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Copy of LED Dimmer Circuit using 555 Timer - on Sat, 02/22/2020 - 20:10

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Description

This LED Dimmer Circuit uses a 555 Timer to control the PWM duty cycle of the current drive. Rather than apply proportional but continuous current to the LED for dimming, which can cause color shifts, modulating the duty cycle allows the LED to operate at its nominal current during the “ON” portion of the cycle. Because the frequency response of human vision is limited, using a PWM frequency of 250 Hz avoids the perception of flicker for the observer.

The LED model has an internal monitor for the "perceived" light output (light-blue waveform), which is a low-pass filtered version of the instantaneous light output. The filter pole frequency is set to 15 Hz to represent the bandwidth of the human eye. The value of the dimmer setting (green waveform) is increased from 10% to 90% at time 100msec. The LED current pulses (purple waveform) are shown just before and after the duty-cycle transition.

Part of this design is based on a dimmer schematic found on-line:

http://www.555-timer-circuits.com/led-dimmer.html

That original circuit is actually incorrect, one of the diodes needs to be reversed. There was a comment from a reader who said he built the circuit as shown and it didn’t work. This is a good example of the value of simulating circuits before building hardware!

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Copy of Joule Thief Transformer Physical Design For LED Lighting - on Thu, 02/20/2020 - 18:53

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Copy of Joule Thief Transformer Physical Design For LED Lighting - on Wed, 02/19/2020 - 19:15

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Copy of Single-string Test - ACME AS123 LED Driver with Dimmer Control - on Sat, 02/08/2020 - 22:12

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Copy of LED Driver with Auto-Dimming for Thermal Protection - on Thu, 01/30/2020 - 18:30

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Description

This LED lighting example demonstrates the value of simulating both the electrical and thermal* aspects of power dissipating circuits together, simultaneously.In this application example, a Vishay NTCS0603 Thermistor provides feedback of the enclosure temperature. This feedback is used to control PWM dimming of the LEDs, thereby limiting the internal temperature when operating at high external ambient temperature conditions.This is a "Live" design, the user can change key parameter values and then run new simulations to see the results. These parameters include "r_mirror", the resistance of the current mirror that controls the capacitor charging rate of the 555 timer, and thereby set the PWM frequency. The user can also change "r_offset" that controls the temperature level at which the dimming operation begins. Finally, the user can set "r_iLED_set", to control the ON-state operating current of the LEDs.----------------* To reduce the time needed to simulate the transition and settling at 6 different temperature levels, all thermal time constants were reduced by approximately 1000x. The actual thermal response time constant of the NTCS0603 is approximately 3 seconds (depends on mounting), not 3 msec! Also, the enclosure thermal capacitance value would more likely be 3 (J/degC) instead of 3 (mJ/degC), giving a thermal time constant for the enclosure of 10 (degC/Watt) * 3 (J/degC) = 30 seconds. This time scaling does not affect the static relationship between the outside temperature and PWM dimming.

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ACME AS123 LED Driver with Dimmer Control - on Sun, 01/26/2020 - 15:13

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LED Driver with Auto-Dimming for Thermal Protection (with realistic thermal time constants)

Designer19

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11 years 7 months

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Member of the PartQuest Explore Development Team. Focused on modeling and simulation of analog, mixed-signal and multi-discipline systems covering a broad range of applications, including power electronics, controls and mechatronic systems.

Description

This LED lighting example demonstrates the value of simulating both the electrical and thermal aspects of power dissipating circuits together, simultaneously.In this application example, a Vishay NTCS0402 Thermistor provides feedback of the enclosure temperature. This feedback is used to control PWM dimming of the LEDs, thereby limiting the internal temperature when operating at high external ambient temperature conditions.This is a "Live" design, the user can change key parameter values and then run new simulations to see the results. These parameters include "r_mirror", the resistance of the current mirror that controls the capacitor charging rate of the 555 timer, and thereby sets the PWM frequency*. The user can also change "r_offset" that controls the temperature level at which the dimming operation begins. Finally, the user can set "r_iLED_set", to control the ON-state operating current of the LEDs.*Note: The PWM switching frequency was intentionally reduced from the practical value of 260 Hz to 2.6 Hz, in order to provide fast simulations and realistic (long) thermal time constants. This was accomplished by increasing the 555 timer capacitor from 100 nF to 10 uF. This should have no significant impact on the thermal feedback or settling behavior that would be observed at the higher PWM frequencies needed to avoid visual "flicker" to the human eye (i.e. &gt; 200 Hz).

