AC PWM Dimmer for Arduino

Before you decide to build this, you may want to have a look at acolomichi's design as well

Warning: this project uses deadly voltages, as well as a sizeable capacitor to store these deadly voltages. Even after you disconnect the circuit from the grid, the capacitor still can give you a jolt to remember!!

Over 3 years ago, I published a simpel TRIAC AC dimmer for the arduino. That proved to be a very popular design. Yet in spite of the simplicity of the circuit the software needed was a bit complicated as it needed to keep track of the zero crossing of the AC signal, then keep track of the time and then finally open the TRIAC. So to avoid letting the arduino just wait for most of the time, an interrupt and a timer were necessary.

So why can't we just use PWM, like with LED's? Well, I explained that in that instructable, but there are possibilities to do that. Someone looking for that would no doubt end up at design by Ton Giesberts/Elektor Magazine that can do PWM of an AC source. This design is on Instructables as well.

That will work, but in spite of my admiration for Giesberts and Elektor, there is something fundamentally wrong with that circuit. I think it is necessary that I explain what is wrong before I come up with improvements. If you are not interested in the technical details, just skip to the next step.

At first glance, the Giesberts circuit seems like a complicated circuit, but we can bring it back to 2 or 3 components: A lamp, and a switch, but as in fact the switching is done in DC rather than AC, it becomes a lamp, a bridge rectifier and a switch. That switch, which is in fact the MOSFET and the components around it is controlled by the Arduino (or PIC or whatever). So, switching that on and off in a certain duty cycle will switch the lamp on and off and if done fast enough the lamp won't be seen anymore as flickering, but as being dimmed, similar as we do with LED's and PWM.

So far so good. The theory behind the circuit is sound. However, the MOSFET needs a voltage on its gate to be switched on and as we cannot get that from an arduino for obvious reasons (it is only 5 Volt, which isnt enough AND you don't want your arduino to be connected to the mains grid), Giesberts uses an optocoupler. That optocoupler still needs a DC voltage and Giesberts is using the to DC rectified AC voltage for that.

And that is where the problems start, because he is feeding the gate from the MOSFET, with a voltage that is shorted by that same MOSFET. In other words, if the MOSFET is fully opened the DC voltage coming from the rectifier is completely shorted. Therefore there will be no voltage anymore to put on the gate and the MOSFET will block again. This effect might not be so outspoken by a low dutycycle (= lamp on a low intensity), because of the presence of C1, that will retain its charge for a while and will be receiving new charge thanks to the low dutycycle, but at 25-80% dutycycle the voltage on C1 just cannot be sustained anymore and the lamp may start to flicker. What's worse is that at moments that the voltage on the gate drops, for a while the MOSFET will be still conducting, but not be fully saturized: it will slowly go from its nominal 0.04 Ohm resistance to infinite resistance and the slower this goes, the higher the power that needs to be dissipated in the MOSFET. That means a lot of heat. MOSFETS are good switches but bad resistors. They need to be switched ON and OFF fast. Currently the circuit heavily relies on D1 to keep the voltage on the gate of T1 at acceptable limits while the voltage is swinging between 0 Volt and Full peak At peak the rectified voltage is 230x1.4=330V The average rectified voltage is 230x0.9=207V

If we forget about the smoothing effect of the capacitor for a while and presume the optocoupler to be fully open the average voltage on the capacitor would be 22/88 * 207 =52 Volts and in peak 22/88 * 330= 83 Volts. That it is not is because of D1 and the fact that the MOSFET will short the Voltage.

