# LEDs 101

[![Screenshot 2026-03-13 at 3.39.05 PM.jpeg](https://wiki.ridgewoodcommons.nyc/uploads/images/gallery/2026-08/scaled-1680-/screenshot-2026-03-13-at-3-39-05-pm-jpeg.jpeg)](https://wiki.ridgewoodcommons.nyc/uploads/images/gallery/2026-08/scaled-1680-/screenshot-2026-03-13-at-3-39-05-pm-jpeg.jpeg)

The anode wire extends into the epoxy/plastic housing of the LED and has small cup which houses a tiny semiconductor. a ~25µm diameter gold or aluminum "bond wire" bridges the semiconductor to the cathode pin.

The semiconductor in the cup of the anode creates the light, and is a "forward-biased p-n junction." This is just a way of describing the mechanism. At a certain voltage (~1.8–3.5V) the p-n junction is activated, releasing light. Brightness is proportional to current, and the forward voltage is a consequence of that current.


The behavior of an LED can be described using the Shockley diode equation. The important part to note is that this equation describes an exponential relationship, such that a tiny voltage increase can cause an exponential increase in current, making it impractical to control an LED via voltage.

So... LEDs are all fundamentally current-controlled devices.

#### Types of LEDs

##### High-power LEDs

Typically have associated datasheets that specify a drive current, and the forward voltage is listed as a *result*, with tolerances. Running these from a fixed voltage is risky, with small Vf variations between chips of the same model, or changes with temperature, translate directly into wildly different currents. These essentially require a CC (Constant current) driver.

##### Low-power LEDs

Typically run with a series resistor from a fixed voltage rail. The resistor functions as a crude current control.

##### Addressable LEDs

Have a CC driver integrated, to control current to each color channel. So from the outside they function as constant voltage design.

##### Strips and arrays

Strips and arrays also typically have resistors integrated, so again can be treated as constant voltage devices, though at the component level are still constant current.

#### Strips in detail (from [https://www.ledsupply.com/blog/ultimate-guide-on-buying-led-strip-lights/](https://www.ledsupply.com/blog/ultimate-guide-on-buying-led-strip-lights/))

##### DC flex strips

Simple. Supply 12-24V, the built-in resistors on the strip do the current limiting.

Can be dimmed with PWM.

Downsides:

- Resistors generate heat rather than light.
- Over long runs, end of strip can be visibly less bright. 24V strips can run longer, but same eventual result. This can be mitigated by powering from both ends, making sure power supply wire gauge is ample.

##### AC strips

Have miniature switching converters that rectify and regulate DC voltage. These are more limited in terms of cutting to length than DC strips.

Requires specifically matched leading or trailing edge triac dimmer.

#### Dimming

The most common LED dimming method is Pulse Width Modulation (PWM). A pulse wave is similar to a square wave, but has more variation. A square wave is defined is 50% low, 50% high. A pulse wave can be high/low in any proportion.

[![image.png](https://wiki.ridgewoodcommons.nyc/uploads/images/gallery/2026-08/scaled-1680-/image-png.png)](https://wiki.ridgewoodcommons.nyc/uploads/images/gallery/2026-08/scaled-1680-/image-png.png)

(from [https://docs.arduino.cc/learn/microcontrollers/analog-output/](https://docs.arduino.cc/learn/microcontrollers/analog-output/))

An LED being controlled by a pulse wave at a high enough frequency exploits limitations in human perception, and appears to at 50% brightness, when in fact it is on a full brightness, but just 50% of the time. The cutoff for perception is around 50-60Hz. In cheaper or poorly designed PWM controls, the frequency of the on/off cycle is lower, e.g. 100Hz, which may not be obvious to the eye, but can leave people with headaches or become tiring. Better PWM controls cycle upwards of 1kHz.

References:

[https://www.ledsupply.com/blog/understanding-led-drivers/](https://www.ledsupply.com/blog/understanding-led-drivers/)

[https://www.ledsupply.com/blog/dimming-leds-guide-how-to-tell-if-your-lights-are-dimmable/](https://www.ledsupply.com/blog/dimming-leds-guide-how-to-tell-if-your-lights-are-dimmable/)