I'm trying to explain some LED-lighting phenomena, that we (me and my family) have observed, while changing light bulbs into LEDs. I've had some courses in semiconductor physics, and I've had a basic course in electronics, but I still have some difficulties in explaining these phenomena.

In our living room, there are 4 light sources, each of which consists of 4 light bulbs. I assume all of those (16 in total) light bulbs are in parallel, because when one of them is broken, all the others (15 bulbs), still shine. We can turn those light bulbs on with a slider. So it is possible to let them shine on f.i. half the intensity.

We now tried to replace those bulbs with LEDs. The first thing we noticed (when replacing one bulb), is that the LEDs intensity can't be gradually changed by the slider. The LED is shining on full intensity or isn't shining at all. Furthermore there were no problems in changing 15 of those bulbs into LEDs, but when we changed the 16th bulb, for some reason all LEDs start to flicker. Furthermore if we just remove the last LED, there is no light at all. Finally if we replace the last LED again by the light bulb, we noticed that all LEDs are now shining (at full intensity), while the light bulb (which isn't broken) does only glow (and refuses to shine).

I hope someone can help us to explain these phenomena:

1) The intensity of the LEDs can't be gradually changed by the slider.

2) When changing all light bulbs into LEDs, all LEDs start to flicker. Nevertheless they work fine when changing only 15 or less bulbs.

3) When just removing the 16th bulb, there is no light at all.

4) When replacing last LED by a bulb, all LEDs are shining, while the bulb doesn't.

I have some guesses at how this results could be explained (for instance I think the LEDs flicker, because the offset voltage dropped to lower voltages, the LED will then get only in some part of the voltage period enough current to shine). But I have problems in seeing the bigger picture. I think my main problem is that I can't guess the electric circuit, which is the main reason I post this question here. Any attempt to solve these questions will be greatly appreciated!

  • \$\begingroup\$ I think the drive circuit inside those LED devices will be the culprit. Interesting problem anyway. \$\endgroup\$
    – Dzarda
    Feb 15, 2015 at 12:43
  • \$\begingroup\$ My guess is the dimmer is the problem. \$\endgroup\$
    – HKOB
    Feb 15, 2015 at 14:19

2 Answers 2


The new LED style bulbs have complex power driver electronics in their bases, the brightness is controlled mainly by this internal circuit not by the socket voltage. Putting these bulbs on a dimmer switch is like slowly reducing the supply voltage on your computer and expecting the display to dim.

Before buying an LED bulb you could check the specifications to see if it compatible with any type of dimmer switch. Most are not. Also since LED bulbs as so new (for residential lighting) there are not yet many standards followed for their operation. Some specialty LED bulbs are beginning to become available with dimming systems specially design for that type of bulb.

If you had used small fluorescent bulbs in this same setup you might also see a strange operating condition.

Standard incandescent bulbs work with simple voltage limiting or pulsed dimmer switch devices because the bulb is basically just a simple low value resistive pieces of wire. Some of these dimmer switches rely on this simple resistance and high current characteristic to create the dimming affect. So when using the newer LED or florescent bulbs the dimmer circuit no longer sees a high current or simple resistance and the dimmer operates strangely.


The problem with the last bulb is this; the dimmer switch itself needs to see a suitable load. i.e. it is happy working into a standard filament bulb. It's driving all your lights in parallel, so whilst there is one regular bulb in there the dimmer is still happy. At the point you replace that last bulb with an LED lamp, the dimmer now sees only LED electronics as its load. The dimmer will not function properly (due to the way it works, it won't fire correct) and the result is it sends occasional pulses to the lights in its confusion and you observe the flickering.

to fix your problem;
(a) replace the dimmer switch with a regular switch, then all 16 of your LEDs will work fine.
(b) go back to using regular bulbs
(c) spend even more money on dimmable LEDs and a install a suitable dimmer for them.


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