When is a Transformer NOT a Transformer?

[Brigham]

For many years, I've had a low-voltage transformer powering a 24v filament lamp in my bathroom, for the purpose of avoiding having a mains light-switch within reach of the bath.
The transformer spent the '60s under a plant pot next to my father's ornamental pond, where it powered a miniature fountain. The laminations had become rusty, and when the last of the 24v 40 watt lamps had been used-up, I decided to try something else instead.
I bought what was said to be a 'transformer' from the B&Q clearance bin, rated 220/230v to 11.5v AC. It was fine with a 12v BC filament lamp, a direct replacement for the 24v one used previously, but I was trying to cut down on the heat build-up in the fitting, and so I bought some 12v AC/DC LED bulbs, again BC, and again a 'direct replacement'.
...Except that they don't light!
I went through every part of the 12v circuit, and could light the filament lamp at every point. As a last resort, I plugged a 2-way BC splitter into the ceiling fitting, and inserted BOTH bulbs, the incandescent and the LED.
BOTH of them lit. Remove the filament bulb, and the LED lamp goes out.
So, when is a transformer NOT a transformer?
Presumably when it's an LP by Lou Reed!

[FIXITNOW]

some info here
http://www.ledbenchmark.com/faq/Tran...atibility.html

and here
http://www.ledbenchmark.com/faq/Tran...for-MR16s.html

[turretslug]

Does the (presumably electronic HF) transformer have a minimum current/power rating on the spec. plate somewhere? LED units are so much more efficient than their predecessors that it can be a bit of a surprise as to just how little they take.

[AC/HL]

I've just scrapped some. Minimum load is 20 watts.

[emeritus]

I think there was a post a year or so back with a similar problem. The poster, who had replaced tungsten kitchen downlighters with leds, had the same problem, solved by retaining one tungsten lamp.

You can get a similar problem with older lamp dimmers, which can produce annoying flicker with tungsten bulbs of less than 40W, and kill non-dimmable leds after less than 20 mins operation (at least, that was my experience with dimmer-fitted bedside lamps at Warner's last year: - 4 bulbs in 4 days, after which the maintenance man managed to find me a dimmable one!).

[broadgage]

Most modern "transformers" are in fact switched mode power supplies with a very high frequency output.
These have lower losses than a traditional transformer, are smaller in bulk and lower in cost.
They work fine for halogen or other incandescent lamps that are close to the transformer.
They often fail to work LED lamps, for two reasons. Firstly as has already been said the transformer often has a minimum load, which may be greater than the demand of an LED lamp.
Secondly, these lamps use DC internally and the very high frequency may be too great for the cheap rectifier diodes in the lamp to handle. Some LED lamps have a very tiny choke coil in series with the input, this has negligible impedance at 50 cycles, but is almost an open circuit at the lower RF frequencies used by electronic transformers.

The makers of these transformers often specify a maximum distance between lamp and transformer, often one metre or less.
Exceeding this distance can result in excessive RFI as the longer connections radiate better. It can also result in excessive voltage drop due to the inductance of longer wires, or overloading due to capacitance between conductors.

I would advise against use of these electronic transformers for any purpose other than the intended use of an incandescent lamp located near the transformer.

For alternative applications, consider either a traditional transformer with a line frequency output, or a switched mode power supply with a DC output.
For a bathroom light, I would use a SMPS with a 12 volt or 24 volt DC output.
12 volt and 24 volt LED bulbs with a B22 base are readily available.

[Brigham]

Thanks for the info.,
I've tried a 12v 21w car bulb across the output with no success, but a pair of them in parallel does the job.
I don't want a number of auto bulbs burning above the bathroom ceiling, so I've decided to abandon the 'device', and use a 240v/12v transformer instead.
A suitable one badged 'Hornby' has been obtained!

[duncanlowe]

These electronic switching supplies can have some confusing characteristics. Transformers they are not.

Some years ago (probaly more than 15 years) I wanted to put low voltage lights in the bathroom and the downstairs loo. Both systems were 6x35W halogen bulbs, the only difference being the diameter of the halogen lamps. Identical 'transformers' same brand of lamp, all bought in the same order from Screwfix.

Bathroom lamps all worked fine. Fitted the downstairs loo, and when switched on the lamps would flash briefly and go off, then flash for a bit longer and go off. This was repeated a few times until they came on normally. Bit of a pain waiting for 30 seconds until the lights came on normally. I assumed a faulty 'transformer' and got a replacement from Screwfix. The replacement behaved just the same.

So I was confused. Tried putting all the lamps from the bathroom downstairs, same result. The same lamps with the same model of 'transformer' worked in one location, not the other. I didn't believe I could have had two faulty and one good device, though of course it is possible.

So I had a think. The behaviour was a bit like an overcurrent trip. Each time it happened, the lamps was a bit warmer so the current surge was a bit lower so it stayed on a bit longer until it was warm enough to not trigger the overcurrent. So I pulled one of the six lamps, and guess what, all fine.

Now the lamp load was well within spec for the 'transformer' and even more confusing, was fine in another location with the same load. What's the difference? Well the downstairs loo is a smaller room than the bathroom, so there's less wire. I'd ended up getting the wiring from the 'transformer' to the lamps low resistance enough that it tripped the overcurrent. Simply by adding a couple of metres of wire between the 'transformer' and the first lamp, the problem went away.

Incidentally, the bathroom has since moved to LEDs, and the particular 'transformer' we have has had no issues driving the lamps. Not worth changing the downstairs loo for the very short duration of operation. So short that only one halogen has failed in the 15 (+) years they have been fitted.

[whyperion]

thanks duncan for that. Mums block of flats has a number of external lighting circuits for lights outside roughly each flat door and stairwell, as does the same build opposite. I noticed in last nights gloom (the lights used to be bulkhead lights mounted upside down under the soffits effectively with normal incandescent 60w bulbs screwed in - the are now CFL as in the U shaped plug in illuminarie Philips/GEC type ones ), while one half block the 15 lights all switch on in a rough instant go, but each with a small fluorescent flicker , the opposite block with 15 on the circuit they all cut out and re-fire about 5 times, the cut out is quite noisy under the stairs for one of her neighbours and no-one has identified a clear reason, or more importantly, solution. Quite why CFLs should heat up or retain the circuit only after a number of goes I dont really understand. Mum's block side flickers too, but without the cut-out noise, there are more lights overall but i think they must be on two circuits albeit on one time / darkness sensor.