The problem with dimming AC LED Light Bulbs || DIY Trailing Edge Dimmer

Warning! these circuits showcased in this video use mains voltage mishandling of such a high voltage can lead to fatal injuries Replicate the circuits at your own risk. In a previous video we found out how traditional light dimming functions by examining a commercial dimmer And afterwards I showed you how to create an improved DIY Microcontroller dimmer that as you can see here still functions without a problem Well, at least with old halogen light bulbs Because when it comes to dimming AC LED light bulbs everything becomes much more complicated and complex Due to the structure of them So to get rid of this confusion, we will experimentally determine the electrical challenges AC LED light bulbs come with Examine a commercial trailing edge dimmer, and finally create our own DIY trailing edge dimmer with and without a microcontroller Let's get started This video is sponsored by JLCPCB Whose goal is to give engineers the best experience when it comes to ordering PCBs Try out their $2 PCB prototypes today and easily track every production step of your PCBs online First off let's have a look at all the LED light bulbs which are labeled as not dimmable After connecting one of them to my DIY dimmer and hooking up differential voltage probes as well as a current clamp We can see that by varying the phase angle, the brightness of the light bulb pretty much stays constant Unless the phase angle is very big at which point the bulb begins to flicker The reason for that is the power supply of the LEDs, which after tearing it apart Turned out to be build around a 9918C LED driver IC.

This is a type of switch mode power supply which comes with a rather big capacitor on the inputs as a power reservoir Now as soon as the mains voltage is somehow available, the capacitor on the input gets charged up quickly And the stored energy from it is enough to illuminate the LEDs for the rest of the half cycle of the mains voltage That is why for the most part of the phase angle control, the brightness stays the same since even a small mains voltage section can deliver enough energy only with a very big phase angle the delivered energy is not enough and Thus the LEDs turn off during the next half wave which results in this flickering behavior.

I also tried the same test with the other non dimmable LED light bulbs Which sometimes offered a bit more brightness control since in the end, we do control the delivered energy amounts but most of the time they behaved like the first light bulb What I realized though while testing those bulbs was the first big problem of such TRIAC dimmers. with traditional halogen light bulbs, We got a constant current flow across the available mains voltage which since it is bigger than the holding current of the TRIAC, Keeps it open But with the LED bulbs the current quickly decreases to almost zero after the impulse Which means since the TRIAC is no longer activated through its gates, and we undershot the holding current the TRIAC is closed, and another charge up of the LED power supply would not be possible The second problem i noticed occurred while testing my almost six year old LG light bulb Whose current waveform looked pretty extraordinary and quickly led to the destruction of my TRIAC The problem was probably the electromagnetic interference filter of the bulb Whose job is to reduce noise generated by the switch mode power supply Due to the big voltage change in a short time, We got a huge current spike due to the capacitors And since there are inductors available, we got ourselves an oscillator, which can lead to big problems So getting rid of the sharp voltage edge would be a good idea since it would also reduce the operating noises generated by, for example the dimmable LED light bulb I got from Amazon Those dimmable like bulbs, by the way All dimmed perfectly fine with my DIY dimmer since their current waveform pretty much all looked like this This time we got a steady current flow whose length and thus delivered energy to the light bulb Can be shorted with a bigger phase angle which results in a dimmable brightness This is accomplished by not relying on simple and cheap power supplies But instead using a more sophisticated and expensive LED driver IC Like the LM3445 TRIAC dimmable LED driver, for example.

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Whose datasheet can tell you lots of secrets about such dimmable LED driver circuits So when you buy properly labeled dimmable LED light bulbs They should function well with leading-edge dimmers we use so far which like the name implies offers a leading edge voltage But like we already determined, such a leading edge creates current spikes Electromagnetic interference and noise and since such designs are also often based around a TRIAC We also got the minimum holding current issue To get rid of all those problems we can get ourselves and modern trailing edge demo from eBay for around 50 euro After receiving it I hooked it up to mains voltage and connected the differential voltage probes as well as the current clamp and As you can see on the oscilloscope This time we got our edge at the end and thus the amazon dimmable bulb We got noise problems with earlier is now completely silent But keep in mind that such trailing edge dimmers do not make not dimmable LED light bulbs suddenly dimmable But they are definitely more likely to offer better dimming results The only question remaining is how such a voltage waveform can be achieved electrically Since a TRIAC with it's given holding current can no longer be used To find that out, I opened up the commercial trailing edge dimmer and found a pretty complex circuits build around a microcontroller The two main power switches are this time those FCPF11N60 MOSFETs Which are connected in series to the power line and load like it's shown here When a voltage is applied to the gates the current can flow through them and as soon as there's no control voltage applied The current flow is interrupted Which makes trailing edge control possible.

Now in order to convert my DIY dimmer into a trailing edge one I firstly searched for suitable high voltage MOSFETs and found this IRF840 Only problem was that five volts as the gate voltage is not the optimal solution So I replaced the 5 volt power supply of my design with a 12 volt one Added a 5 volt linear regulator for the control electronics and a 4N35 optocoupler to activate/deactivate the power MOSFETs according to this final schematic of the microcontroller trailing edge dimmer I then designed a proper PCB for it And as soon as that was done, I exported the Gerber files and ordered five pieces of my PCBs from JLCPCB for only two dollars plus shipping after week of waiting my PCBs arrived and looked like always, very attractive So much so that I could not resist soldering all of the required components immediately to the board Which took me around half an hour, as soon as that was done I hooked up my Arduino Uno as the programmer to the ISP header and adjusted the code for the dimmer by simply activating the MOSFETs at every zero crossing point and then turning them off after a set time by the timer with those changes implemented I uploaded the code to the ATTiny and then hooked the circuits up to mains voltage and the light bulb After powering the circuits the dimming of the halogen light bulb was no problem as well as he dimmable light bulbs including the light bulb I originally got from my friend which started this whole LED dimming adventure On the oscilloscope, we can also see that the trailing edge dimming works flawlessly Now at this point I could end this video and you should have already learned quite a lot about the subjects but as a bonus, I searched for a simpler trailing edge dimming circuits and found this FL5150 MOSFET AC Phase Cut Dimmer controller IC, whose functional principle you can find in its data sheet according to its given example circuits I created my own schematic in easyEDA and turned it into a proper PCB design Which I then obviously ordered through JLCPCB Just like before, the PCBs looked promising So I soldered all of the components onto its and afterwards tested these circuits which also worked like a charm and the cool feature of this IC is that it does not only offer a trailing edge dimming control But also a leading edge dimming control by simply flipping a switch on the PCB so feel free to rebuild any of those circuits, but make sure to handle mains voltage with care As always thanks for watching.

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