Constant Voltage LED Power Supply

About the driver circuit

The biggest concern when driving the LED is the constant DC power at the LED input. Any unwanted fluctuation in voltage at the LED input can damage it. Excess current flow through the LED greater than the forward current rating of the LED may increase its temperature and damage the LED. Therefore, the role of constant voltage LED supply is very important in this scenario. In this experiment, a driver circuit is designed that provides a constant voltage of 12V at the output with a maximum current of 1A.

Overview

In this experiment, a constant voltage LED power supply is designed to provide a stabilized and regulated DC output. This driver circuit can provide a constant voltage of 12V at the output. Thus, a single LED or a combination of LEDs can be connected to the output that requires constant 12V as input. At the output, current of up to 1A can be supplied by this source.

Required components

Required components	Specifications	Amount
TR1 Transformer	Lower 18V-0-18V/2A	1
Voltage regulator	LM7812	1
Diode D1-D4	1N4007	4
Capacitor C1	470uF 50V	1
Fuse	1A	1

Power supply basics

Every DC power supply needs to follow a few steps to get the proper DC voltage at the output. The diagram below shows these basic steps by which we obtain an AC regulated DC power supply.

Working

• AC to AC Conversion

The mains voltage (electricity arriving at our house from the government) is approximately 220 V, but according to the circuit requirements, only 12 V is required at the output terminal. To reduce this 220V to 12V, a step-down transformer is used.

The circuit suffers some drop in the form of resistive loss and by IC LM7812. Therefore, a high voltage transformer higher than the voltage required for the application (12V) is used and which can supply 1A current at the output. The most suitable step-down transformer that meets our voltage and current requirements is 18V-0-18V/2A. This transformer reduces the main line voltage to 18V as shown in the image below.

• Rectification

Rectification is the process of converting AC to DC. There are two ways to convert an AC signal to DC. One is through half-wave rectifier and another is using full-wave rectifier. In this circuit, we are using a full wave bridge rectifier to convert 18V AC to 18V DC. As full wave rectifier is more efficient than half wave as it can provide complete use of negative and positive side or part of AC signal. In the full-wave bridge rectifier configuration, four diodes are connected in such a way that it generates a DC signal at the output, as shown in the image below. During full-wave rectification, two diodes are forward biased and another two diodes are reverse biased. We chose 1N4007 diode because they can allow 1A current through them when forward biased and in reverse biased condition, they can sustain 12V reverse supply. Due to this, we are using 1N4007 diodes for rectification purposes.

• DC smoothing

As the name suggests, it is the process of smoothing or filtering the DC signal using a capacitor. A high value capacitor C1 is connected to the output of the rectifier circuit. As the DC that must be rectified by the rectifier circuit has many AC spikes and unwanted ripples, to reduce these spikes we use a capacitor. This capacitor acts as a filtering capacitor that shunts all the AC through it to ground. At the output, the remaining DC is now smoother and ripple-free.

• Voltage regulation

To provide regulated 12V at the output, an IC LM7812 is used. This IC is capable of delivering current up to 1A. It will provide regulated and stabilized voltage at the output regardless of variations in input voltage and load current.

Output voltage and current

In IC LM7812 input voltage of 14.8V to 27V is required so it will provide constant output voltage in the range of 11.5V to 12.5V. The IC is capable of providing maximum current of 1A in the exit.

LM7812 Internally Tolerable Power Dissipation

Pout = (Maximum IC operating temperature)/ (Thermal Resistance, Junction-Air + Thermal Resistance, Junction-Case)

Pout = (125) / (65+5) (values ​​according to technical data sheet)

Pout = 1.78W

The 7812 internally can sustain up to 1.78W of power dissipation. Above 1.78W the IC does not tolerate as much heat and starts to burn. This can also cause a serious fire risk. Therefore, a heatsink is required to dissipate excessive heat from the IC

Practical results

1. 1.8V LED

We can connect a maximum of 6 LEDs in series at the output with a 68 ohm limiting resistor. Each LED consumes approximately 1.8V to operate in forward bias condition.

Input voltage to the circuit, Vin = 12V (from 7812)

Total voltage drop across 6 LEDs, V = 1.8 *6 = 10.8V

Output current supplied by this power supply/Current consumed by the circuit

I = (Input voltage – voltage drop across LEDs) /R1

I = (12 – 10.8)/68

I = 17.6 mA

For a 1.8V LED, approximately 20 mA of forward current is required for adequate illumination without breaking its forward current limit. For this purpose only, a series resistance (in this case 68 ohms) is used to limit the current.

We can also calculate the power dissipation of IC LM7812 by –:

Energy dissipation

P out = (Vin – Vout)*Iout

Pout = (12-10.8) *(0.0176)

Pout = 21.12 mW

2. LED 2.2V

We can connect a maximum of 5 LEDs in series at the output with a 47 ohm limiting resistor. Each LED consumes approximately 2.2V to operate in forward bias condition.

Input voltage to the circuit, Vin = 12V (from 7812)

Total voltage drop across 5 LEDs, V = 2.2 *5 = 11 V

Output current supplied by this power supply/Current consumed by the circuit

I = (Input voltage – voltage drop across LEDs) /R1

I = (12 – 11)/47

I = 21.2 mA

For a 2.2V LED, approximately 25 mA of forward current is required for adequate illumination without breaking its forward current limit. For this purpose only, a series resistance (in this case 47 ohms) is used to limit the current.

Energy dissipation

Output P = (12-11) *(0.0212)

Output P = 21.2mW

3.LED 3.3V

We can connect a maximum of 3 LEDs in series at the output with a 6 – 7 ohm 1W limiting resistor. Each LED consumes approximately 3.3V to operate in forward bias condition.

Input voltage to the circuit, Vin = 12V (from 7812)

For a 3.3V LED, approximately 300-350 mA of forward current is required for adequate illumination without breaking its forward current limit. For this purpose only, a series resistance (in this case 6 ohms) is used to limit the current.

From the practical result above, we can see that the power dissipation is less than 1.78W (internal tolerable limit of 7812), but it is still recommended to use a heatsink to assist in cooling the IC and to increase the IC lifespan.

Other LED combinations are also possible using a perfect current limiting resistor and considering that the input current requirement of the circuit (LED combination) should not be more than 1A.