/ Replacing halogen MR16 bulbs with LEDs?

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Martin W 08 Aug 2019

We have four 50w halogen MR16 downlighters in the bathroom.  Each one is supplied by its own 12v dimmable transformer.  The whole lot is controlled by a dimmer switch outside the bathroom door.  I've been pondering replacing the setup with LEDs for some time.  Last week one of the transformers failed, which has prompted me to look seriously in to making the change.

Now, first things first: the advice I've had so far is that, because of the location of the light fittings wrt the bathroom fittings, I need to stick to 12v lights.

We would like to retain the ability to dim the lights.  As I understand it, this will mean having to use dimmable LED-compatible transformers.  Questions I have are:

1) Given that the LED bulbs will be running off the 12v supply from the transformers rather than directly from the 230v mains, does the dimmer switch need to be LED compatible or can I use an ordinary (cheaper) dimmer switch of the kind that works for incandescent bulbs but not LEDs?  (My guess is probably not.)

2) Given the much lower power draw of LEDs of equivalent light output to halogens, is there any point in sticking with the one transformer per bulb approach, when a single 50w transformer should be plenty for 4x8w LEDs?

TIA for any advice or guidance folks may be able to offer.

wintertree 08 Aug 2019
In reply to Martin W:

If the wiring goes 230 v > dimmer switch > transformers > bulbs, I think you’ll be crap out of luck with LEDs in terms of dimming.  I’m no expert but that’s weird and I can’t see how it’ll work nicely with LEDs.

I’d pull the dimmer switch out and put a normal switch in, drop down to one transformer and use smart bulbs - loads of people do smart (WiFi/Bluetooth) MR16 bulbs, and I expect at least one does a remote control you could wall mount.

Your second question - if someone else fitted 4 halogen bulbs in the future, they could unknowingly overload the single transformer and potentially set it on fire.  I’d imagine there’re buildings regs on that sort of thing but I don’t know.  The problem with standard 12 V and 240 V fittings is you have to assume someone is going to put halogens in - this meant some ”unnecessary” changes with respect to fire safety buildings regs when doing in-ceiling 240 V LEDs in GU12 fittings in our house.

Post edited at 17:39
Hooo 08 Aug 2019
In reply to Martin W:

1) Your new dimmable LED "transformers" will actually be switched-mode power supplies. Some of them will work on normal dimmers and some will need a special dimmer (trailing edge). You'll need to check the specs to see whether you need a new dimmer switch or not. Dimmer switches are pretty horrible things electrically, so even if your transformer is compatible it still might not work. If I was dealing with an old unknown dimmer I'd be inclined to buy the transformer and a new dimmer switch from the same supplier and get them to verify that they are compatible.

2) I would fit one transformer for all lights. I don't know about regs, but a modern electronic LED transformer will be protected against overload and will shut down if someone puts a halogen bulb in. If you're worried you could always fuse the output (4x8W @ 12V = a 3A fuse).

3) Don't be tempted to buy the cheapest transformer you can find. I bought one for my bathroom from what I thought was a reputable supplier. I switched it on and my DAB radio went dead - it killed all reception within about 5m. I complained to the supplier and they had the cheek to say that all LED transformers did that! 

Steve Clark 08 Aug 2019
In reply to Martin W:

Check the actual bathroom zone where your light fittings are. Unless they are actually IN the bath tub itself or shower tray, they will be in zone 1 or 2. 

In zones 1 & 2, you can use standard 230v dimmable LED downlighters that are miniumum IPX4 OR SELV (Separated 12v). 

Assuming your lighting circuit is RCD protected, I'd buy 4 new sealed unit 230v IP65 LED dimmable downlighters and fit those instead. More reliable, no remote transformer and less likely to start a fire. Check compatibility of your dimmer with the LED datasheet. 

Post edited at 21:49
Martin W 09 Aug 2019
In reply to Steve Clark:

> Assuming your lighting circuit is RCD protected, I'd buy 4 new sealed unit 230v IP65 LED dimmable downlighters and fit those instead.

Hmm, those seem to run pretty pricey.  And am I right in thinking that, being sealed, if the bulb fails then you have to replace the whole thing?  I know that LED bulbs last a loooong time but still...

wintertree 09 Aug 2019
In reply to wintertree:

> if someone else fitted 4 halogen bulbs in the future, they could unknowingly overload the single transformer

This applies to the old transformers - as Hoo says, if you replace them with a modern LED power supply (not really a transformer despite the name), that should provide in-built overload protection.

Mike Stretford 09 Aug 2019
In reply to Martin W: My gf got some 240V dimmable LEDs from IKEA, I was surprised these were on the market but they seem to work ok. These were cheap as chips, if you pay more I would assume you would get something that lasts longer (the switch mode ciruit not the LEDs).

Steve Clark 11 Aug 2019
In reply to Martin W:

Depends what you consider expensive.

https://www.cef.co.uk/catalogue/products/4688226-5w-led-dimmable-mini-fire-rated-downlight-white-4000k

These are £10 and have a 25000h life.

If you use your bathroom for an hour a day, you’re good for the next sixty years.


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