Improving an existing LED Driver

[DSpeegle]

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?

 

[mcgyvr]

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..

 

[ApexM0Eng]

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.

 

[zappedagain]

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

 

[DSpeegle]

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?

 

[VEBill]

MHz, not Mhz.

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.)

 

[ApexM0Eng]

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.

 

[DSpeegle]

Here's the link to it on amazon ;

http://www.amazon.com/Cooligg-Wirel...41838707&sr=8-1&keywords=wifi+led+controller.

I'm using this and some LEDs. It looks like its being controlled by a MCU in the STC11F/10Fxx Series.

 

[IRstuff]

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
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[mcgyvr]

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.

 

[reson8r]

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.

 

[DSpeegle]

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?

 

[IRstuff]

I would suggest that there's something else going on. Any flicker over 240 Hz should not be visible.

TTFN
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[DSpeegle]

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.

 

[IRstuff]

If you are designing your own LED drivers, synchronizing them to the camera frame sync should get rid of the flicker.

TTFN
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homework forum: //

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[DSpeegle]

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?

 

[zappedagain]

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

 

[LionelHutz]

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

 

[DSpeegle]

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

 

[OperaHouse]

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.