Recently I bought myself this big autotransformer. In case you do not know then let me tell you that such a device can take mains AC voltage on the input and transform it into a lower adjustable AC voltage on its output. This can come in handy when you want to test your DIY electronics project with a lower AC voltage like I did during a previous video with a Tesla coil.

Now I actually already owned a smaller autotransformer before but it was only capable of delivering 500W on its output which is much less in comparison to the new one that can output 3000W. And let's not talk about how I connected mains wires to my old autotransformer which is now way easier and safer to do with the new one. So, I had to upgrade my setup to accommodate the increased power output of the new autotransformer.

So, when I first started using the new autotransformer, I noticed that it would sometimes trip the circuit breaker. This was due to the initial current spike that occurs when the transformer is powered up near the zero point of the mains voltage waveform. The initial current spike can be quite high and causes the circuit breaker to trip. To solve this problem, I needed a circuit that turns on the appliance near the zero point of the mains voltage waveform.

The easiest way to do this would be with such a solid state relay. After connecting it to mains voltage and the autotransformer according to this little schematic, we can see that whenever I hold down the pushbutton on the input side, the autotransformer powers up without a problem. If we observe the applied voltage waveform on the oscilloscope then we can see that the transformer does in fact only switch on near the zero point.

However, even though this solution works for me, I was not yet happy with it because every inductive load is different and your might even trip your breaker when it switches at the zero crossing point. Instead, I wanted to create a proper soft starter that uses phase angle control in order to slowly increase the RMS voltage value for the connected load. That's why I looked through all the components I had lying around and came up with this schematic.

The heart of the system is an Arduino which notices when a zero crossing point is reached and then it waits a slowly decreasing timedelay before it activates a triac. The zero crossing detection is achieved by an optocoupler which at low voltage values at the mains input side does not activate and thus the output of it at the Arduino side gets pulled up. This is how the detection looks like on the oscilloscope.

I also added a red LED that indicates that the system is ready, a push button to start the soft start sequence and a green LED to indicate that the soft start process is over. With the schematic being completed, I gathered all of the required components for the build and started soldering them onto a perfboard. The circuit assembly was not really hard to complete but all in all I spent around 3 hours working on it before it was done.

Next, I moved on to the Arduino programming which was also pretty straightforward except maybe the timer configuration for the phase angle control delay. You can read comments about those lines if you download my code from the video description. After uploading the code, I wired everything up for a first test with a light bulb because this way the soft start is easier to observe.

As you can see the light bulb slowly lights up and on the oscilloscope we can see how the phase angle control works flawlessly. Next, I did the same test with the autotransformer which also passed it without any problems. That's why I continued by designing a housing for the project in Fusion 360, 3D print it with my 3D printer, attached an outlet to it along with an IEC socket for the input, added the push button and LEDs, wired everything up and finally closed it all with the lid.

As you can see everything still works fine without any problems. With that being said I hope you enjoyed this video and learned something new along the way. If so don't forget to like, share, subscribe and hit the notification bell. Stay creative and I will see you next time.

