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I've got a string of LV lights in my kitchen that over the last few weeks have started to behave oddly. They will turn on and the one farthest from the power supply are dimmer than the ones closest (there are 5 12-volt lights in the series and they stretch over about 10 feet all told). Then after about a minute they start to flash, and the period of the flashing gradually increases.
Any idea what might be going on here?
It is a common problem with Triacs on LV loads. I am assuming you have a 12Vdc wall convertor on perhaps 1-2A for white LEDs. The Triac conduction can fail to have the necessary holding current at low levels. The load is insufficient and nonlinear in that it does not present any load below 9V or a certain phase angle on the triac. The threshold is thermally sensitive so the Triac triggers a flash then skips a few cycles until it cools which then repeats like a porpoise or motor-boat bouncing in waves. The thermal resistance and dynamic impedance of the triac with a small nonlinear load causes these time constants and frequencies with the LED load.
It is a complex stability problem from insufficient load on the Triac dimmer. Adding one small normal bulb would fix that. Perhaps only 8W or 25W.
Better dimmer designs, can also prevent this with trailing edge phase control instead of leading edge or smart current sensing with better triggering.
EE since 1975.
Your transformer will have a minimum load of say 20 watts, your bulb will be say 5 watts. Hence the transformer flash's as it constantly restarts looking for the load. Some low wattage LED bulbs are designed to bluff the transformer into thinking they are say 25 watts when they are only 5 watts. I think Phillips make such bulbs.
I see that this question is still getting some attention after all this time so I thought I'd come back and share what I found to be the problem.
The question itself was predicated on my assumption that since I had recently replaced the transformer it was unlikely to be a transformer issue. As it turns out that was a poor assumption.
After some time we ended up replacing this set of lights with another of a similar kind. As I was doing to work to replace them I noticed that I'd never trimmed the excess LV wire from the end of the run (these are the type with bare braided wire that carries the LV current as well as supports the fixtures) and at some point, probably when someone was dusting up there, the 2 sides of the wire got stacked on top of each other creating a big old short. I suspect at this point that that short burned out the transformer much more quickly then I considered likely and the now-faulty transformer caused the odd behavior.
In any case the replacement system (with its nicely trimmed LV conductors) has been working flawlessly for quite a while now.
