INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Improving an existing LED Driver

Improving an existing LED Driver

(OP)
Hello ! I have a WiFi LED driver that we purchased that has visible flicker when its dimmed below even 80% using its own dimming software. What would you suggest for me to upgrade in order to remove the flicker? What would cause the flicker? In general, how can I improve on an existing driver to help reduce flicker?

RE: Improving an existing LED Driver

increase PWM frequency? above 60hz

Many drivers use PWM to turn the LED on/off quickly.. Persistence of vision makes it look dimmer/brighter to human eyes.. Some are more sensitive than others..
I can't stand LED Christmas lights because I can see the mains freq flicker.. My wife does not notice it at all..

RE: Improving an existing LED Driver

PWM is a means of obtaining an analog signal from digital means. It is a technique that utilizes ratios and is independent of frequency.

But the speed of your driver may be an issue.
Just how many LEDs are you running, and what driver? Does the driver have it's own internal oscillator?
These are just some questions that could be part of the problem.

With more information you can address what can be done. If at all possible get into a frequency range where
a duty cycle of even 5% won't cause flicker.

RE: Improving an existing LED Driver

Watch out though. The green area in the chart is the start of where you will start to hear the PWM as piezoelectric effects can translate the power into the audio domain. Then you get to hear your LED control instead of see it! Proper component selection and PCB layout design can resolve those issues. How many layers of this 'onion' are you willing to peel?

Have you tried this controller with different LEDs? That might be the simplest fix.

Z

RE: Improving an existing LED Driver

(OP)
So it has a 16Mhz Crystal serving as the clock for a microprocessor which put its output as the LEDs. It's wired up to a couple LED Strips that total 42 small LEDS in Series. The Microprocessor can handle up to 36 Mhz, would increasing the clock help reduce flicker?

RE: Improving an existing LED Driver

MHz, not Mhz. smile

The same crystal might be used for other purposes, so changing the crystal might have other unwanted impacts.

(edit: e.g. the same crystal might be used as a reference for the wifi chip, or something else in the system. It's very unlikely that the xtal is used only to set the PWM frequency.)

RE: Improving an existing LED Driver

Maybe. But unknown atm.

Was this a breakout board you purchased?
If so a link to it could help.
If not a link to your driver and MCU could help.

RE: Improving an existing LED Driver

42 LEDs in two strings? Your device only outputs a maximum of 24V, which means you're pushing the voltage limit. What LEDs are you using? I would think that you ought not have more than about 6 LEDs per string, particularly for blue.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com: http://www.engineering.com/AskForum/aff/32.aspx

RE: Improving an existing LED Driver

Those 5050 LED strips are typically 3 LEDs in series with a resistor for each series string. Not all in series. They typically accept a 12VDC power supply.
regardless.. there is very little you can do short of making your own controller or finding a different one as you would need to reprogram it and I'm sure you don't have access to the original code nor would it be worth your time to rewrite it from scratch.. (thats not an LED driver BTW).. its simply a controller that just controls each R/G/B channel via PWM'ing a mosfet or transistor.

RE: Improving an existing LED Driver

You might consider putting a filter inductor in series with your LEDs. That would smooth the current through them and reduce flicker. If V is the voltage your PWM switches between (in volts), I is the current your LED string draws when on (in amps), and t is the time period of the PWM frequency (in seconds), you can start with an inductor of L = 2 * V * t / I henries. Make sure the inductor will handle the LED string current.

For example, if your PWM switches 0 - 24 V, your LEDs draw 20 mA, and your PWM switching cycle is 10 us (100 kHz rate), L = 2 * 24 * 10e-6 / 20e-3 = 24e-3 or 24 mH (millihenries). Larger inductors would smooth the current more.

RE: Improving an existing LED Driver

(OP)
I'm going to bump this topic since I'm still working on this project. I've found that I can eliminate the flicker if I can bump the frequency of the signal to 10khz. It outputs at 600 hz and is picked up by our cameras. I designed my own LED driver and when I output 10khz from the MCU its not picked up by the cameras. Thing is, I still want to modify these existing drivers to simply output a higher frequency. I was thinking that I could use a Phase-Locked Loop to simply increase the frequency. Here's an example http://www.eetimes.com/document.asp?doc_id=1230513 I tried but it has issues and doesnt work the way I need it to, essentially the leds never turn off. Do any of you guys know of a fairly simple method for increasing frequency but maintaining the singal?

RE: Improving an existing LED Driver

(OP)
It's not really noticeable by looking at it unless you apply the photoelectric effect. The issue is with how the cameras record the leds. The image comes out as a wave moving down the screen. When I drive LEDS at 10khz the wave doesn't appear because its faster than the exposure of the camera.

RE: Improving an existing LED Driver

(OP)
That's a great idea which I've actually been working on trying out. The idea that I need help with is how to modify an existing driver to output at a higher frequency. We're in the process of developing our own driver but in the mean time I wanted to modify the existing ones to function better. There no reason I cant add a MCU to the output and tell it to read the signal and output the same signal at a faster frequency? Would I need to condition the signal before it hit the input of the MCU?

RE: Improving an existing LED Driver

Not exactly condition it, just adapt it to make sure it meets the input parameters of the MCU (Vih, Vil, Vmax, Vmin, etc.). If you are sending it over a cable to the MCU you'll most likely need a cable driver on the transmitting end. Otherwise the capacitance of the cable could throw your circuitry into oscillation.

Z

RE: Improving an existing LED Driver

Have you tried contacting the seller to ask if they would sell you a version with a higher PWM frequency?

RE: Improving an existing LED Driver

(OP)
What a brilliant and simple solution. I will contact them right away haha.

RE: Improving an existing LED Driver

Even at higher speeds it may not be consistent. A filter was talked about before but what is really needed is a buck converter output stage, diode, inductor and large filter cap. That will give you filtered DC and should work with most output devices.

RE: Improving an existing LED Driver

(OP)
Buck converter.. Ok, thanks for the information. I'll certainly look into that. Do you think that running it through a microcontroller would eliminate that step? Since it would be outputting square waves anyhow?

RE: Improving an existing LED Driver

"Have you tried contacting the seller to ask if they would sell you a version with a higher PWM frequency? "

Are you sure they haven't already fixed this problem because others have complained about it? If the issue is on the web or social media there is a good chance that they paid attention and fixed it fast with firmware, so a new unit may not have the same problem.

Z

RE: Improving an existing LED Driver

(OP)
Thing is that they have it output at 600 hz which is high enough for most normal applications. It's just not good enough to use on video. I will contact them and see if they would be willing to modify the coding for me. Next, I'm going to run the signal through a microcontroller and output it at a faster rate, does anyone know of a reason that wouldnt work?

RE: Improving an existing LED Driver

That's a lot of work just to get rid of "flicker."

The issue that you're having with the camera is that the actual time that the detector is actively integrating photons is extremely short, typically less than a few milliseconds. At 10 kHz, you'd get a minimum of 10 PWM cycles within the integration time of the camera, which is what minimizes the observed "flicker."

The simpler solution is to stick a neutral density filter to drop the amount of light coming into the camera to force the camera's AGC to increase the integration time. A 1.0 ND would decrease the light by a factor of 10 and force the integration time to correspondingly increase. If the camera lens has a physical iris, then stopping down the lens would be equivalent.

TTFN
I can do absolutely anything. I'm an expert!
homework forum: //www.engineering.com/AskForum/aff/32.aspx
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close