how does changing frequency causes led to dim?

Thread Starter

clarelluffyjjang

Joined Jun 8, 2017
34
i am currently building this HV9910B led driver. The typical application looks like the diagram below. upload_2017-6-17_1-5-26.png
I wanted to build an application that can dim when the surrounding is dark. So , for the first trial i built a 555 timer circuit with 2 diodes with the circuit looks like the diagram below. i connect collector leg output to the led driver(HV9910B) PWM pin. i also replace R1 of the 555 timer circuit with LDR. The pulse width is sent to PWM pin and it does dim when i cover the ldr. i know that it is because when Ton the switch keep on switching and when toff it doesnt switch so then average power decreases, the problem is that the frequency also changes when duty cycle changes and i realize that just by using 555 timer it can already dim the led. so i decided not to use PWM pin and connect the led driver PWM pin to VDD(HIGH) and start manipulating the RT(oscillator pin). I used a 1M POT as the datasheet says it needs at least 226k resistor and max 1M resistor and i realize by changing the RT the frequency changes and so does the LED . But the problem with it is that i dont understand how it works. I know that impedance of inductor is 2pi*f*L but that is applicable for sine wave only, the oscillator give out pulse to the mosfet and cause it switch at high frequency. so how does changing this switching frequency cause led to dim?



upload_2017-6-17_1-9-6.png
 

Papabravo

Joined Feb 24, 2006
12,695
At a sufficiently high frequency the LED is neither, all the way on, nor all the way off. It just switches back and forth at reduced intensity. The optical response is slower the the electronic response.
 

crutschow

Joined Mar 14, 2008
23,764
Normally you change the pulse duty-cycle, not the frequency, to control the LED brightness.
PWM stands for Pulse Width Modulation (typically at a constant frequency).
 

Papabravo

Joined Feb 24, 2006
12,695
Normally you change the pulse duty-cycle, not the frequency, to control the LED brightness.
PWM stands for Pulse Width Modulation (typically at a constant frequency).
Finding a frequency and duty cycle range to provide satisfactory performance can be a challenge depending on the type of LED. There are some aspects of the selection process that are less than obvious. Choose poorly and you end up blinking instead of dimming. Choose poorly in the other direction and you get almost no dimming at all.
 

crutschow

Joined Mar 14, 2008
23,764
Finding a frequency and duty cycle range to provide satisfactory performance can be a challenge depending on the type of LED. There are some aspects of the selection process that are less than obvious. Choose poorly and you end up blinking instead of dimming. Choose poorly in the other direction and you get almost no dimming at all.
I would think that a PWM frequency of a few hundred Hz would work for just about any LED.
 

Thread Starter

clarelluffyjjang

Joined Jun 8, 2017
34
Finding a frequency and duty cycle range to provide satisfactory performance can be a challenge depending on the type of LED. There are some aspects of the selection process that are less than obvious. Choose poorly and you end up blinking instead of dimming. Choose poorly in the other direction and you get almost no dimming at all.
but the weird thing to me is that since 555 timer alone can already drive the led to dim or bright why do i need to use hv9910b? so i connect my Pwm to high so that my gate output totally depends on oscillator which is RT pin. i am doing this because my assignment require me to dim many leds to dim and bright. my practical result shows that changing Rt value does change the led brightness. but the only thing that changes if i change rt is the switching frequency. don't get why changing switching frequency changes the brightness of led
 

Papabravo

Joined Feb 24, 2006
12,695
The optical response time of an LED is much slower than the rising or falling edge of the driving current. Full brightness may be delayed by several milliseconds after the current starts to flow. The optical persistence of the LED will maintain the optical output for several milliseconds after the current drops to zero. Get the picture?
 

Thread Starter

clarelluffyjjang

Joined Jun 8, 2017
34
The optical response time of an LED is much slower than the rising or falling edge of the driving current. Full brightness may be delayed by several milliseconds after the current starts to flow. The optical persistence of the LED will maintain the optical output for several milliseconds after the current drops to zero. Get the picture?
do u mean that if i wait longer the led will slowly going back to its original brightness mode even if i decrease the frequency?
 

Papabravo

Joined Feb 24, 2006
12,695
do u mean that if i wait longer the led will slowly going back to its original brightness mode even if i decrease the frequency?
When you decrease the frequency and adjust the duty cycle appropriately you will reach a point where the LED achieves its full brightness just as the current is cut off and returns to dark just as the current is turned back on, and you will be able to see it blink.
 

Thread Starter

clarelluffyjjang

Joined Jun 8, 2017
34
it
When you decrease the frequency and adjust the duty cycle appropriately you will reach a point where the LED achieves its full brightness just as the current is cut off and returns to dark just as the current is turned back on, and you will be able to see it blink.
it wont blink the minumum frequency of the oscillator is 20kHz
 

Papabravo

Joined Feb 24, 2006
12,695
20 kHz has a period of 50 μsec. I doubt that the optical response of the LED would allow it to be fully on or fully off in 25 μsec So you never even turn the LED on or off in effect.
 
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