LEDs driven by AC without transformers or rectifiers

Robin W Hughes,Mark Warner

doi:10.1038/s41598-020-80617-2

Abstract

We explore the driving of LEDs by untransformed AC. An extreme case is driving 1.9 V threshold (red) LEDs with UK mains, peak voltage 325 V. Commonly, driving is by transformed, rectified (DC) supply with a series resistor (where a significant fraction of the power is wasted) to limit current in the LED. With AC, one can instead reactively limit to a maximum current safe for an LED by employing a series capacitive impedance. Cheaper and simpler supplies can thus be employed in some cases. We analyse such non-linear circuits, and also explore questions of duty cycle and power experimentally.

Highlights

Light emitting diodes, LEDs, ubiquitous in modern electronic devices and lighting, are typically driven by low voltages, rectified to give DC, with a current limiting series resistor, in which a significant fraction of the supplied energy is dissipated
This paper is concerned with an obvious, very low tech alternative to drive LEDs that is never taught to physics and engineering students when they are introduced to these low voltage components
Given that the point a of the capacitor is pinned close to either ±Vc during conduction, the current flow is determined by the rate of voltage change of the other end of the capacitor, that is of the supply

Summary

Introduction

LEDs, ubiquitous in modern electronic devices and lighting, are typically driven by low voltages, rectified to give DC, with a current limiting series resistor, in which a significant fraction of the supplied energy is dissipated. LED lamps, by contrast, have power supplies of considerable sophistication; see a review[1] and here in the context of c apacitors[2,3]. Current limiting can instead be achieved reactively by a single capacitor, even when mains voltage AC is directly applied to individual LEDs, eliminating transformers and rectifiers, and resistive losses in associated resistors. DC-driven LEDs need series resistors to limit current flow because of the extremely steep I–V variation after their threshold voltage Vc. See Fig. 1a,b. AC (mains) driving reduces the need for control/power circuitry by using a series capacitor, see Fig. 1c, which offers lossless current limiting, independently of load. Current flows when either LED is conducting, changing the charge on the capacitor and the voltage across it. The maximum current flows when dV /dt is Scientific Reports | (2021) 11:963

Methods

Conclusion