Cheap DC To AC Inverter Circuit Using Power Transistor

Thread Starter

q12x

Joined Sep 25, 2015
622
(just curious)
- How reliable is this?
- If not very reliable? how can it be improved?
- Is this like the basic circuit? I have the impression it is. I'm not really into this category of electronics, so it's my first, but i also have the impression that those 2 transistors and their resistors are forming an oscilator, a very weird one but very simple, also using the coil from the transformer i might guess. Feel free to chop my head if im wrong. Classically, I imagined this kind of circuit having a bistabil flip-flop in the secondary of the transformer, but this circuit is bluntly more simple that my imagination. Thats why im so perplexed. I will try build it sometime but before doing that i need to know if its even a correct one, from you.
Thank you as always, you are the best.
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Video link here: (scroll fast through the video, made by some indian, but to get the idea)
This MJE13007 power transistor pdf here : https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiCzbKEoLHtAhU5i8MKHeLNAF0QFjAAegQIAxAC&url=https://www.onsemi.com/pub/Collateral/MJE13007-D.PDF&usg=AOvVaw2plLabh3AvG5-aSTZbgtOB
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_Cheap DC To AC Inverter Circuit Using Power Transistor copy 1 - Copy.jpg
 
Last edited:

Alec_t

Joined Sep 17, 2013
12,089
How reliable is this?
Not very. The transistors will be damaged because their bases are driven negative far below their Veb breakdown point. Respective diodes in series with the bases could prevent that.
Imbalance in the circuit could result in net DC current in the transformer windings and consequent core heating.
If the circuit fails to oscillate (it's a relaxation oscillator) there could be excessive heat developed in one of the transistors and a transformer winding.
The properties of the transformer are important to the performance/efficiency of the circuit.
There will be horrendous EMI generated.
 

Sensacell

Joined Jun 19, 2012
2,836
As with most stuff electronic, the really simple circuits seem appealing to beginners but are actually very tricky and don't work well.
Look inside a commercial inverter, you will see a lot of parts!! They are there for a reason.
When I was starting out in electronics, I was always attracted to those simple-looking projects, but quite often they were very frustrating- they just didn't work.
Once I overcame my fear of complexity, I realized complex circuits are easier to get working- because they are made of BLOCKS that can be analyzed and understood separately.

You could learn some stuff by building this thing, but it's a terrible design if you want a real inverter that actually works driving useful loads.
 

MisterBill2

Joined Jan 23, 2018
8,803
certainly posts 2,3,and 4 are all correct, although I am not familiar with that name for the oscillator. The circuit s lacking all of the additional components that are added to provide stability and protection, and would probably damage the power supply of any PC connected to it. In addition it is probably very dependent on having exactly the right load and the right transformer in order to even work. The name assigned to this style of circuit assigned by another group would be a "blooper" oscillator, meaning some collection of parts that happens to work sometimes, under certain conditions. It functions because the Motorola MJ13007 is a fairly rugged device.
 

DickCappels

Joined Aug 21, 2008
7,443
The MJE13007 was designed for AC Line powered inverters and not reliable with a lot of collector current, also this trick of driving a transformer "backwards", that is driving what was intended to be the secondary as the primary keeps the output voltage from getting to what may be the expected value because the intended secondary was not designed to drive the magnetizing current of the transformer, which might approach 50% of the load current. This is because an IR drop because of the magnetizing current in the secondary was not planned for.

One little thing, if you are going to power something from a device that can supply substantial current, like a car battery, use a fuse or you may wind up with a charred circuit or much worse.
 

crutschow

Joined Mar 14, 2008
27,241
Also the oscillating frequency of that circuit is determined by the transformer saturation characteristics and likely will be significantly different than the normal 50/60 Hz main's frequency.
Typically that circuit is used to generate DC at the output by rectifying the signal, not to generate AC.
 

Thread Starter

q12x

Joined Sep 25, 2015
622
It's a basic Royer Oscillator without diode protection for bemf spikes ..
- WOW. Fantastic reading ! https://en.wikipedia.org/wiki/Royer_oscillator
- Thank you very much for this information !
I can't say i understood All there is there explained, but i have a better idea now after reading it. It is indeed a very specific and dedicated circuit. They also say "... which has been extended recently to drive low voltage LED lamps too".
- I know there are tens of models of transformers these days, and for sure I have no clue which is which, especially after their names. I only have a very general, basic idea about inductors and transformers. Not in depth for sure. In this page about Royer oscillator, is mentioned very clearly that all this circuit needs a "saturable-core transformer", and i have no idea what is that. MY basic transformer that i know for decades, is that one with a iron/steel core made from sheets cut as E and I or U and I and assembled stacked and glued. I think in my young wild life, I think I may even constructed such a transformer all by hand, and i know for sure to run from building another one, because ... its hard as hell. These are step (up or down) only transformers I put my hands on and had experience directly. I also played with some I core (also steel but then replaced exclusively by the ferite core) transformers, but i didnt find anything practical to use them, because they involved too much complicated electronics so i give up in playing with those.
- Back to the saturable core transformer, which is a very important component for this thing to work, also named as "saturable reactor", from the reading i just did, I searched over the internet to actually see how it looks like. I was disapointed to not find anything very clarifying, but i get over this diagram
1607005956786.png
(though im not sure i find the right one, but i have a broader idea about this subject, now). Why i searched for its physical look, because i wanted to see if its the same with the one from the indian movie. I dont think the indian used a correct transformer even if it worked for him. As @MisterBill2 here said, its a "[ ..."blooper" oscillator, meaning some collection of parts that happens to work sometimes, under certain condition ]". So its how i think about the indian circuit as well, as a blooper. But it works and i like it, even if its a blooper.
Very fascinating subject indeed.
Thank you very much so far.
 

Alec_t

Joined Sep 17, 2013
12,089
Given enough current through the coils, the core of any transformer will saturate. The transformer in the vid is nothing out of the ordinary.
 

MisterBill2

Joined Jan 23, 2018
8,803
The circuit shown in post #8 is a magnetic amplifier, a device used for power control long before transistors were discovered or invented. It uses magnetic saturation of a core to vary the impedance of an inductor used to control AC current through a load. Although the range was limited it served the purpose adequately at the time.
The saturation of transformers in inverter circuits is presently not used, at one time some inverters used a separate transformer that would saturate as the feedback source, and a non-saturating transformer as the power delivery device.
The challenge posed by core saturation is that suddenly the impedance drops, the output voltage stops rising, and the current may spike. So there is a great deal of waveform distortion when that happens.
There is a huge amount of literature and information about inverters that was created back in the later sixties and early seventies, so continued research will be educational. But avoid yootoob postings, they will not help much.
 
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