How to dim an LED

Dimming in the past was often done by lowering voltage to the light source by which the light source would decrease in output. This works well using traditional light bulbs but does not function with LEDs. Although decreasing voltage can alter the light output levels of an LED it can also adversely affect color, efficiency and present other issues.

Now a days, LED circuits come in two variants:

  • Constant Voltage
  • Constant Current

Constant Current

Constant current means that you present the LED circuit with a variable voltage but limit the amount of amperage going to the LED. By varying this amount of current you can make the LED more dim or give more light. The LED circuit will automatically find the best voltage in the range combined with the given amperage to make it’s light.

In theory, this is the best way to drive an LED. It should always run optimally and thus give out the most light and the least heat possible even if conditions change such as temperature, etc..

An advantage of this method is that the LEDs don’t flicker, at all. If you think you are susceptible to this, this type of setup is for you! Personally I believe that with a high enough PWM frequency this advantage becomes a moot point because it’s impossible to see the flickering in person (or even with a camera) but that is my opinion.

Constant current has a big downside though, driving an LED with constant current scales very bad to multiple LEDs. Basically you will need a driver per LED. If you then want to dim that LED, you often get a very “cluncky” setup where you use have to use an AC TRIAC dimmer to send a dimming signal to the Constant Current LED driver which then translates that to constant current dimming. Some drivers can also use a seperate dimming signals.

I looked at Constant Current and building hardware for it, but because of the driver per LED you need, it quickly becomes expensive. Especially because to dim them you need a “dimmable driver” which in turn becomes more expensive again, so I looked further.

Constant Voltage (and PWM dimming)

Constant voltage means that voltage instead of the amperage the LED sees always stays constant. Max Amperage is often limited by resistors so that the LEDs can’t overdraw themselves. Most LED strips work this way.

As I mentioned above, although lowering voltage can lower the light output this comes with undesired effects. For this they invented something called PWM dimming. PWM stands for “Pulse Width Modulation” and it basically means that the LEDs are switched on and off in a rapid pace.

PWM has a frequency and a duty cycle. The Frequency determines how many times the LED is pulsed a second and the duty cycle determines how long this pulse will be. If say the duty cycle is 20% and the frequency is 1000Hz, the LED will only be on 1/5th of a second in total. But because this happens so fast, the human eye (or even a multimeter) perceives a lower brightness (voltage) from the LED.

Many commercial dimmers operate between 100Hz and 400Hz. Although the human eye cannot perceive 100 individual frames a second, fast movement in front of the light source can still cause a visual strobing effect.

For this reason QuinLED-OG ran at a frequency of 1000Hz. Absolutely impossible to see for the human eye, no matter how sensitive you are to perceiving flickering.

1000Hz is still not enough for a Camera sensor

Have you ever noticed where during some videos on the internet you see this weird stripe pattern in the room or over the picture? That is PWM dimming! Although the human eye might not be able to see or even perceive frequencies around 400Hz or 500Hz a camera sensor still can. Because of that PWM dimming often is not usable in any photo or video situation.

But since I want to use LED strips in my new office, where I’m also going to film, this wouldn’t do.

To try and “fix” this issue, the new QuinLED-Quad and QuinLED-Deca are going to PWM not at a frequency of 1000Hz but 40.000Hz (or higher, up to 150.000Hz) in an attempt to not only making the PWMing of the LED invisible for the human eye but also for cameras (at reasonable shutter speeds).

Concluding remarks

So that is why QuinLED boards are Constant Voltage based PWM dimmers. That way they can control a large variety of lights (often including multiple Constant Current lights in a string) and using the very high PWM frequency you should be able to use them in most if not all situations.