MR16 to GU10

21 Jan 2014
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United Kingdom

I have been advised to change the spotlights in my lounge from MR16 to GU10's as there is a greater variety of dimmable LED's on the market for GU10's.

I have seen some conflicting information online with regards to the replacement of these fittings.

Some sites say that you can cut off the current MR16 transformer and wire the GU10 socket straight into the mains, others state that you should still use a transformer with GU10's.

Could someone please help clarify this?
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You are, not surprisingly, mixing up the names.
MR - multifaceted reflector - refers to the glass type and GU to the fixings.
The 16 is diameter of the lamp in eighths of an inch.


L to R
MR16 with GU 10mm. base. 240V
MR16 with GU 5.3mm. base 12V
MR11 with GU 4mm. base 12V

The straight pins are 12V.
Thanks for that.

So it would appear that i am swapping out the 5.3mm base 12V for the 10mm 240V.

By that rational is it correct that no transformer would be needed?

One website i visited suggests that inside the GU10 is a mini-transformer but it is not suitable for the job (which seems a little odd!)
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One website i visited suggests that inside the GU10 is a mini-transformer but it is not suitable for the job (which seems a little odd!)

Don't believe everything you believe on the web. There is no room for a transformer inside a GU10 that would do the job. Some LEDs have capacitive droppers, some may have an electronic regulator of some sort. Either way no external transformer is needed as they run on 240 volt. Whether they will work on dimmers is another matter entirely.
Well you have still missed one there a special with hole so one can only fit LED lamps in the holder as well.
The GZ10 holder will take all the bayonet bulbs and has to have provision to get rid of the heat at the back. The GZ10 bulb has a reflector which reflects light but not heat. Designed to light without warming item being lit.

The GU10 holder will not take GZ10 bulbs I seem to remember there is a T2 version of the GU10 designed to comply with house building rules where the builder has to use a holder which will not take tungsten lamps. There is a pin in the holder which goes into a hole in LED bulbs but the problem is not all LED bulbs have the hole so you end up paying a fortune for bulbs with the hole in base.

In the main we think of GU10 as being 230 volt and MR16 as being 12 volt and buying from the high street that works well. However from the internet you can get a variety of voltages including 110 volt.

The quartz halogen lamp works by keeping the envelope very hot so tungsten from the filament will not be deposited on the envelope but back onto the filament. As a result the voltage had to be controlled between some very close limits. The 12 volt lamps also had thicker filaments so lasted longer anyway. Hence using MR16 and 12 volt using a high frequency AC power supply which contained a transformer.

As cold cathode (florescent) and LED lamps came out the advantages of 12 volt system evaporated. With the LED the voltage to the LED it's self is around 1.2 volt so yes the GU10 has some method of reducing voltage. There are many ways but main method is again a very small switch mode regulator all in an integrated circuit because it's switched mode this means the voltage has a huge latitude often 150 - 250 volt.

Likely it will use some type of transformer but at high frequency so they become much smaller. But even using a capacitor can limit the power and an LED is more worried about current than voltage. Also since you can't access any parts it does not need to isolate as a separate transformer and light would so could use auto transformers. Which is really just a coil of wire with a tapping.

It is the method used by different manufacturers of dropping the voltage which means there is a huge difference in light output per watt used. This is also the case with discharge lamps (florescent) and the cure was to publish the lumen output however the standard was too wide as with the exception of automotive lighting which has much tighter standards the lumen is useless as two lamps with same lumen output allow us to see what is lit very different. It was suppose to be what the human eye could see but the designers of the standard got it wrong.

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