Looking to buy a wall mount swing arm lamp, and it has a 3 way switch in it with a 60 watt minimum rating to 150 watt maximum rating. I understand the reason for maximum rating but not minimum rating ? I want a lower low setting around 30 to 40 watt but mfg. said 60 minimum....Why
A standard 3-way bulb consists of two filaments (or light sources in non-incandescent bulbs). Low is when the dimmer of the two are on, medium is when the brighter of the two is on, and high is when both are on. So, you'll find that the high wattage is equal to the low plus the medium wattages.
Take a look at Amazon's stock of three-way bulbs. There are lots of 50-100-150 bulbs, plus smaller numbers of 50-200-250, 30-70-100, and 100-200-300 bulbs. In each case, high = low + medium, and generally the medium wattage is twice to four times the low.
If you want a very low low setting, then that has to be the same as the difference between the brightest and the next-to-brightest setting. So, let's say you want a 150W bulb whose lowest setting is 10W. To get that, the bulb will have a 10W filament and a 140W filament, giving you a 10-140-150 bulb. They don't make such bulbs, because the high and medium would be indistinguishable, and consumers would complain.