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MR16 LED Spots flickering and flashing?

Customer Question

I wonder if you can help. A few of my Energy Saving LED bulbs (5W MR16, mixture of natural and warm white) start flashing or flickering after they’ve been on for a while – perhaps 20 or 30 minutes.

I’ve tried replacing the electronic transformer on one of the fixtures with an LED driver rated for up to 12W, but this made the flashing even worse.

In an attempt to figure out whether it’s caused by the bulb or the fixture I swapped a bulb that flashes and one that doesn’t. The same bulb starts flashing even when it’s in a different fixture.

Could it be that these few bulbs are faulty? The others are perfect so if it were possible to replace them with working ones that would be great.


Technical Tim

This is most unusual and we are perplexed. Whilst it could be the transformer or transformer/LED combination we doubt it as you have carried out some excellent diagnostic work and the issue appears to be time related due to the delay before the units misbehave. If you have has some photographs of the transformers used (need to read the writing on the units) it would be appreciated.

Generally transformers intended for use with halogen lamps don’t work correctly with LED lamps due to the very low power requirement of the LED lamps. However this usually shows up immediately and not after a delay period. Again there are differences in LED transformers and we doubt the transformer is the cause of the issue, a photo would be a lot of help.

Sorry for the inconvenience.



That’s excellent, thank you.

You’re correct ref your other email:

5w Spot – MR16, Natural White x 5
5w Spot – MR16, Warm White x 8

I’ve attached a few photos showing the transformers. There are two slightly different types of fixture so I’ve included photos of both, although the problem doesn’t seem to be isolated to a specific type of fixture / transformer.

I’ve also included a shot of the DC LED driver I tried swapping in, in case that’s useful.

Hope this helps.


Technical Tim

Many thanks for the time, effort and input from your end regarding this issue. The photos have been most helpful and have probably revealed the source of the problem, let me explain further.

Referring to the photographs of your two transformers you will notice that the specification for the transformers is 20 – 60w. This would tend to indicate that the transformers are/were intended for tungsten halogen lamps and would cater for 20w, 35w and 50w versions. The minimum load that the transformer would work with effectively is around 20w and relies on the metal filament of the lamp to smooth out inconsistencies in the generated voltage; won’t go into technical details here.

The LED units require only 5w which is far below the minimum 20w the transformer is happy supplying, any less than 20w and the transformer generally doesn’t work correctly, assuming the lamp has failed. Usually operating a transformer of this type with a low load as provided by a 5w LED with no metal filament causes the transformer to “complain”, the usual symptoms we’ve come across are constant flashing on and off or just one major flash at switch on; there are others. However as there are more different transformers out there than there are seeds in a pomegranate all is not so usual and we’ll look into this further once we have the LED units back.

We suspect that the combination of transformers and lamps has worked for some strange reason but outside normal operating conditions and that some damage may possibly have occurred of an unknown nature, most probably to the LED lamps which I understand are being totally replaced. I would most strongly suggest that the transformers are replaced with LED compatible types to prevent further failure and inconvenience to yourself.

However before you do this I hear you say “ What about the one specific, special LED transformer? There is no difference in effect”. Could I ask you to confirm that the LED transformer you have works correctly all the time with a ”good” LED lamp and confirm what it does with a “suspect” LED lamp? Does the suspect lamp flash immediately or after a period of 20 – 30mins? This latter case would tend to confirm that the lamp(s) are damaged somehow.

Look forward to your comments on this matter.

If you require any further information or assistance please do not hesitate to contact me.



Thanks for your comments and for taking the time to investigate so thoroughly. That all makes perfect sense.

I’ve just tried reinstalling the LED driver in one of the fixtures as requested. All four of the “good” lamps I tried (i.e. I’ve never seen them flash before) started flashing rapidly as soon as they were switched on, exactly the same as the known “bad” lamp. Perhaps this indicates that they’re all damaged to some extent, or maybe the LED driver itself is faulty – perhaps caused by being connected to the original “bad” lamp?

I don’t have another LED driver to hand, but could order one. However I do have plenty of regulated 12V DC bench power supplies – would you like me to try some experiments with “good” and “bad” lamps using one of those? Fully understand if you’d rather I don’t.


Technical Tim

Thanks for the info, seems things aren’t going in the right direction, let me elaborate.

1. The 20-60w transformers you have should NOT work with the low power LEDs. They should either flash on or off or just flash once at power on. They should not continuously illuminate the LED due to the minimum power requirement. We live and learn though.
2. The LED specific transformer you have should work correctly and not flash at all.

Seems that life isn’t performing as it should though.

We’re erring on the side of caution with regards the LEDs being “damaged” by the transformers intended for halogen lamps, we’ve never had it occur previously but we should factor the possibility that it might have caused issues, hence the replacement. My concern is that there is something more amiss and the replacements will perform just as the units you already have.

Could I as you try a little experiment “Blue Peter” style as shown in the attached, you won’t need the radio. You’ll need a PP3 battery and connect the LED(s) as shown, do they flash or illuminate normally? I know it is only 9v not the 12 volts that the LED needs but you will find that the lamps do work as shown.

Sorry you are having this problem with what should be relatively straight forward.

Let me know the outcome if you would.



Thanks for this. I’ve tried connecting one of the bulbs that’s known to flash on the LED transformer to a 9v battery – it worked fine with no flashing.

It took ages to find the right model of Sony radio and an Audrey Hepburn poster 🙂

Would it be useful for me to send you the LED transformer along with the bulbs so you can take a closer look? Of course it’s possible that it’s simply faulty as I’ve never seen it work correctly.


Technical Tim

Thanks for your feedback, doesn’t actually prove 100% but certainly heading in the right direction.

Here is an update with some additional tests we have run with more electronic transformers received today; our original tests were based on transformers from a few years back.

We’ve run a series of tests with different LEDs and transformers, some electronic transformers work with 5/6w LEDs even though the minimum load is stated as 20w; we’ll run them for extended periods to determine the long term effect. A slight flicker from the LED may be noticeable with some transformers due to the “incompatibility”. Please note we refer to flashing as an on/off situation where as flicker is a slight rhythmical change in the level of brightness.

Some transformers won’t work correctly at all with LED lamps.

We suspect that the transformers you have, two different types I believe, will perform differently. The flashing (or is it flickering?) will most likely be some imbalance/incompatibility between the LED and the transformer load showing up once either/or or possibly both the transformer and the LED have warmed up. We haven’t come across this before but again theoretically the 20-60w transformers shouldn’t work at all although we suspect if they do they are not working correctly hence the issues arising.

The LED specific transformer you have which flashes is most unusual and we suspect something is amiss with it. You certainly can send it over to us if you would and we can let you know our findings but effectively the 12v dc LED transformer should work exactly in the same fashion as the 9v battery test. It may be the transformer is defective but that should be easy to confirm.

We can certainly recommend http://www.myswitchshop.com/index.php?main_page=index&cPath=44_50 transformers. The 50w version should power up to 4 LEDs although this would obviously require additional low voltage wiring run from the transformer to the lamps. Mention us if you propose purchase.

Needless to say please ensure that the circuits you are working on are electrically isolated and don’t rely on the switch to isolate the circuit, it may be wired in the Neutral return rather the Line Feed.

If you need further assistance or information please don’t hesitate to contact me.

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