**Building an Inverter: A Cautionary Tale**

As you may know, building an inverter to power your phone charger and a light bulb from a 12V lead acid battery is a reasonable thing to do. However, I decided to take the cheaper route and search for an inverter schematic that can be built at home. My intuition told me, "Why not? It's just a few components, how hard can it be?" Little did I know, I was about to embark on a journey of discovery and frustration.

My search yielded a simple circuit diagram that seemed like a great idea at the time. The circuit consisted of a transformer, two MOSFETs, resistors, and capacitors. It was easy to understand, and I was convinced that it would work flawlessly. After all, who needs expertise or experience when building an inverter?

I began by assembling the circuit, using the components as instructed. As I connected the wires and placed the components on a breadboard, I couldn't help but feel a sense of excitement and pride. This was going to be easy!

The first issue I encountered was with the input voltage. The 12V lead acid battery would slowly discharge over time, causing the output voltage to fluctuate wildly. It seemed like the circuit was responding well to the changing input voltage, producing a square wave that looked remarkably like a sine wave.

However, as soon as I started using my smartphone to charge it, the output voltage began to drop, and the frequency of oscillation changed dramatically. The MOSFETs were alternately pulling current through the low-voltage transformer winding, creating a higher AC voltage on the high-voltage side of the transformer. And to my surprise, the oscilloscope showed that this was indeed happening!

But as I continued to experiment with different loads and input voltages, several problems became apparent. The frequency and voltage drift were significant issues, especially when connecting a load. Moreover, the MOSFETs were experiencing unwanted and parasitic inductances, causing them to overheat.

To solve these problems, I had to add an RC snubber circuit to the drain source path of the MOSFETs. This reduced the voltage peaks to relatively safe levels, but it also highlighted another issue: the sharp square wave voltage used in the circuit was causing ringing and spikes on the output voltage.

To mitigate this problem, I attached a constant load to the output, using a 40kΩ resistor network. This reduced the voltage peak values to around 350V, which seemed like a manageable level.

Despite these fixes, several minor problems remained. The circuit did not produce a proper sinusoidal output voltage, and there was no undervoltage protection that would safeguard the 12V battery from overdischarge. Furthermore, most people would use the wrong kind of transformer for this build, leading to collapsed output voltages or funny-looking waveforms.

In conclusion, while my intention was not to bad-mouth the website where I found the schematic, I hope this experience serves as a cautionary tale. Building an inverter from scratch can be a fun and educational project, but it requires expertise, patience, and attention to detail. Don't believe that every circuit you see works flawlessly – always expect the unexpected!

