Halogen to LED Downlights

4 Jan 2022
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United Kingdom
I recently tried to replace all my halogen downlights with LED's. I checked the transformers and found the bit that said they support 20-60W bulbs. (20-60VA). I then ordered 5W LED's (50W Equivalent) and replaced them.

About half the fittings i replaced work. Except 2 circuits, the kitchen and utility. When i turn the lights on they turn on for about 1 second then turn off.

The previous bulbs were Halogen MR16 12V 35W i replaced them with 12V 5W (50W Equivalent) LEDS

Should i just order replacement Halogen bulbs? Can anyone explain why they are behaving like they are to me just for my interest?

My leading theory is the new LED bulbs require AC, and the old bulbs(transformers) are creating DC. Hence the brief spec of power when turned on then nothing.
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Do you have one power supply per lamp? ,(I'm not going to call them transformers, with that minimum load they are probably switch mode PSUs of some sort).
If yes then your new (5w) lamps aren't loading the supply enough for it to work properly- it tries then gives up cos no stable voltage.
And no, your theory is poo. If you had lighting transformers they'd be emitting AC. Light Emitting Diodes will run on AC but (without rectification) they might flicker and be a bit dim.
EDIT That 20va minimum load is very likely minimum resistive load (which you'd get from a tungsten filament). LED lamps likely have a highish inductive load...
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As said load needs to be at least 20 watt, 4 x 5 watt just on the edge. But AC v DC can also be a problem, although unlikely, the G5.3 is a package, as well as LED there are other components, there are DC only versions, mainly used with boats and caravans, but most are designed for 50 Hz AC. This can be also a problem, many of the electronic transformers are in the kHz range and have warning about the length of cable, often limited to 0.5 meter. In theory it could become a transmitter, although not seen it happen personally.

So to move to LED we often have two options, one is to swap the electronic transformer for a toroidal transformer, although using a DC (often called a driver) or kHz electronic power supply which can work from zero is possibly an option, but not as certain to work as a toroidal transformer. The other option is to move to low voltage (230 VAC) however this also has problems.

The main problem is the regulations which says
A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lamp holder having no exposed-conductive-parts and suspended from such a point.
this means we need a earth wire to every lamp even if not used as a GU10 is not suspended, as to if you comply with rules is of course up to you, BS 7671 is not law, but can be used in a court of law.

On the plus side however low voltage lamps can be "Smart" with built in dimming etc.

However neither option is without problems, I was lucky mine already have toroidal transformer supply.

Personally I would want to move to GU10, and before 1966 it was not required to have an earth to lights, however any EICR (electrical installation condition report) could highlight lack of earth wires, and often when there are no earth wires it means wired with rubber cable, so it could have a knock on effect if reported, with people reading the report assuming pre 1966 wiring.

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