My stove's vent hood has a pair of 120VAC 50W MR16s with GU10 bases, e.g. these halogen bulbs. These are some of the last incandescent bulbs in my house, and I'd like to replace them with something that uses less power and lasts longer, e.g. these LED bulbs.

The curveball: the vent hood's light switch has three positions, "Off", "Bright" and "Dim". I'm 99% sure that the "Dim" position just puts a diode in series with the bulbs, so that you get half-wave rectified AC. The LED bulbs I'm interested in are "dimmable", but I'm sure the expected application is with the usual triac dimmer that gradually blocks parts of each half-pulse, rather than completely discarding half of the pulses. (BTW, we usually use the "Dim" setting, as it's plenty of light for us.)

How likely is it that a "dimmable" 120VAC LED bulb would function (hopefully at half brightness) when supplied with half-wave rectified power?


closed as off-topic by Nick Alexeev Dec 20 '15 at 3:00

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  • \$\begingroup\$ It's not certain but it's very likely that it will not work. If the bulb works on lower voltages it will try to struggle into life on each full amplitude half cycle. It may have a "hill & valley" circuit to extend the internal voltage across the low voltage in the zero crossing area. So it may come up to full brightness at the start of the half cycle, continue on all the way to end of half cycle using internal store and then fade away sometime in the unsupplied off cycle. Depending on how much energy it stores in the internal cap this means it will probably make at least half power .... \$\endgroup\$ – Russell McMahon Dec 20 '15 at 7:31
  • \$\begingroup\$ .... in the half cycle and some more in the off 1/2 cycle. Depending on the design it may not like the constant "hammering" of the 1/2 cycles. And say it makes 60% of light overall - there will be a largish dead portion at 50 Hz so you will get nasty 50 Hz flicker. AND as eye response to light is logarithmic actual light level perceived will be close to original. So the two positions are probably full-steady and almost-full-flickering. | IF one only LED is OK in dim mode, better is to disable one only LED on dim settings. This is possibly able to be done with the switch alone .... \$\endgroup\$ – Russell McMahon Dec 20 '15 at 7:36
  • \$\begingroup\$ .... and certainly with a relay or SSR and maybe a few passives. | \$\endgroup\$ – Russell McMahon Dec 20 '15 at 7:36
  • \$\begingroup\$ Daniel - This is a design question BUT needs a little rewording. The question is ~~= -> Given xxx conditions switch that does this etc -> How do I design a circuit that will allow LEDs to operate at x% of power in one state. Do that and ask for question to be reopened. | A phase control dimmer circuit could easily be mde with minimal components with setting controlled by switch. Giving details of what switch did exactly will help. eg SPDT (probably) but may be progressive shoerting eg looks like 2 x progressive spst etc. \$\endgroup\$ – Russell McMahon Dec 20 '15 at 7:39
  • \$\begingroup\$ Daniel: (1) What is the exact functionality of the actual switch by itself (SPDT, ...?) ? - this should be easy to determine with an Ohmmeter - the question is NOT "what connects to the switch?" or "how is it used?" BUT "If I consider terminal A B C how many modes are there and what are the connections between terminals in each mode. eg a SPDT would have two mode with AB and BC connected in each mode and B is common. A SPDT with centre off may give connections AB/-/BC. | A switch with a progressive sliding short and 3 positions may give -/AB/ABC etc || Aim is to know what is available. .... \$\endgroup\$ – Russell McMahon Dec 20 '15 at 8:01

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