WEBVTTKind: captionsLanguage: enSo you got a 12V lead acid battery that youwant to use to power your phone charger inorder to obviously charge up your phone andyou also want to power this light bulb whileyou are at it.The reasonable thing to do would be to simplybuy a proper inverter, connect the battery,hook up the appliances and that's it.But you are not reasonable and want to savemoney which is why you went to google imagesinstead and looked for inverter schematicsthat you can build at home for cheap and thisone right here attracted your attention.You thought to yourself: „Only 7 components,what could possible go wrong?“Well, in this episode of testing circuitsI found on the internet I will show you exactlywhat can go wrong by building up the is circuit,properly testing it and finding out what problemsthe design comes with that the schematic willcertainly never tell you.Let's get started!This video is sponsored by Brilliant whichis a website and app that I would describeas an interactive storyteller where you canlearn all about math, science and computerscience.Now since my videos require knowledge aboutelectricity, I had a closer look at theirelectricity and magnetism course and I waspleasantly surprised that while this coursewas still easy to understand, there were lotsof important details and interactive puzzlesthat kept everything very interesting andengaging.So if you want to try out Brilliant for yourselfthen go to Brilliant dot org /GreatScott andsign up for free.And the first 200 people that sign up foran annual subscription through this link willalso get a 20% discount.First off, I drew my own schematic based onthe chosen one.But I had to replace the BJT and MOSFET componentswith other types since I didn't have the intendedcomponents lying around which does not reallymatter though since my replacement parts arevery similar to the original.And with that being said I gathered all thecomponents mandatory for the circuit.For the transformer part I went with thissafety transformer that comes with a 230Vwinding as well as a 24V winding that I cancentre tap and thus the transformer shouldbe suitable for the circuit.Now the cost of those components equals around40$ which is kind of budget friendly.And building up the circuit on a breadboardaccording to the schematic was also simpleand fast to do as long as you remember thatyou can find the pinout of the more complexcomponents in their data sheets.So after double checking all of the connectionsit was time to test the circuit with my labbench power supply.And it seems like the circuit didn't feellike working yet.But after adding a bit of capacitance to theinput of the circuit as a buffer, the circuitconsumption seemed normal and thus it wastime to hook up the 12V battery.The good thing was that nothing blew up andall the components remained pretty cool whichwas a good sign.We were also getting around 202 V AC RMS onthe output which basically means the circuitworks; but I also have to warn you that workingwith such mains voltage levels can lead tofatal injuries if not handled correctly.OK, so after connecting the light bulb tothe system and powering it all up once againyou can see that it does light up withoutany problems.At this point you might be thinking that poweringthe phone charger should work just fine aswell, right?Well, before doing that let's rather findout how this circuit works and why it is actuallynot a good inverter even though it seemedlike it so far.Luckily the schematic I found on google imagesled to a website where the circuit and itsapplication is explained in detail.And one of the first things I read was thatthe circuit is a multi vibrator that createsa closely resembling sinusoidal waveform.So let's just hook up my oscilloscope to thecircuit and have a look at those sinusoidalwaveforms...............OK, I do not feel like trusting this websiteany more so let me explain how this astablemulti vibrator circuit functions.When 12V is first applied to the circuit,both BJTs and MOSFETs are turned off; butcurrent can flow through each base of theBJTs.But since every transistor is a bit differentone of them will conduct first and thus turnon which in my case is the right one.At that moment the right MOSFET Gate is pulledto GND which means it is off, while the leftMOSFET Gate is connected to a now freshlycharged up capacitor and thus turned on.Now the right capacitor on the other handis connected to GND on the right side andthus slowly charging up through the left sidewhich is connected to the base of the leftturned off transistor.And at a certain threshold voltage, currentwill flow though its base and turn it on.At this point the left capacitor will getdischarged and thus the left MOSFET turnsoff, meaning that the capacitor now has tocharge up with the reverse polarity whichtherefore means that the right BJT turns off.That however means that the right capacitornow quickly got charged up with a reversepolarity and therefore now the right MOSFETgate sees a positive voltage and thus turnson.This process basically repeats over and overagain creating a more or less nice lookingsquare wave at the gate of the two MOSFETs.The frequency at which this oscillation occursis set by these resistor and capacitors andcan be calculated approximately like this.In my case the used components deliver a frequencyof around 50Hz which is the same as with themains AC voltage here in Germany.Last but not least we got the two MOSFETswhich with the frequency of 50Hz pull currentalternately through the low voltage transformerwinding which therefore should create a higherAC voltage with the same waveform on the highvoltage side of the transformer.And according to the oscilloscope it doesjust that, so overall the circuit works.BUT after doing a couple of tests I noticedsome rather big problems.First off, if we alter the input voltage whichis pretty common considering that a 12V leadacid battery also discharges overtime, theoutput voltage as well as the frequency ofit changes quite a bit due to the obviousnature of the circuit by using a transformerand an RC timer circuit.This frequency and voltage drift does alsooccur over time without changing any voltagessince the components do heat up a bit as well.And last but not least, the output also changesnoticeably if we connect a load.This can be a problem for sensitive electronicand can be pretty much only solved if we monitorthe voltage and frequency on the output andadjust the input voltage as well as maybethe resistance of the RC timer circuit witha digital potentiometer in order to alwayshit that 50Hz 230V RMS sweet spot.So basically put, a feedback circuit is missing.Next we can observe while looking at the drainpin of the MOSFETs, that there is a big voltagespike with values of up to 65 to 70V whichis above the maximum rated drain source voltageand thus this could destroy the MOSFETs overtime.The reason could be unwanted and or parasiticinductances and to solve that problem we wouldhave to add an RC snubber circuit to the drainsource path.After doing just that the voltage peaks werereduced to relatively safe levels.But if we have a closer look at the outputvoltage of the transformer than we can alsoobserve spikes with values of around 650Vthat have the potential to kill your appliances.The reason for this ringing is the utilizedsharp square wave voltage which by the wayyou can hear way more through the transformerin comparison to for example a sinusoidalwaveform.To get rid of this problem we would have toattach a constant load to the output for whichI chose a 40kΩ resistor network .As you can see it reduced the voltage peaksto values of around 350V.Now these 3 issues were my main problems withthe circuit of which 2 we easily solved.But there are still tons of minor problemslike not featuring a proper sinusoidal outputvoltage or not having an undervoltage protectionthat safes your 12V battery from getting overdischarged.And since most people will be using a wrongkind of transformer for this build, most ofthe times the output voltage will just collapsewith certain power loads or in some casescreate funny looking waveforms that can alsoharm your electronics.So all in all while your phone charger mightpower up with your online schematic inverter,it can still do some serious damage like inmy case completely frying my old smartphone,no joke.But in the end I want to say that my intentionhere was not to bad-mouth the website thisschematic came from or all of the other schematicsyou can find with google images.I think it is great that you can find so muchelectronics knowledge on the internet nowadaysbut I just wanted to prove to you that youshould not believe that every circuit yousee works flawlessly.And with that being said I hope you enjoyedthis video and if you find other schematicsthat I should test then let me know in thecomment section below.As always don't forget to like, share, subscribeand hit the notification bell.Stay creative and I will see you next time!