FAQ - Magnetic vs. Electronic Transformers for MR16 LEDs

This article assumes you are considering retrofitting existing low voltage halogen downlights with LED MR16 lights (MR16 lights are also known as GU5.3 sometimes). LED MR16 lamps require external driver circuitry in order to step down from the 120/240VAC mains supply to 12VAC. The LED chips inside LED lamps are driven by direct current (DC). As a result of this, MR16 LED lamps contain electronics to convert from AC to DC (generally a bridge rectifier).


There is also a different class of low voltage LED lights that do not contain a a bridge rectifier and are DC only (12V to 48V DC in general). These lights are not suitable for retrofitting to existing low voltage halogen tranformers and are not considered in this article. Generally these lights do not have a MR16 fitting and are instead sold with a compatible hard wired transformer (a LED driver).


This article looks at if existing wire wound transformers should be kept when switching to MR16 LED lighting or if a full or partial replacement with electronic transformers should be done. The compatibility differences between electronic transformers is examined in another FAQ article LED Compatibility across Electronic Transformers.


Electronic Transformers

Electronic transformers for halogen lighting, including the OSRAM Redback transformer used in our test setup, often require a minimum load of 20-60W or 10-60W throughout the line-voltage cycle otherwise the transformer will turn off and the lamp light can flicker. Due to the low power consumption characteristic of LEDs, a single LED light cannot meet this minimum load, needing additional "intelligent" circuitry to draw an input current compatible with the electronic transformer's load requirements. Therefore, unless the LED lamp is specifically designed for backwards compatibility with halogen transformers, it may exhibit issues such as flicker, buzzing or poor light output. The best solution is to test with a single fitting for compatibility before bulk replacement.


An alternative solution for the insufficient loading problem is to connect several LED bulbs in parallel to the same transformer or include a halogen bulb in the circuit. While this may solve flickering and buzzing issues, it should be kept in mind that these transformers often have a specified maximum length allowed in cables connected to the transformer's secondary in order to limit voltage drop. Lamps installed in the same transformer must also have cables of the same length to avoid variation in their light output.


Wire wound Transformers

When used with LED lamps that are 12V AC/DC compatible, wirewound (also called "iron core", "electromagnetic" or "magnetic") transformers exhibit little to no issues regarding dimming, flicker and buzzing. As their construction is simpler they should also be more durable. They are, however, very inefficient in terms of power consumption compared to electronic transformers. They are also harder to buy new as they are being replaced by electronic transformers. If purchasing a new magnetic transformer then consider a toroidal magnetic transformer as they are considerably lighter, quieter, and more energy efficient than the traditional brick style.


Power consumption using different transformers, when powered

Wattage and power factor were measured for different LED lamps when connected to a ATCO TM50-2 wire wound transformer and a OSRAM Redback 20-60 Watt electronic transformer. The results are provided in the table below. Testing across 13 LED lamps from different manufacturers, the lamps drew an average of 4.5W more when supplied by the iron core transformer.


Name ID Wirewound Electronic Difference (W)
Watts PF[?] Watts PF[?]
GU5.3-MR16 14 8W @ 245V 0.25 3.4W @ 245V 0.63 4.6W
GU5.3-MR16 5W LED 31 9.2W @ 244V 0.29 5.2W @ 244V 0.69 4.0W
12W Cool White MR16 35 9.8W @ 244V 0.33 5.3W @ 244V 0.62 4.5W
COB5WMR16CW120*ND 50 11.8W @ 245V 0.36 5.1W @ 245V 0.83 6.7W
Veco 10W Dimmable MR16WW 52 11.5W @ 244V 0.36 6.5W @ 244V 0.55 5.0W
MR16 9W HO WW 57 12.6W @ 244V 0.31 8.4W @ 244V 0.73 4.2W
10-50W MR16 58 15.4W @ 244 0.44 11.4W @ 244V 0.98 4.0W
6W LED MR16 62 10W @ 244V 0.31 5.0W @ 244V 0.65 5.0W
MR16 Hi-Output 71 15.9W @ 244V 0.46 8.2W @ 241V 0.98 7.7W
7W Focused Beam 89 12.1W @ 244V 0.38 7.9W @ 244V 0.99 4.2W
DR700 Retrofit 94 18W @ 244V 0.53 13.9W @ 244V 0.95 4.1W
MR16 LED Lamp 95 10.1W @ 243V 0.31 5.9W @ 243V 0.65 4.2W
MR16 35 Advanced R WW 104 15.6W @ 244V 0.49 11.7W @ 244V 0.97 3.9W
Average 4.8W


Power consumption using different transformers, when turned off

Even when there is no load connected (e.g. the light has been removed from the socket) transformers draw a small amount of power. However when switched off at the light switch there will be no power usage.

Transformer type (W)
Wirewound brick 4.8W
Wirewound toroidal 2W
Electronic 0.4W


Light output using different transformers

For performance comparison, the total lumen output (luminous flux) of several LED lights was measured when using electronic and wirewound halogen transformers. With low voltage leads of the same length, the light output from LEDs was very slightly better on the iron-core transformers with an average of 2.3% improvement in Lumen output - almost below the margin for measurement error.


Name ID Lumens (Wirewound) Efficacy (lm/W)Lumens (Electronic) Efficacy (lm/W) Lumen % Difference
GU5.3-MR16 5W LED 31 185 23 182 35 1.6%
12W Cool White MR16 35 206 21 198 37 4.0%
MR16 Hi-Output 71 387 24 374 46 3.5%
7W Focused Beam 89 256 21 255 32 0.4%
MR16 LED Lamp 95 428 42 420 71 1.9%
Average 2.3%


Return on Investment when replacing Wire-wound Transformers

Power usage was on average 4.5W higher with a wire-wound transformer, a significant increase in power consumption considering MR16 LED lights generally use between 4-10W. With 60 of these transformers using 5 Watts extra each for an average of 3 hours per day, this will consume an extra 328kWh per year. At an average cost of $0.20 per kWh, this will cost you $65.70 a year just to run the transformers. Replacing them with a quality electronic transformer you might pay $7/unit ($420 for 60 units).


Assuming no installation costs and no unit goes faulty, you would get your money back in about 6.4 years. If installation costs are added at $10/unit, then the return is 15 years.


Since the average user would be interested in balancing their energy savings with installation costs, a recommended approach would be to:

- Replace the units that get used for many hours each day (>5 hours)

- Replace all the units if you pay a lot for electricity (>$0.30/kWh)

- Replace none of the units if you have cheap electricity (<$0.15/kWh)

- Replace none of the units if you can't get the installation done cheaply (>$10/unit)

- Replace any wire transformer that goes faulty. (Old wire-wound transformers should be recycled)


It is, however, a complex question as there are also options to switch to GU10s or LED kits with proper drivers. Using proper drivers makes a lot of sense technically, except they are still comparatively expensive.


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