Conflicting advice on transformers for 5W MR16 LED bulbs

[MM]

Currently I have lots of 50W MR16 halogen downlighters, but since the latest exorbitant lecky price rise announcement I plan to replace these with LED equivalents.

Each halogen bulb at present is connected to its own transformer, which is in turn connected to the mains. The transformer is a JC4030

10-60VA electronic transformer Auto-Reset Dimmable (reading off an empty box) or equivalent. I've also used Knightsbridge IP20 Electronic Dimmable Transformer for 12V Low Voltage Halogen Lamps 20-60VA

Converning replacement with LED, one website states:

"A room has 4 downlights using 50w Halogen bulbs each has its own transformer all rated at 60VA. You could successfully replace the Halogens with the Crompton 5w LED MR16 which gives out light equivalent to the 50w Halogen bulb."

But on the Screwfix website, in answer to a customer's question: "Does this mean that I must remove the transformers from all of the fittings (21 of them throughout the house) currently in place with the

12v halogens?"

their response is:

"If you are going to fit MR 16 Led Lamps you will have to change all your transformers, the one you require would be either: Halolite Led constant current driver 1-9W (66599). Halolite Led constant voltage driver 1-16W (85503)."

So I'm confused. Can I leave all the existing transformers in place and simply swap out the halogen for the LED?

Thanks.

MM

[Harry Bloomfield]

MM formulated on Thursday :

A replacement 5w LED lamp would be below the 10-60va and even more so the 20-60va lower limit. Having said that, I suppose you could buy one and see how it goes, before spending more.

[Martin Brown]

Assuming that the MR16 pin compatible LEDs you buy are designed to work off a nominal 12v supply then it ought to work - although any in circuit dimmer may baulk at the load not being sufficient to work properly. You may need to connect 4 or 5 LED's in parallel across a single old transformer to meet the transformers minimum load expectations (likewise for any dimmer on the circuit).

The LEDs might also be subject to some over voltage if the tranformers minimum load is not met (won't harm the transformer but might well shorten the LED lifetime). I suspect this is the main failure mode of some cheap LED refits in hotel corridors.

What makes you think they know what they are talking about?

A constant voltage driver is potentially death for LEDs. They need to be operated in a current limited mode or they will die. They also don't like being fed AC if they have been designed for DC only operation.

Short answer is it depends. The downlighter market is now complete madness with 240v, 12v ac and dc devices on offer so you really need to read the spec sheet(s) for the specific choice of bulb or fixture carefully. I think that MR16 LED bulbs claim to be pin compatible replacements for 12v halogens should be OK but they will come in dimmable and non-dimmable versions (the latter should be a bit cheaper).

Some are designed to be pin compatible with existing installations and so will run off existing transformers others are intended for rip out and replace entirely. The latter tend to work better longer term since the thick insulation designed to protect a ceiling from the hot quartz halogen bulb tends to shorten the lifetime of LED based devices.

[Dave Plowman (News)]

The problem here is you're not using a traditional transformer. But an SMPS to do the same job. A traditional transformer will produce (roughly) the same voltage with any lower load than maximum. A SMPS will usually have both a minimum and maximum load. In your case 10 VA minimum, and a 5 watt LED should be approximately 5VA.

But try it and see. You may be lucky.

If the wiring is easy to get to, you could run two or more fittings off the one SMPS.

BTW,a 5 watt LED isn't going to give the same light as a 50w halogen. More like a 35w halogen.

You can get 50w equivalents, but they'll be 6.5w and cost about a tenner each.

[MM]

They are these:

They're selling LED bulbs and a potential customer asked a question.

Perhaps they just want to sell extra stuff whether it's needed or not.

Replacing just a bulb (halogen with LED) is relatively inexpensive, but if I have to replace all the transformers as well, then we're talking about a lot of money. There are 29 halogen downlighters in my house.

If I bought one pack of the five on offer from Screwfix (URL above) to try, would I

1. Damage the new LED bulb?
2. Damage the transformer?
3. Cause a potentially dangerous situation with the the rest of the house wiring?
4. Cause overheating in the ceiling space to occur? Fire risk?

If the LEDs didn't work, either I could just forget about it, or replace each downlighter with an LED and the proper LED driver (though God knows what that would be) as and when the next halogen bulb blows.

Mind you, I've lived here in this house from new (12 years) and most of the MR16 halogen bulbs are still the originals. They blow very infrequently, mostly in the kitchen where the downlighting is often on, especially in the winter months.

MM

[MM]

SMPS?

If anything, the current lighting is overbright anyway.

[ARW]

Why? Assuming you are using the voltage the manufacturer specifies.

[Brian Gaff]

The problem is that there seems to be leds with constant current circuits and some where its in the transformer psu and no doubt yet others that work on the full mains. I always smile at the way making things modern simply complicates things. Brian

[Brian Gaff]

And of course if there was a constant current circuit in the lamp it would not play nice with a dimmer. Brian

[Harry Bloomfield]

Current limiting should be within the LED lamp anyway.

[Harry Bloomfield]

MM brought next idea :

Switch Mode Power Supply.

Rather than a simple transformer, a more efficient method is to use a SMPS, which controls the voltage supplied at the output, by rapidly switching on and off, then filtering the output. Most easily they produce a fixed DC voltage.

[Martin Brown]

Bare LEDs are current driven devices. You can get away with driving them without current limiting for small power levels but when you want serious light output they have to be protected somehow.

Their semiconducting resistance decreases as they get hotter and on an insufficient heatsink they will fail by thermal runaway when driven at constant voltage if they ever get a bit too warm.

It really is very bad practice to drive them hard at constant voltage.

[ARW]

Would you use a constant current driver on the lamps the OP has suggested?

[Andrew Gabriel]

The LEDs will be too low power for the electronic transformers to stay on - typically they'll flash on and off in this situation.

I would suggest changing the fittings to GU50 (mains) ones which don't use transformers, and use GU50 LEDs. There's a much better range of GU50 mains LEDs available than MR16 12V LED replacements.

[ARW]

Are they what I call GU10's?

[PeterC]

Thank you! I've just been looking for GU50, having never seen that before. I should have read your post first.

[Andrew Gabriel]

Yes!

[Martin Brown]

I'd probably use whatever is already there but with 4 or 5 lamps connected to each transformer rather than a transformer per lamp. (making a note somewhere that going back to halogens isn't possible)

It is likely that the consumer use bulbs are intended to be pin compatible with 12v halogen bulbs but using a lot less power. They might well be OK on any sort of PSU that yields roughly 12v ac or dc provided that the minimum load requirements of the PSU are satisfied.

[Harry Bloomfield]

Andrew Gabriel explained on 17/03/2017 :

Aldi had a stock of GU10's in the last two weeks, 5w for around £9 per box of 10 and marked 240v.

[Dave Plowman (News)]

The snag is that with many a ceiling light, getting at the wiring to alter things may not be easy. A single SMPS per lamp is common so it can be replaced if needed by just removing the fitting. If the wiring was easy to get at, you'd use a SMPS for more than one lamp even with halogen.