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LED Driver with Auto-Dimming - Low PWM frequency but realistic thermal time constants

Designer19

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11 years 7 months

1,833

designs

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Member of the PartQuest Explore Development Team. Focused on modeling and simulation of analog, mixed-signal and multi-discipline systems covering a broad range of applications, including power electronics, controls and mechatronic systems.

Description

This LED lighting example demonstrates the value of simulating both the electrical and thermal aspects of power dissipating circuits together, simultaneously.In this application example, a Vishay NTCS0402 Thermistor provides feedback of the enclosure temperature. This feedback is used to control PWM dimming of the LEDs, thereby limiting the internal temperature when operating at high external ambient temperature conditions.This is a "Live" design, the user can change key parameter values and then run new simulations to see the results. These parameters include "r_mirror", the resistance of the current mirror that controls the capacitor charging rate of the 555 timer, and thereby sets the PWM frequency*. The user can also change "r_offset" that controls the temperature level at which the dimming operation begins. Finally, the user can set "r_iLED_set", to control the ON-state operating current of the LEDs.*Note: The PWM switching frequency was intentionally reduced from the practical value of 260 Hz (used in the companion design https://www.systemvision.com/design/led-driver-auto-dimming-thermal-protection), to 2.6 Hz, in order to provide fast simulations and realistic (long) thermal time constants. This was accomplished by increasing the 555 timer capacitor from 100 nF to 10 uF. This should have no significant impact on the thermal feedback or settling behavior that would be observed at the higher PWM frequencies needed to avoid visual "flicker" to the human eye (i.e. &gt; 200 Hz).

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ACME AS123 LED Driver with Dimmer Control - Application Note

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The AS123 LED Driver from ACME Semiconductor (fictional) provides fully integrated PWM dimming and a programmable current set-point. This IC is specifically for automotive rear combination (tail/stop) lighting applications. It tightly regulates the desired string current under conditions of widely varying applied DC voltage.The PWM dimming function, which switches the LED on and off at just over 300 Hz, is active when the line input pin is high and the "PWM_disable" pin is low (The AS123 supports current programming using a single external resistor (see r_iset in the schematic). The resistor value can be selected to give the desired LED string current, using the following formula:r_iset = 1.85/i_desired The AS123 is capable of regulating up to 100mA per string continuously. Therefore the value of r_iset should be no less than 18.5 Ohms. The user can change this resistor value and run a new simulation, to see the effect of this change on the LED string current. See a "functional block diagram" schematic model of the ACME AS123 here: https://www.systemvision.com/design/acme-as123-led-driver-schematic-model-------------- End of Example Application Note ------------------If you are a component supplier, this type of "Live Application Note" can help your potential customers better understand the features and benefits of your components.

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CAT4104 LED Controller Application Example

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This is an application example for an ON Semiconductor CAT4104 Quad Channel Constant Current LED Driver. It demonstrates the part's resistor programmable and independent current control for each channel, as well as its PWM dimming capability. By choosing the value of r_set from 768 Ohm up to 12 kOhm, the current in each channel is controlled from 175 mA down to 10 mA. That is, the channel current is 100 times the current in r_set, which is driven by an internal 1.2 V source from the CAT4104 "rset" pin. The LED strings can be driven by parallel channels to reach even higher string current levels. For LED dimming, the CAT4104 "en_pwm" pin can be driven by a PWM signal. In this application circuit, that signal is provided by an LM555 timer module. The potentiometer setting controls the PWM duty cycle, and the element's total resistance value determines the PWM frequency. The initial potentiometer wiper ratio of 0.5 and resistance of 20 kOhm, gives a 50% PWM duty cycle at 500 Hz.The LED string model, for which the user can select the number of LEDs in the string as well as the individual LED operating parameters, has an internal quantity that gives the "perceived" luminous output. This includes a 15 Hz Low-pass filter, to represent the light-averaging effect of the human eye. This is shown in the dark blue waveform. The user can explore the effect of changing the potentiometer settings, which in turn change the PWM characteristics, on the perceived light output. The initial LED parameters for this example represent a CREE XPCWHT-L1-R250-009E6.

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