We have a Vent-a-hood over our stove with halogen lights. There's a switch for low level and high level lighting using the same two light bulbs. Would like to replace with LED GU10. Our first attempt to use a 'dimmable' bulb resulted in great light at the high level and a flicker at the reduced level. Worth trying another LED and/oror are there specific two-way LED bulbs to look for? Thanks!

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    I had that problem with a range hood, and found that it just had a diode in series with one of the switch outputs (the diode was soldered to the switch and covered by heatshrink tubing), I took out the diode and connected the "dim" wire directly to the switch, so now instead of "dim-off-bright", the switch is "bright-off-bright", which is fine because I never really found a use for the dim setting, I just didn't want to potentially damage the LED by someone running it on "dim". – Johnny Nov 17 '15 at 19:51

The high and low settings of the lights on your range hood are most likely not controlled by standard semiconductor triac dimming technology. Such technology is typically used where continuously variable brightness control is desired.

In the case of your range hood the high setting applies the full AC waveform of the mains voltage across the light bulb. For the low setting the light bulb is typically placed in series with a diode that cuts out half of the AC voltage waveform thus operating the light bulb at half the typical average voltage.

The graphs below show the difference of the AC voltage waveforms for the two cases.

enter image description here

The high setting allows the bulbs to be energized for the AC line voltage being first positive and then negative during its 60Hz cycle (16.66 msec total per cycle). The low setting with the diode only allows one polarity of the AC voltage to be applied across the bulbs.

(Note that in the above pictures the AC waveforms show that the voltage attained by the sine wave at its peak is about 170 volts. The typical 120VAC we are familiar with in our homes is an average value that would deliver an equivalent amount of total energy as the sine wave if the voltage waveform was a flat line at 120 volts.)

Typical incandescent light bulbs that give off light by heating a filament to white hot temperatures have a thermal response time that is much longer than the 16.66msec period of the AC waveform. This averages out the light emission of the light bulb to the extent that we do not see flickering when viewing the light bulb.

LED lights on the other hand are a semiconductor component and are able to respond to changes in applied voltage much more quickly. So quickly in fact that most LED bulbs that are designed to run right from the mains power line have to have internal circuitry that converts the AC line voltage into a DC voltage to operate the LED. The reason that you will see flickering with the LED bulbs on the low setting is that the AC to DC converter circuit built into the LED bulb is not designed to handle the case where the AC voltage disappears for half of each cycle and is thus turning on and off (at least partially) at the AC line frequency rate.

Could LED bulbs be designed to work without flicker at a "low" setting like this?

The answer is that yes they could be and such LED bulbs may very well be available. However such circuits cost a bit more to build and may take more space in the base of the bulb.

  • So is the answer not, "Get a dimmable LED bulb" ? Great description of the problem. – JPhi1618 Nov 17 '15 at 15:00
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    @JPhi1618 - Dimmable LED's generally expect a portion of the waveform to be cut off -- either the leading or trailing edge, but on both the positive and negative halves. You might get lucky and find an LED that will still work with half the waveform cut off, but it'd take trial and error and even if it appears to work, it could stress the circuitry and cause premature failure. – Johnny Nov 17 '15 at 19:45

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