If the optocoupler is not in saturation and its impedance therefore infinite, the capacitor C1 would charge up to full rectified voltage if not for D1. On average 3mA will flow through R3,R4 and R5 (207-10)/66k which equals a power consumption of 0.6 Watt in the resistors R3,R4, R5

Improvements

The problems mentioned with the Giesberts circuit can be remedied by putting the lamp somewhere else: remove it from the AC line and put it in the Drain of the MOSFET. For the lamp it doesnt really matter if it is receiving DC or AC. You could make more improvements, as now there is no need to cater for a a voltage swinging between 0 and 330 Volt

Step 1: AC PWM Dimmer: IGBT or MOSFET

But as I was changing the design, I might as well take it a step further and use an IGBT (Insulated Gate Bipolar Transistor) Simply put, an IGBT is a device that is a MOSFET at its gate and a bipolar transistor at its Collector and Emitter, making it an ideal switch. Thus we can come to the following circuit:

The IRG4PC30 acts as a fast switch that either switches the lamp on or off. It needs about 12 Volt on its gate to do that. The voltage divider R1/R2 should put about 13-15 volts* max on the Gate of the IGBT, switching the lamp fully ON. As there might be some fluctuations on the grid 4k7 is a safe value. If you want to be safe, make sure you have an IGBT with a Base Emmitter breakdown voltage of >= 20 Volt and put a zenerdiode of 15 V parrallel to R2. Possible IGBT's are IRGPC40W or IRG4PC30, but basically any will do provided they have a Base emmitter voltage rating of at least 20 Volts When the optocoupler receives a signal, it opens and pulls the voltage on R1/R2 to zero, effectively closing the IGBT. The PWM signal of an Arduino is faster than the 50Hz Frequency so you will basically see the PWM signal modulated on the 50Hz rectified sine wave, making the effective voltage lower. This circuit is ONLY for incandescant bulbs. It is NOT for any inductive load as it is DC biased. With regard to the capacitor C1, I have tested it with 100uF but will probably work with lower capacity as well.

• Although the average voltage will be 230*0.9, C1 may eventually charge to 310-330 hence 4k7 is a safe value.

Reader acolimitchi pointed out to me that if you add the zener, the 6k8 resistor isnt really necessary anymore, which is true, so the circuit becomes like this:

With regard to MOSFETs vs IGBTs both have their pro's and con's and the voltage and switching rate this circuit is operating under may just be in an overlap of both spectra. Using a MOSFET rather than an IGBT is therefore possible. MOSFETS are generally also cheaper than IGBT's. A tried and tested MOSFET is the STP10NK60Z (Thanks Pavel). This MOSFET has a gate-source breakdown voltage of 30 Volt and has clamping diodes protecting the the gate. Another one that worked quite well is the relatively cheap IRF 730. MOSFETs usually need a bit of a higher voltage than IGBTs to switch so a 6k8 resistor should be fine. If you use a MOSFET without clamping diodes a zener of 15 Volt is adviseable.

I couldnt embed a video, but you will find it here. It is a 60 Watt lamp. The slight flicker you may see when the lamp is turned up, is an artefact. It is caused by my cellphone camera trying to adapt to the changing light intensity

Conclusion: stick to the MOSFET, they are on average 4 times cheaper than IGBT's

BOM:
Bridge rectifier. I used a 1 amp 400 Volt DIL pack 0.25 euro
Diode 1n4001 or 1N4007 0.10 euro
Capacitor 100uF 350-400 Volt 1.36 euro
resistor 100k 0.5-0.6 Watt (actually it dissipates abt 480mW max) 0.12 euro (play it safe with a 1 Watt resistor)
resistor 6k8 1/4 Watt 0.10 euro
Zener 15 Volt 0.5 Watt 0.08 euro
MOSFET IRF730 or STP10NK60 0.58 euro
Optocoupler 4n35 0.25 euro
Resistor between 330ohm-470ohm (possibly even 1k dpending on the Optocoupler) 0.10 euro

So, alltogether at single piece retail prices the cost is 2.94 euro

Step 2: AC PWM Dimmer for Arduino: Thoughts

Without any input signal the lamp will burn at max.

If you notice that When writing a zero PWM value to the circuit the lamp burns a bit less than when you switch off the Arduino, that means that the 4n35 is still opened a bit. Play around with t470 Ohm serial resistor, maybe you need a higher value.
The 100uF 400 Volt can be quite big. A 50 uF may work as well and maybe even a smaller value. However, you do not want the value to be too small thus as not to be able to deliver enough voltage for the Gate of the MosFet in fast switching.