WEBVTTKind: captionsLanguage: enRecently I bought myself this big autotransformer.In case you do not know then let me tell youthat such a device can take mains AC voltageon the input and transform it into a loweradjustable AC voltage on its output.This can come in handy when you want to testyour DIY electronics project with a lowerAC voltage like I did during a previous videowith a Tesla coil.Now I actually already owned a smaller autotransformerbefore but it was only capable of delivering500W on its output which is much less in comparisonto the new one that can output 3000W.And let's not talk about how I connected mainswires to my old autotransformer which is nowway easier and safer to do with the new one.So in a nutshell everything is better nowand I just wanted to show off, right?Well not exactly; because often times whenI plug the new transformer into an outlet,my circuit breaker trips.Only due to pure luck I can power up the newautotransformer from time to time.And that is why in this video I will not onlydemonstrate why my circuit breaker keeps trippingbut also how I created a DIY AC softstarterwhich like the name implies can softly startup my autotransformer every time without trippingmy circuit breaker.Let's get started!This video is sponsored by JLCPCB!Feel free to visit their website JLCPCB.comto not only find out what awesome PCB andAssembly services they offer but also to easilyupload your Gerber files and thus order affordableand high quality PCBs quickly.First of why not open up my old autotransformerin order to find out how it works.As you can see it mainly consists of one bigferromagnetic core around which partly enamelledcopper wire is being wound.And I say partly because a sliding brush seemsto make electrical contact with the wire whichis connected to the output terminal of thetransformer.The input terminal however is connected tothe whole length of the copper wire whichtherefore means that the diagram of this transformerlooks something like this.Now like probably already mentioned a thousandtimes before we are dealing with a transformerhere whose job it is to obviously to transformhigh mains AC voltage on the input to a lowerAC voltage on the output.And if you have no clue how a transformerworks then make sure to watch my video aboutthe topic.But anyway this autotransformer type is specialbecause it only uses one winding instead oftwo like a traditional transformer does.Its functional principle however is stillpretty similar with the output voltage beingthe multiplication of the input voltage withthe the turns ratio.And since we are using a moveable brush forthe secondary turns we can fine tune the outputvoltage very precisely.This design also has the advantage that itrequires a smaller core and fewer windingsin comparison to a two winding transformerwhich makes it smaller, lighter and cheaper.But on the other hand there is no electricalisolation between the input and output sideof the transformer which means that if youhook up mains voltage incorrectly then youcan get a nasty shock if you are not careful.And that brings me to a small warning thatwe are dealing with mains voltage here whichif mishandled can lead to major injuries.You have been warned.But anyway now that we more or less understandhow the autotransformer functions, why exactlydoes it trip my circuit breaker?It must have something to do with the currentbecause protecting from over-current is inthe end the main job of circuit breakers.So to investigate I hooked up my energy multimeterto my big autotransformer.And after powering the system, the transformerseems to draw an apparent power of 27VA with13W of real power, so nothing suspicious here.Next to further investigate I added a 1 ohmcurrent shunt in series to the live wire whosevoltage I monitored with my oscilloscope.Because as you might know the current flowingthrough such a resistor is the voltage acrossit divided by the resistance and since theresistance is 1 ohm, the monitored voltagewill equal the flowing current.But anyway after once again powering up thesystem, my oscilloscope found the culpritby recording a fast voltage peak with a maximumvalue of 38V so 38A.Now not only my autotransformer does createsuch a current spike problem at the powerup though because all kinds of motor do alsocreate a similar problem.The reason is the inductive characteristicof those devices.You see when no current flows through ourcoil or winding then the initial current flowis only limited by the resistance of the windingwhich is in my case 2.2ohm.So if we due to back luck connect the transformerto the grid at the highest peak of our mainsvoltage waveform which equals a value of 325Vthen we would get an initial current spikeof 148A which is why the circuit breaker trips.But, and here is my first solution, if wepower the transformer near the zero pointthen the initial current spike will be muchlower and hopefully not trip the breaker.And due to the now flowing current, the magneticfield also builds up which opposes the currentflow and thus creates an additional resistancewhich will lower the current to the previouslymeasured values.And fun fact; my smaller autotransformer didnot come with this problem since it featureda way higher real resistance because the morepower your transformer has to output, thelower its real resistance will be.OK, now so to solve this problem we basicallyneed a circuit that turns on our appliancenear the zero point of our mains voltage waveform.The easiest way to do this would be with sucha solid state relay.After connecting it to mains voltage and theautotransformer according to this little schematicwe can see that whenever I hold down the pushbutton on the input side, the autotransformerpowers up without a problem.And if we observe the applied voltage waveformon the oscilloscope then we can see that thetransformer does in fact only switch on nearthe zero point.The reason for that can be found in the schematicof the solid state relay.As you can see it uses a triac in order toturn on the load but it can only do so nearthe zero crossing point which basically solvesmy problem.And if you want more information about suchsolid state relays then make sure to watchmy video about it.But anyway even though this solution worksfor me, I was not yet happy with it becauseevery inductive load is different and yoursmight even trip your breaker when it switchesat the zero crossing point.So instead I wanted to create a proper softstarterthat uses phase angle control in order toslowly increase the RMS voltage value forthe connected load.That is why I looked through all the componentsI had lying around and came up with this schematic.The heart of the system is an Arduino whichnotices when a zero crossing point is reachedand then it waits a slowly decreasing timedelay before it activates a triac.The zero crossing detection is achieved byan optocoupler which at low voltage valuesat the mains input side does not activateand thus the output of it at the Arduino sidegets pulled up.And this is how the detection looks like onthe oscilloscope.Then I also added a red LED which indicatesthat the system is ready, a push button tostart the soft start sequence and a greenLED to indicate that the soft start processis over.And with the schematic being completed, Igathered all of the required components forthe build and started soldering them to aperfboard.The circuit assembly was not really hard tocomplete but all in all I spent around 3 hoursworking on it before it was done.So next I moved on to the Arduino programmingwhich was also pretty straightforward expectmaybe the timer configuration for the phaseangle control delay but you can read commentsabout those lines if you download my codefrom the video description.Anyway after uploading the code, I wired everythingup for a first test with a light bulb becausethis way the soft start is easier to observe.And as you can see the light bulb slowly lightsup and on the oscilloscope we can see howthe phase angle control works flawlessly.So next I did the same test with the autotransformerwhich also passed it without any problems.That is why I continued by designing a housingfor the project in Fusion 360, 3D printedit with my 3D printer, attached an outletto it along with an IEC socket for the input,added the push button and LEDs, wired everythingup and finally closed it all with the lid.And as you can see everything still worksfine without any problems.With that being said I hope you enjoyed thisvideo and learned something new along theway.If so don't forget to like, share, subscribeand hit the notification bell.Stay creative and I will see you next time.