took off in the mid-1960s when General Electric (GE) introduced an innovative new form factor, dubbed the MR16, whose light could be accurately focused by the reflector. The device was different from the common MR16 lamps sold today, primarily due to the fact it was not actually multifaceted, and secondly because it was not targeted at the lighting sector, instead being designed for use in film projectors¹ (Figure 1). But what GE’s lamp did share with its modern equivalent was the use of a tungsten-halogen (“halogen”) bulb.

Figure 1: GE’s original MR16 light from 1965 was the first to feature a focused output but was yet to adopt a multifaceted reflector. (Courtesy of General Electric.)

The halogen gas in the bulb reacts with the tungsten evaporated from the filament to form a halide. The reaction is reversed when the halide comes into contact with hotspots on the filament, depositing the tungsten and freeing up the halogen to repeat the process. Because of this chemical reaction, the lamp glass stays cleaner and the filament lasts longer than in a conventional tungsten-filament bulb.

The MR16 fixture is popular because it is compact, directional, bright, and suited to being embedded into ceilings for a clean and modern look – allowing the bulky transformer required to step-down the mains voltage to the 12 to 24 VAC used by the lights to be hidden in the roof space.

There were and are some downsides. Because the reversible halide reaction only occurs at high temperatures, a 30 W halogen bulb, for example, can reach 300oC, so care must be taken to ensure that materials such as roof insulation cannot come into contact with fixtures recessed into ceilings. Second, the bulbs contain gas at high pressure and must be handled carefully to limit the risk of explosion.²

Another disadvantage is the relative inefficiency compared with other modern lighting technologies. The power consumption of the most commonly used MR16 lamps ranges from 10 to 50 W for luminosities of 150 to 800 lm. Efficacy, therefore, is about 15 to 16 lm/W, and while halogen bulbs last at least twice as long as conventional bulbs (at around 2,000 hours), this is still well short of the longevity of contemporary alternatives.

Swapping halogen for LEDs

LEDs are one of those modern alternatives. These solid-state lights boast far higher efficacy (50 to 100 lm/W) and much longer life – typically 50,000 hours before illumination drops to 70 percent of the original value (see the TechZone article “Determining LED Rated Life: A Tricky Challenge”). LEDs are available in a range of correlated color temperatures (CCT) to suit the user’s desired ambience, and the devices are robust enough to withstand rough handling.

LEDs also run cooler than halogen lights; however, the junction of the active device can still reach temperatures in excess of 125oC so careful attention does need to be paid to thermal management (see the TechZone article “LED Heat Dissipation and Lowering Thermal Resistance of LED Lighting Substrates”). However, the outside of a well-designed MR16 LED fixture is unlikely to exceed 100oC – much cooler than the typical figure for a halogen fixture.

Ever alert to such lucrative opportunities ¬presented by potential renewal programs for the 20 billion MR16 halogen fixtures in service, LED manufacturers are starting to produce fixtures suitable for direct substitution for the conventional MR16 fixtures. And because LEDs can be produced with directional light output, there is no requirement for the multifaceted reflector used to focus a halogen’s beam.

Lumex, for example, offers its SunBrite MR16 fixture comprising four LEDs producing 315 lm from a 12-VAC source while consuming 5 W for an efficacy is 63 lm/W (Figure 2). Lumex claims that SunBrite will last for over 17 years if used for a typical 4 hours per day.

Figure 2: Lumex SunBrite MR16 fixture offers much longer life than a halogen equivalent.

While MR16 LED fixtures duplicate the form factor of the halogen versions, they are not direct “drop-in” replacements for the older types of light. Halogen lamps are driven from a 12 V AC source, but the same source is not suitable for LEDs. LEDs require a constant current, constant voltage supply to ensure that they operate correctly.

However, there are a number of proprietary solutions on the market for driving MR16 LED fixtures. Maxim’s MAX16840 LED driver (voltage regulator), for example, is purpose-designed for powering 12-to-24 VAC lights. The company says that with the addition of a few external components, the MAX16840 incorporates all the features needed for an LED driver system. The IC also features an internal overvoltage protection to protect the internal switching MOSFET from damage if the LED string is open or if the voltage on the LED string is too high.

Another option for the designer looking to develop an MR16 LED product to replace halogen versions is to dispense with the need for a LED driver completely by considering a fixture that can be powered directly from the mains. Seoul Semiconductor manufactures a suitable module in its Acriche A3 range, the AW3211, which can be used as the basis for an MR16 LED fixture (Figure 3).

Figure 3: Seoul Semiconductor’s Acriche 3 module can be powered directly from the mains supply.

The module measures 30 mm in diameter and can be powered directly from a 110 VAC source (a 220 VAC version is also available). The module provides 215 lm (at 40 mA) while consuming 4 W for an efficacy of 54 lm/W.

The environmental choice

Since their inception, MR16 halogen fixtures have proved very popular. Their compact form factor and pleasant, directional light has made them popular for fashion-conscious consumers and commercial premise owners alike.

But to today’s environmentally aware public, the relative inefficiency of these halogen devices has come under scrutiny. And the operating temperature of the lights introduces a risk of fire from the ignition of materials such as building insulation.

Replacing the halogen light source with LEDs retains the advantages of the MR16 form factor while addressing the drawbacks. LEDs are far more efficient and last much longer while also being naturally directional. Electronics companies are now introducing complete products and component parts for designers to develop their own MR16 fixtures in an attempt to take a slice of this growing market.

References:

1. http://www.lamptech.co.uk/Spec%20Sheets/TH%20RM%20Q150MR16-21%20EJM%20GE.htm
2. http://en.wikipedia.org/wiki/Multifaceted_reflector