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Although exhibiting better dimming performance than CFLs, commercially available LED lightbulbs seem to exhibit poor linearity compared with incandescents (especially in the bottom 20% of the dimming range).

On the workbench, I've seen that it's possible to dim LEDs to very low effective outputs using pulse width modulation. It's also reasonably straightforward to envision using comparators and zero-crossing detectors (or something more complex, like a microcontroller with an A/D converter) to analyze the AC input coming from a triac/thyristor, and generate an appropriate pulse train.

So what is the big challenge here? Why does it seem that all commercially available dimmable LED bulbs have poor linearity (especially at the low end) compared to incandescents? What am I missing here? Is it the voltage drop across the LEDs that's "offsetting" the linearity?

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The light output of an incandescent bulb depends upon its temperature (black body radiation). This is not the case with LEDs that emit photons of a particular energy - more photons more light.

Depending on the method used to control the light output you will get different results.

PWM generally gives better control and works well with LEDs. Some controllers may simply be limiting current by controlling the series resistance or the voltage supplied.

PWM with incandescent bulbs works well up to the point so little power is being fed into the bulb that it fails to achieve sufficient temperature to radiate light - most of the energy is now infra red.

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  • \$\begingroup\$ so in the case of LED , it requires to control the current instead of voltage. For the VI characteristic of the forward bias. It's difficult because it's exponential curve. It's linear to the current , but almost exponential to the forward bias voltage. I think that should be edited and there if this fact is right. \$\endgroup\$ – Standard Sandun Jun 25 '13 at 17:29
  • \$\begingroup\$ @sandundhammika I was referring to the type of controller in my answer not the LED i.e. the entire circuit, so my facts are correct. A simple 'controller' may consist of a variable resistor and fixed resistor from a fixed voltage power supply or it may be a fixed resistor from a variable voltage source. A more sophisticated controller may consist of a variable 'constant current' element. Whichever way the analogue type controller works it essentially changes the current through the LED between some max and min limit. \$\endgroup\$ – JIm Dearden Jun 25 '13 at 17:53
  • \$\begingroup\$ Your facts are right, that's why I've +1ed. But it lacks the important point Why it linear to current and exponential to Voltage \$\endgroup\$ – Standard Sandun Jun 25 '13 at 18:33
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LEDs need low voltage DC constant current supply but in the grid you have 110/230V AC voltage with phase angle control dimming signal. You need to convert this AC signal to DC as cheap as possible, possibly using a single transistor. Because of this, the current supplying LEDs is very often pulsating. I haven't seen a cheap LED driver that would convert the AC voltage into DC current and control the brightness with high frequency PWM signal controlled by the AC phase angle. Dimming linearity is one of this things that can easily be sacrificed in favor of cost/size/lifetime of the LED driver.

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It is a complex problem. AC dimmers work by cutting out part of the AC cycle, so (for example) a lamp at 50% brightness has full voltage (ie the usual AC waveform) for half of each cycle, and no voltage for the other 50%.

To use this to drive a LED requires a circuit that will sample the incoming AC waveform, sample it to determine the dimming level, and then use that dimming level to drive a PWM signal that drives the LED.

Maxim makes a the MAX 16841 to do exactly this. It is supposedly inexpensive (

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