Hi, I replaced two 12V 20W halogen bulbs (wired in parallel) by two 3W 12V LEDs also in parallel. The problem is that the transformer (in a kitchen fume hood) seems to shut down when both LEDs are connect. It can take one but the two cause problems. I guess this might be caused by the very low resistances of the the 2 LEDs connected in parallel that short circuit the transformer? How can I reduce this short circuit without causing significant voltage drop for the LEDs? I tried using them in series but they won't work because they will then each have half the required voltage. Of course the idea is o find a solution that will also not significantly interfere with the energy savings associated with the change of halogen by LED. Btw, the LEDs are MR16 and have their own little voltage regulator/stabilizer circuit (or whatever that is).Thanks!
What does 12 volt LED mean? LEDs operate at voltages around 2.8 to 4 volts, not 12 volts. If it means the LEDs have a resistor built in, so they operate from 12 volts, then you should be fine, and the transformer should not shut down, as the current would be a low lower than the halogen lamps drew. So I suspect these are NOT 12 volt LEDs, and they need a series resistor which is missing, so the current is much higher than it should be. Probably means the LEDs are cooked already. Another possibility is that the 12 volts for the halogens is AC, not DC, and the LEDs require DC. So you need to look at the specs on the LEDs, and also check the 12 volt supply. I suspect it is AC. .
Low Voltage transformers (ELV or MLV) are tricky. Just because you have the voltages the same doesn't mean you have the right current being drawn by the LED's.It could be too low or too high. Were the original halogens MR16 form factor as well? I'm guessing the xfmr was specifically designed for a set voltage/current/power. Consider this: Two 20W halogens 40W. PowerVoltage*Current 2012*I I 1.67 amps through each bulb, 3.33 amps total drawn from the xfmr Transformers operate on the principle of Power in Power Out. The xfmr is probably designed to provide 40W, regardless of whatever current draw you put on the transformer. The LED's only need 6W from the xfmr. However, the xfmr probably doesn't care. It's still spitting out 40W at 3.33A. 0.5A (2*3W/12V) is all the LED's need. If the xfmr is still trying to provide 40W at 0.5 amps, the output voltage is compensated up to 80V coming from the xfmr. Most LED bulbs have protection for this in the engine, (the regulator/stabilizer as you call it), called over-voltage protection. This is probably whats causing the bulbs/transformer to fail. The problem is, especially with transformers, is that you must match the load. If the transformer is expecting a specific load and you give it something it can't handle, it'll either destroy the load or damage the transformer. The easiest way to avoid this is to always match the power of the original load to the load you intend to put in. 40W of halogens should be matched with 40W of LED. This is the gimmick with retrofit bulbs like the MR-16. You're not really saving any energy when you use retrofit bulbs. You just get a longer bulb life and a lot more light from the same power draw. LED lighting is a niche market. If you can't figure it out, wait a few months and LED lighting fixtures will decrease as the market is flooded with new product and becomes more competitive. I'd recommend investing in something like that instead of retrofitting.
