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I've been playing with LEDs for video work for a while. I made some LED panels using strips and off the shelve controllers which always worked nicely while on full brightness. Everytime I dim then down, I start to get some weird "scanline" flickering. That happens to all sorts of leds and dimmers (Arduino included) Ive ever played with.

Yesterday a friend told me he got a modded 12-24V 8A Dimmer which extinguish this kind of problem. Once I got it on hands I was surprised it looked just like a 12-24 Dimmer I got from eBay. Googling around I found out it's a very popular circuit, with MANY different PCB layouts and different vendors.

enter image description here

I tried to find Flicker related mods online but couldn't find anything. Obviously I opened his circuit and it's surprisingly similar to mine, apart from 3 differences I spotted:

My friends modded PCB I tried to transpose this mods to my PCB. I think I got two of them right (Yellow and Green), but I just can't find where to do the mod which I circled in red (the trail that has been interrupted).

My PCB and where I think I should place the mods

I tried to follow the trails but they go underneath the LM358 and the 3 pinned connector. The other IC is a 555.

I have two questions regarding this situation, which my restrict electric engineering knowledge is not helping me out:

  • Where I should interrupt the board on my PCB Layout?
  • What exactly this MOD did that eliminated the flickering on video?

Here are a few other pics of the boards:

Bottom of friends board

Bottom of my Board

Any help or tips are extremely appreciated! Docs and Papers regarding LEDs and Flickering are also welcome :D

Thanks in advance!

EDIT: I managed to unsolder the pot conector and the two ICs so I can get a better view of the board. I did destroyed one pad on the U1, which apparently didn't connect anything, just the jumped pins.

There's also a solder blob between U1 and the middle pot pin. It's a jumper to rejoin a track I manually interrupted and repaired afterwards.

Overview

758 Side 555 Side [![Another Overview shot][9]][9]

I'll be posting demos of both circuits working so I can show you the visual difference too.

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    \$\begingroup\$ "I tried to follow the trails but they go underneath the LM358 and the 3 pinned connector." - you have two choices: 1. remove the part that is obscuring the trace you are trying to follow. 2. Use a continuity tester to look for connections between components. \$\endgroup\$ – Bruce Abbott Nov 14 '17 at 6:13
  • \$\begingroup\$ found this smokedprojects.blogspot.ca/2013/09/led-dimmer-pwm-hack.html \$\endgroup\$ – jsotola Nov 14 '17 at 8:42
  • \$\begingroup\$ the cut trace "red" most likely connects to one of the bridged pins "yellow" \$\endgroup\$ – jsotola Nov 14 '17 at 8:45
  • \$\begingroup\$ Thank you both! Ill be looking into the circuit and checking if I can solve it our based on your feedback. Unfortunately I wasn't allowed to make changes to my friends PCB and I don't have access to it anymore... It has been shipped abroad :/ \$\endgroup\$ – Tejada Nov 16 '17 at 3:22
  • \$\begingroup\$ I just remembered do have a broken board back home. Will be unsoldering every component of mine to get a better understanding of the circuit. Will edit my post as soon as I do it! \$\endgroup\$ – Tejada Nov 16 '17 at 4:36
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Adding more info I couldn't on my first post:

I edited my tests to show you guys. Different cams but same results. First one is my friends circuit, 100% flicker free (I had to crop it cause there was sensitive content on frame):

Flicker Free Circuit

The little blackouts are just me not being able to hold the supply wires. not circuit flicks...

My Flickery Circuit

As you can see, it only stays flicker free on max power. As soon as you dim out, it starts flickering.

Both tests were powered by a regular 9V battery (not ideal, but what I had on hands).

Based on the link pointed out by jsotola, I could gather a few more infos:

  • Frequency is governed by the 555 astable. The capacitor between pins 2 and 6 of the 555 should be responsible for that frequency.
  • This calculator helps to find the right cap for the frequency I want:Calculator
  • Assuming my R1, R2 and C values are respectively 1K, 10K and 100nF, the output frequency should be around 680hz, which matches the links video osciloscope teste. I will try to get one and verify my circuit.
  • A comment by brokenmonkey on the same link gave some video related tips:

I know 6 kHz is a safe frequency for video but it can cause audio interference, and 20 kHz is safe for audio but maybe not all types of video.

Following his logic, replacing C1 with a 11nf capacitor should do the trick. Still, my friends circuit have a couple of other stuff applied, as shown on the images.

My friend was also emphatic that the waveform should be square. I guess it comes from factory that way too...

Thanks again for all the help :D

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NAILED IT!

As I described on my last post, based on the link suggested by jsotola, replacing the 100nf capacitor with a 11nf capacitor should do the trick. As I didn't have a 11nf, I used a 10nf polyester one and it just did the trick!!!

So yeah, if you are intending to shoot video/digital cinema, try to set the PWM around 6Khz.

Mod picture: My mod

PS: notice that I did not apply any of the other mods from my friends unit.

Cheers and again thanks for all the help!!!

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These are very poor dimmers. A decent dimmer will have an inductor and fairly large capacitors to smooth out the flicker.

The dimmers you have just turn the LED power off and on. As you dim the power is turned off for longer lengths of time. That is why they flicker.

What you need is an adjustable constant current regulator.
You will need at least a 36V power supply.
You need one driver for each strip.

Inexpensive: PicoBuck LED Driver, FemtoBuck LED Driver

Good quality: Infineon EVALLED-ILD6150

The easier way is to use a Mean Well HLG-xxH-Cxxxx driver with the strips wired in series. Which one depends on how many strips and the current required.

Example: 3-5 strips @ 210 - 350mA use a HLG-60H-C350 Type A


Any ideas on what exactly the mod may have done to the circuit?

Really cannot say without schematics and the number of LEDs per strip. The pictured boards have different circuitry and is doubtful a mod from one will apply to another.

Your mod is somehow changing the shape of the switching waveform.
Most likely the mod would increase the switching frequency.

With a 24V output you should not drive more than 7 or 8 LEDs in series.

These dimmers turn the LED off and on to regulate the current. The frequency and duty-cycle have a significant effect on the Stroboscopic Effect. Higher frequency and higher duty-cycle will minimizes the flicker.

The max current will affect the duty cycle. If the max current is too high the duty-cycle must be decreased. When the dimming is at full output and it does not over-drive the LEDs, the duty-cycle may be 99%. If at full output the current over-drives the LEDs the duty-cycle must be reduced to reduce the current to the max. Further dimming reduces the duty-cyle further increasing the LED off time (flicker).

duty cycle



Modulated Dimming

In this look at the "Signal" as the level of dimming.

modulation

White paper: Understanding Stroboscopic Effects

IEEE paper: Designing to Mitigate the Effects of Flicker in LED Lighting

US Dept. of Energy paper: What You Need to Know about LED Flicker and Dimming

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  • \$\begingroup\$ Thanks! Im aware that they are cheap stuff. Unfortunately that's what I could get one day before my film shooting down here in Brazil. These FemtoBuck Led drivers looks nice, I'll be getting a couple of both to perform some tests. Still, I shot the filme using my friend dimmers (which unfortunately are already abroad) and indeed it completely solved the flickering issues (shooting ag 23.976fps, 1/48). Any ideas on what exactly the mod may have done to the circuit? Thanks again! \$\endgroup\$ – Tejada Nov 16 '17 at 3:20
  • \$\begingroup\$ Thanks again for your answer! I managed to unsold the 2 ICs and the potentiometer conector, updated my post. Actually, I'm driving single 10W leds. Will be looking more carefully into your information! Thanks again! \$\endgroup\$ – Tejada Nov 22 '17 at 1:13

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