Does a dimmable 6W LED lamp, dimmed to 50%, draw c. 3W? Or does it still draw the full 6W?

  • \$\begingroup\$ It depends on how your dimming it and if your dimming it by PWM, voltage control or current control. \$\endgroup\$ – Voltage Spike Oct 8 '18 at 19:33

It depends on the LED driver, but unless it's of a bad (or very specific need) design, a dimmed LED lamp will draw less power.

Usually, dimming on LED lamps is done by PWM (Pulse Width Modulation). The LED will actually turn on and off faster than the eye can see. By modulating the ratio of the time the led is on and off, it simulates the dimming to the human eye.

This method is widely used because it is the cheapest and easiest method to dim an led but it can also cause some eye fatigue.

With this method the power will reduce with the light intensity (or perceived intensity).

Another method of controlling the light intensity of an LED is to control the current flow through the LED, and there are mostly 2 ways of doing it:

  • Through DC/DC conversion like a current controlled buck converter or similar circuit.

in this case the current will reduce with the intensity of the LED. With this method, there will be little flicker, which will be roughly equivalent to the DC/DC converter ripple.

  • Using an analog ballast (adjustable current source), like a transistor, mosfet or specialized IC with required regulation.

In this situation, the current will diminish but this circuit will likely have more losses.

The latest has the advantage of providing a very stable light intensity (if needed for some specific application perhaps like photography), so we can imagine this exists on the market.

The power will be reduced when the light intensity is reduced, but it's not a linear factor. F.I. 50% dimming might not have a 50% decrease on the power consumption.

The reason is that the control electronic will have some non-linearity and won't have the same efficiency at different operating condition and the second reason is that the eye intensity response is not linear to the actual light.

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  • 1
    \$\begingroup\$ In this situation, the power that is not used for the led is dissipated by the ballast that controls the current, in that case, the power input will be the same even when dimmed. No, the power input will be less because the current through the LED is less (unless the dimming is done by shunting the LED, which would be a strange way of doing it unless there was some very good reason to). As you say though, the relationship between power and brightness won't necessarily be linear. \$\endgroup\$ – nekomatic Oct 8 '18 at 10:26
  • \$\begingroup\$ You are right, thanks for pointing it out @nekomatic \$\endgroup\$ – Damien Oct 8 '18 at 11:19
  • \$\begingroup\$ but it can also cause some eye fatigue was true for older dimming bulbs. These would run at twice mains frequency, so 100Hz or 120Hz depending on country. For LEDs which use regulated DC though, they can run PWM at any frequency, and this will be much faster than any eye can perceive. So not an issue these days. \$\endgroup\$ – Graham Oct 8 '18 at 16:02

In general yes, the LED itself will use less power when "dimmed". But we have to be a bit more specific what "dimmed" means.

So the answer depends a bit on how the dimming is done. If a pulse width modulation is used to control the LED's brightness, then the dissipated power scales directly with the pulswidth. This means full brightness is reached with max pulswidth what equals max power dissipation whereas as a reduced brightness requires less pulswidth and therefore dissipates less power.

Back in the days when incandescent light bulbs where used, dimming was done by so called phase angle control, comparable to something like AC pulse width modulation. And before that, you would just add a potentiometer as a pre-resistor. In that case, the bulb itself would dissipate less power when dimmed, and the resistor would burn the rest.

But long story short, when you use PWM, the power will scale with the pulse width. But consider that brightness doesn't scale linearly with dissipated power.

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I will use about half (except for the controller using a bit).

Probably PWM is used, this means a duty cycle of 50%, means the light is on only half of the time, many times per second though.

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