There should be no problem with a magnetic transformer. AC Electronic transformers which were previously used with halogen MR 16 bulbs usually require a minimum of 15 or 20 watts to operate effectively. LED wattages are so small that they usually do not meet this minimum. A DC electronic transformer might be the answer or a very low output AC transformer.   For those who still have MR 16 12v halogens, you might consider switching to 120v MR 16 LED bulbs and bypassing the transformer altogether.