Should you wonder if perhaps you can use a capacitor of a lower voltage -say 25 Volt- and connect that over the zener/6k8 resistor in order to replace the 400 Volt one... maybe that will work, but the 4N35 will make it lose its charge almost instantaneously and due to the large RC time (remember, there is 100k resistor) it might not have time to fully recharge again. At least it will make keep the MOSFET in its resistive phase longer, leading to extra heat development.

The Arduino has a PWM frequency of about 500 Hz. That is ample enough. My guess is that around 100 would be more than enough. The Attiny85 delivers I think 280 Hz on its PWM pins. Software ofcourse can always elevate that if necessary.

This dimmer is not suitable for inductive loads

This dimmer does NOT need zero cross detection.

Heat development:
I tested this with a 60 Watt lamp at full brightness, without any heatsink: the temperature rose with 9 degrees above ambient after half an hour and an hour.
Then I tried with continous dimming from zero to full and back again.: The temperature rose with 10 degrees above ambient after about 10-15 minutes and stayed like that for the hour I tried.

With a 150 Watt halogen spot the temperature went up 15 degrees. It reached max temperature after about 10 minutes and then stayed the same for the hour tested.

This was measured with a DHT11 sensor directly clamped to the MOSFET

Comments

[nsummy]

I'm somewhat new to building circuits, and considering this was made 6 years ago I am wondering if there has been improvements to mosfets. Case in point: There are plenty of mosfets out there that have a max voltage gate threshold of 4 volts or less. Is it possible to skip the voltage divider and send the pwm directly from a 5v pin on an arduino?

[MoisesBoj]

it is possible to change the mosfet, as long as it can withstand the current at the voltage you give it.
you cannot send the pwm pulse directly without the voltage divider because of how the optocoupler works, you would burn the 4n35 led, see how optocouplers work.

[JavierP27]

Hey, could you let me know how what I need to do to check this circuit with the arduino? I mean, the arduino program

[MoisesBoj]

Arduino ide → File → Examples → 01. Basics → Fade

[MoisesBoj]

this is the circuit i built and tested for 110v-120v ac

[FranTIC]

Hey guys! I just made the cicuit but something is very odd. On the output it only gets to 100VAC and on the gate I only have around 5V. At 0% PWM the Gate goes down to 0VAC and on 100% also 0VAC but the output stays at around 95-100VAC. Does anyone have an Idea what is wrong?
Also if I measure the output voltage with my voltmeter the lower I go with the PWM the loader it begins to buzz. Any thoughts on this?
Thanks in advance.

[FranTIC]

So I just noticed that the voltage is not AC but DC of course.... after powering a light bulb it functions very well but the light is flickering at a PWM frequency of 250Hz. What can be done to reduce the flickering?

[mi-y]

Hi, thank you so much.
I made this circuit and it works so well.
I've got a problem by the way.. The light bulb is always on even when the microcontroller is off! Is there any way to make sure that the light won't turn on until the microcontroller is on and ready to perform the dimming action?
Thanks

[FranTIC]

NVM.... I just noticed that it actually is DC not AC voltage. Just plugged in a light bulb and it is working like a charm. Just a little bit flickering but this is ok since I want to power a heating cable.

[renearduino]

Olá amigo, muito bom seu circuito! Fiz um dimmer com Triac para controlar a velocidade de um motor AC de 1CV, mas notei que o Triac esquenta muito. Será que o seu circuito funcionaria neste caso? Agradeço sua resposta (renearduino)

[Abdalrahman_]

Hi Bloke, thanks for this great circuit, if I changed the MOSFET and the bridge with higher power ones and add a suitable heat sink, so can I use this circuit for higher power loads like 220 AC heaters (about 5000 Watt)?

