LED Circuit

Wednesday, October 7th, 2020 - Electrical Circuit
Since LED circuit are basically same as diode circuit, they are current dependent devices and do not have a current control function. A Light-Emitting-Diode (LED) is a P-N junction device (diode) that gives off light radiation when biased in the forward direction. The light emitting phenomenon makes use of the recombination within the P-N junction instead of thermal radiation, therefore, LED’s are free of waste and wear and can be expected to have a long life time.

Basic LED Circuit

When operated from constant voltage sources, protection should be provided by incorporating a current-limiting resistor as shown in the image below.

Design Example to determine R for figure 1a: Suppose a voltage source of five volts (Vcc) is used and a forward current (If) of 20mA is desired. The data sheet for the LED should indicate an expected voltage drop (Vf) across the LED and for this example 1.5 volts will be used.
$R=\frac{Vcc-Vf}{If}=\frac{5-1,5}{0.02}=175\Omega$

Led Circuit For Constant Current Regulators

This type of DC drive will regulate the current flow through the LED. There are numerous constant current regulator circuits and the circuit shown in the folowing picture can be used for a single LED as shown or a number of devices in series.

Led Circuit For AC Network

Since LED’s should be operated in the forward direction, reverse voltage protection is required to assure the reverse voltage (Vr) maximum is not exceeded. Inthe folowing picture illustrates the placement of back-to-back silicon diode across the LED, the silicon diode will clamp the voltage across the LED to approximately .6 volts. The picture below shows a different approach that uses two (2) LED’s in parallel which alternately light relative to the device under a forward bias condition.

LED Circuit With Basic Transistor Drive

The picture (a) shows a transistor drive circuit that lights the LED when the transistor is conducting into saturation. And the picture (b) shows a transistor drive circuit that lights the LED when the transistor is not conducting or is considered to be open. The equation below should be used to determine the value of R. Both of the transistor drive circuits shown in picture below can be configured to drive multiple LED’s in series.

$R=\frac{Vcc-Vf-Vce(sat)}{If}$

Pulsed operation of LED’s can typically be achieved by the transistor drive circuits shown in the picture above. Significantly higher peak LED light output can be realized from large drive current pulses at a low duty cycle. The maximum tolerable limits should not exceed the LED junction temperature above that obtained by operating the lamp at the maximum DC current of LED circuit.