Lighting 12V AC drivers weird behaviour.

[paulca]

I bought OSRAM smart bulbs for my kitchen downlighters.  The are GU4.5 / MR16 12V fittings.  12VAC lighting with transformers used to be all the rage, they are discouraged these days for many reasons.

I took my working IKEA 5W LED bulbs out and put my new OSRAM Smart+ in.  Turned them on and the new lamp blinks once and then nothing.  Tried 2 different bulbs in two different sockets.  Same.

Took one of the bulbs to the lab and hooked it up to 12VDC.  Worked perfectly.

Took the meter down to the kitchen lights and got a reading of 3.8VAC 0VDC.  50Hz. 

I phoned my electrician.  He's out with me now, talked me into switching to 240VAC GU10s....  for many reasons.  So he is off buying modules to convert them.

I am still puzzled as to how there is only 3.8VAC.  I thought it was my measuring as it was really fiddly to get probes on the tiny pins and I ended up using some pin headers on croc-clics....  janky AF.  So while the spark had them out of the cieling and the "12V" output exposed, he put the meter on a different one again and it read 6.2VAC.

I thought there was a single transformer for all 5 lights, however there is a driver per lamp. 

How can they ALL be putting out less than half the spec voltage?  The LED lights work. 

Is it likely that using 5W LED bulbs with transformers designed for 50W halogens has broken them? 

I don't know how old these transformers/drivers are, but they do indeed say 11.5V AC 50Hz 20-65VA.

It's that "20-65VA".  Is the minimum important?  If I am not pulling 20VA through them are they likely to not produce full voltage?

Hopefully this evening I shall have 5 of them to play with and find out.

[tom66]

Electronic transfomer would be my guess. 

They are rubbish with LED lamps.  Sometimes they work but if the LED does not draw enough current they will often shut off as they are a self-oscillating type converter which requires load to continue switching.  Some LED bulbs work better with them than others, but often you get flickering lights, buzzing noise from the transformer, and unstable start up.   Due to their output waveform - something like 50kHz AC modulated at around 100Hz - they tend to be pretty brutal to any power supply that is not designed to accommodate that operation.

I second your electrician's recommendation to move to GU10 240V bulbs.  I have a couple of GU10 smart bulbs and they work fine (Zigbee type). 

I have a large wardrobe/cabinet in the dining room which came with halogen bulbs.  I wanted to replace these with LEDs for efficiency but found that the integrated electronic transformer - which for reasons that escape me is not replaceable without tearing the unit apart - did not like LED bulbs. They would flash on and off randomly.  I found the solution to this was to build a small PCB with a 5W resistor on it and a relay.  Once the rectified output from the LED transformer is sufficient the resistor gets switched out of the circuit.  This works very well, the electronic transformer is fine once it has started, it just needs that kick in load to get it started.  However in your situation since you can replace the transfomers I would just do that.

[paulca]

Well, they are all replaced now with GU10s.  £85 which I upped to £100 to keep him sweet.

Now I have 5 OSRAM Smart+ bulbs that cost me £65 to figure out how to return or resell.  I could power them in the office with a 12VDC supply, but I really don't need 5xSpot bulbs in the office.

[Zero999]

There are LEDs which are specifically designed to be compatible with electronic transformers.

Here's how it's done.
https://pdfserv.maximintegrated.com/en/an/AN5372.pdf
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjxi9bg4N-EAxXsRkEAHU65BBoQFnoECBEQAQ&url=https%3A%2F%2Fe2echina.ti.com%2Fcfs-filesystemfile%2F__key%2FCommunityServer-Discussions-Components-Files%2F24%2F3058.Transformer-application.pdf&usg=AOvVaw0PKfbIusOR9GVeZSp3RHvW&opi=89978449

Around 10 years ago, I replaced some 12V halogens, run off 12V electronic transformers, with LEDs, in my parent's bathroom and they're still going strong today.