[wolfkeeper]

Looks good! I was thinking about using this circuit but uprating it to make an immersion heater controller for use with solar panels. I figure that together with a house import/export sensor, dimming a standard immersion heater down to 500 watts through a smoothing inductor should be about right for that application. I'll try to do an instructable if I get it to work.

[diy_bloke]

Looking forward to that instructable. Was just wondering though that if you use solarenergy you do not need the AC part.
A FET with some PWM would suffice ( but ofcourse a large part of my circuit is just that)

[BerryZ]

1st off all I love this post, I made your old design once. but I think this is much better no more interupt pins ;-)

So I try to make the circuit 2 channels to drive 220v Cooling fans for a reef tank.
I'm bit new so I was wandering if someone could have a look if This will work.
most of the parts I used double because I'm not sure if 1 Cap and the 15v zenner and diode would work. Also I added 2 Relay switches just to take sure power is off when not needed.

Cheers

[diy_bloke]

Sorry for my late reply. Thank you for your kind words
Your circuit is a bit small on my phone so i can't say much about it but i hope you have the answer already

[diy_bloke]

Thank you for your kind words. Your circuit is a bit hard to read, but it should be OK if you just copy the circuit behind D1

[hassanfire14]

How can i alter the design so that it may work with LIGHT bulb and FAN

[wolfkeeper]

Fans are inductive, so you'd need to add a flyback diode across the load to the power rail or something like that otherwise you'll overvolt the semiconductor from the kickback during switching transients.

[deva_rishi]

I am having an issue with the 100uF (450v i have) Capacitor, maybe it is just broken, but then how did i break it ? I am gonna try another one, but it seems that the current it draws to first fill up exceeds the 1.0 A of the 1n4007 (i am also using 4x1n4007 as a rectifier, and half of that blew out earlier as well, i was doubting that they were actual 1n4007's but my magnifying glass confirms they are.) I am thinking it may be a good idea to add an extra current limiting resistor between the 100K resistor and the diode. I am using a p10nk60zfp mosfet, and really i am just intending to control filament led bulbs with it. I first went for the zero-cross dimmer first, got that to work, but adding more channels started to cause to much noise on the main input, and the filament leds should work fine if i go for a 100hz PWM signal. But first of course i want it to work with the Tap fully open.

[deva_rishi]

Ok so now i made the 15v power supply inclusive. i built a small capacitor based power supply (technically speaking the diode -100uF -100K - 15.1v zener is one but expressed my objection) I decided to go for a symetrical design on the AC - side, and since i didn't have any 1uF capacitors that are rated for 400v, but did have a couple of 100nF - 600v ones i decided to go for those.Those power supply circuits claim they will provide about 30-50mA, and i figured i hardly need any. The 2 resistor across the capacitors are there to drain the energy out of the capacitors when you unplug the power (or you may get a jolt from it when you touch the terminals) and in the original design they were 1M already, i could up them to 10M and still have the same drain time, but i didn't have any. So left them as they are. then i tested it. No ripple on the on the 15v rail, so i guess 47uF (25v i have ) is enough. And when i put an LED and a resistor, i got to a 3mA maximum which i had at about 1K, I used the 15.1v zener from the other project, i don't know what the rating should be, but i recon 500mW should be ok, though when i walked into the corner shop they only had 1.3W, and that's what's in it. When i measured the voltage between the GND of the power supply and the GND of the High-Power DC rail it showed 75v, (DC) and then i was happy i went for the symetrical design, but just i case i decided put a fuse holder and a fuse in between first (200mA) just in case. Then i connected it all and all is good. I am keeping the fuse on the AC line, but i clearly don't need the one on the GND line. I still have the power supply on the breadboard, and i don't like high power on a breadboard, the leads can come loose, there is a lot exposed and a momentary lapse of focus and you put your fingers on something before you unplug. Also the breadboard can have bad connections and particularly with AC a bad connection can cause sparks and crackle. fix that later. I've added the schematics, i left the High-Power Rail as it is, and really want to thank you for this learning experience, i think i finally have this capacitor- based- power supply thing down,