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

ATCO TM50-2 Wirewound Transformer 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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