UPC EETAC Bachelor's Degree in Telecommunications Systems and in Network Engineering. Bachelor's Degree in Aerospace Systems Engineering EEL

 

Project: Power LED dimmer

Project specifications

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Our goal is to design a dimmer to control the level of illumination or light intensity for a 10 W power LED.

Commercial dimmer

Fig. 1. Commercial dimmer for power LED.

Let us reuse and chaining previous content and projects. Let us imagine several design phases as if the circuit has to evolve adding new features and modules. For instance:  

- Design phase A: Analogue system

0.- The actuator: a commercial power LED. Heatsink. Bias point.

1.- LED driver. Equations. Modelling. Simulation. PWM driver using a 555 IC.

- Design phase B: Digital

0.- FSM installation. ON (100% light intensity / OFF (0%, no light).

1.- Digital PWM. PIC18F version using TMR2. Arduino version.

2- UP/DOWN push-buttons or digital potentiometer to set the PWM duty cycle.

- Design phase C. Mains adapter and 12W switching power supply

 

In phases A and B laboratory power supplies will be used as energy sources for driving circuits and LED. 

 

References and additional questions

A good reference on what we are indenting to discuss in this introductory project can be read in subject CESA on [1] "Introducción al laboratorio: Dimmer", [2] "CESA: Ejemplos de actividades dirigidas".

 

 

Design phase A0: LED characteristics

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A0: LED characteristics

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Characterise and experiment with a power LED lamp. Measure its forward current versus forward voltage function iL = f(VD).

Power LED

This actuator is an array of several LED connected in series and in parallel. Example datasheet. 

Characteristics

Characteristics LED power

Fig. 2. Typical power LED lamp.

 

Heatsink

This kind of actuators require designing a heatsink to keep the temperature in the specified range when working in nominal conditions. Therefore it has to be attached to a metallic heatsink and optionally to a fan even if it means an efficiency loss.

 

Fig. 3.  Example heatsink and fan ready for experimentation.

Characterisation

We need a laboratory power supply to observe how the LED works and which may be the correct bias point. We will validate the experiment checking datasheet data. We need to observe the light intensity generated by a single LED, in case we decide to connect several of them in the same luminary or panel.

 

Fig. 4.  Experiment for obtaining the function iL = f(VD).

The exponential diode characteristic shows us that in order to obtain an effective light intensity control from 0% to 100%, it will be much better to control the LED current than its voltage. 

 

 

Design phase A1: analogue control system

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Let us propose a power driver for the LED. This device will be a transconductance amplifier, capable of controlling the output current proportionally to the much convenient input control voltage VCNTL. The circuit floating load is the LED lamp. An active feedback path will be used to sense the LED current while keeping a low voltage VS.

Transconductance amplifier

Fig. 1. Transconductance linear amplifier.

We have adapted such circuit from these references. Controlled current sources are typical OpAmp applications.

[3] Murnane, M., "AN-968: Current Sources: Options and Circuits", Analog Devices.

[4] LT1492 datasheet, Analog Devices. 

[5] Other references on controlling LED light intensity

 

2. Planning

Hardware

The proposed block diagram is shown in Fig. 1. It is based on using an OpAmp as high-gain linear amplifier. The NMOS transistor will drive the high current required by the lamp from a standard voltage power supply.  

To keep the driver efficiency high we must use a very low resistor, for instance RS = 0.2 W, for sensing the LED current. Therefore, the amplifier feedback loop will include another OpAmp to condition the low voltage measured by the sensing resistor RS.

At this point we also must choose a correct level of VDD to power the circuit. A value too low will not allow a correct current control through the amplifier. A too high value will represent excessive power loss and high temperate on the NMOS transistor. 

 

Fig. 2. Example power LED driver using a transconductance amplifier.

Project location:

C:\DEE\Dimmer\PhaseA0\(files)

 

 3. Development    4. Testing      5. Prototyping

Fig. 3 is a Proteus simulation of the analogue Dimmer.pdsprj circuit.

Simulation results

Fig. 3. Transconductance amplifier simulation.

We can measure the range of current regulation, which VDD range keeps a constant iL current?

 

6. Laboratory experimentation and measurements

Let us validate the circuit performing measurements using laboratory instrumentation. Let us verify that the circuit works correctly in nominal conditions for a long period of time keeping the light level over small variation of the power supply voltage VDD.

 

Fig. 11. Circuit working.

 

As a second step, we can add to the circuit a simple PWM oscillator based on the typical 555 IC. In this way modulate the LED light intensity while keeping it working in nominal conditions.

 

 

 

 

Design phase B0: ON/OFF

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Microcontroller board

To visualise two alternative design and be able to compare features, we can imagine two options:

Option #1: Let us use a CSD_PICstick board to provide a digital control of the lamp, as if this project were a CSD continuation.

Option #2: Let us use an Arduino board to provide a digital control of the lamp. as it is usual in many hobbyist  applications.

Control panel

We can imagine two different methods fro inputting the light level. How will be the control panel?

Option #1: Buttons UP and DOWN that dynamically adjust the duty cycle to the light level required.

Option #2: Potentiometer to input the duty cycle from 0% to 100%.

 

2. Planning

Each option will require a design project t location accordingly to the options:

Option #1

Project location accordingly to the options:

C:\DEE\Dimmer\PhaseB0\(files)

Software

 

3. Development    4. Testing      5. Prototyping

Hardware

 

Software

 

6. Reporting

Documenting the project. 

 

 

Design phase B1: PWM light control

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The same of design phase A replacing the analogue control by a digital ON/OFF control based on Arduino.

 

 2. Planning

Our goal is .

 

3. Development     4. Testing      5. Prototyping

Hardware

 

Software

 

6. Reporting

 

 

Design phase B2: PWM hysteresis control

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The same

2. Planning

 

Development     4. Testing     5. Prototyping

  

6. Reporting

 

 

 

Design phase C: Power supply

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Design phase D: Communication interfaces

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Our goal is to command the dimmer using a wireless interface and computer or Android apps.