MR16 LED bulb not working as halogen replacement

5 Oct 2005
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United Kingdom
I want to replace some 20W dichroic halogen MR16 down lights with LED versions but the bulbs I've bought just flash for a split second when switched on (pic attached of bulb spec). Why this is happening and what could I do to solve the situation.

Thanks for any help.


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Your electronic transformer(s) are not suitable for the lower load. It/they need replacing with one suitable for leds or the whole setup changed over to 230V.
Thanks DeltleSchmitz, I was thinking it may be something like that. Is there a specific spec I need to look out for, it's for 5x lights.
If you're sticking with 12V and using those lamps, then it needs to produce 12V ac and have a wattage range that covers the range you will have attached. If you use those lamps in the picture then ideally from 0W to 20W, but I should add a bit at the top in case you end up with higher power lamps. I've never used 'transformers' so don't have any recommendations.
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I used toroidal lighting transformers, these work from zero to rated output without a problem, however their physical size means they are too big to fit through a 2 inch hole.

MR16 means multi faceted reflector at 16/8ths of an inch across, in other words 2 inches. LED MR16 lamps are rather rare, as in the main LED replacements don't have multi faceted reflectors, there are one or two.

But leaving the pedantic to one side, the problem is 12 volt lamps did not tend to run earths, and the regulations state "A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point." (411.3.1 if interested) this came in 1966, but it causes problems converting SELV (separated extra low voltage i.e. 12 volt) to low voltage i.e. 230 volt.

As to if worried by regulations is up to you, but clearly an electrician needs to follow the rules, so converting to low voltage can be a problem, but likely the best method, as low voltage allows the use of smart bulbs.

Today you can get both toroidal lighting transformers and pulse width modulated often called electronic transformers which will work zero to rated voltage, so often you can simply swap the transformer, however an electronic transformer often has an output in the kHz range, and many bulbs state 50 Hz, what we as the user have to decide is does it really mean 50 Hz or does it mean you should not use a 20 - 50 VA power supply? Likely the latter and the 50 Hz does not matter. But unfortunately we can only guess.

There are many ways to limit the current through a LED, pulse width modulated power supplies, resistors and capacitors to name a few, the capacitor is dependent on frequency common with low voltage (230 volt) but not with extra low voltage, with ELV most common is a simple resistor, but there are versions that use a switched mode power supply and work with DC only, often rated 10 - 30 volt and really designed for motor homes, caravans and narrow boats, normally expensive and not normally for sale in high street stores.

The worry with kHz is the whole unit can become a transmitter, but as long as leads are kept short between power supply and bulb unlikely.

So the main thing is access, and second is there any reason like use in a bathroom when ELV is a requirement, if you can go low voltage (230 volt ac mains) that is likely the best option, but if something stops you there are both AC and DC power supplies which will work from zero output, often the DC version is called a driver, not technically correct, but manufacturers don't seem to worry about that.

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