Electrical – Why would an LED stop flickering when connected in series with an incandescent

Dimmers have to be compatible with the types of lights in use. The bog standard dimmers that have been in use for years generally work fine for incandescent type bulbs and the halogens that work at direct line voltage.

With the advent of CFLs (compact florescent lamps), low voltage halogens and LED types it talks special types of dimmers for these to be used in a variable brightness mode. Do note that not all items are even dimmable and you can get into serious overheating issues in either the dimmer switch or in the light module if you try to operate with a dimmer. Similar issues can happen when a non-compatible type of dimmer is in use.

My suggestion for you, if you need to keep the mix of light source types, is to convert over to simply using a switched on-off control instead of dimming. If dimming is desired then switch all of the light sources on the circuit to the same type and then select a dimmer that is compatible with that type of lamp. Modern dimmers should show on their box what types of lamps they should work with. In the realm of the light sources themselves you may need to do some additional research to determine if a particular low voltage halogen, CFL or LED lamp assembly is actually dimmable. It may come down to specifically only purchasing items that are marked as dimmable on the box or product information sheet.

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.

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.