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Inverter Circuit 12V DC to 230VAC Sine Wave

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Hi Guys,
I found the schematic of Steven's super-simple inverter kit and I have figured out why it wastes half of its supply power and has an output power of only 60W to 70W:

1) The BD234 PNP driver transistors are in a multivibrator like I thought. They have avalanche breakdown since their bases try to swing 12V above the supply voltage but the absolute max reverse voltage of their emitter-base junction is only 5V. This wastes a lot of power that should be used to turn on the output transistors harder.
2) The BD234 driver transistors don't have enough base current to saturate well which wastes power.
3) The 2N3055 output transistors also don't have enough base current to saturate well so they also waste power.

Paralleled pairs of output transistors would increase the output power only a little because each one receives half the base current as one. Of course the transistors must be matched when paralleled or the one with the most gain will hog the load and will fail first.


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thankyou for that diagnostic audio guru , it sounds like that kit dont deliver what the makers say it dose, 140 watts they say , well anyhow ill find a use for what i can get out of it when i get the transformer and heat sinks, the 2 transistors that originally came with it had tesla written on them, i have no info on them , and dont know how good they would really be so maybe ill try them first , and see how they go

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it sounds like that kit dont deliver what the makers say it dose, 140 watts they say

Hi Steven,
My first comment about the kit was about the first line in its instructions, "max 120 watts input, output power max. 60...70 watts". The circuit's output cannot provide anywhere near 140W. So I calculated that its input power is about 105W and its output power about 62W. Depending on the very wide gain range of its transistors it could be better or be worse. Maybe the maker tested the transistors and used only the high gain ones, or ordered high gain transistors from a manufacturer. Then it will be a little better.

It is difficult to guess how long the driver transistors and coupling capacitors will survive since the transistors are constantly having avalanche breakdown. At my first job at Philips I was told to never design a circuit that lets a transistor have avalanche breakdown. I have seen the same comment many times on the internet.

The output voltage of this inverter isn't regulated and could go up to 300V. The maker says so. They also say that its output voltage is higher in Australia. 240V/220V x 300V= 327V.
I don't see protection diodes to clamp inductive voltage spikes from the transformer or from the load.
It has a square-wave output so it will be difficult for you to measure its output voltage if you have a meter that is calibrated for sine-waves.

Please let us know if you find a good job for it to do. ;D 
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:) heres the latest results of the super simple inverter circuit,
notice the size of the holes in the circuit board for the wires , they can only take certain size wires ,of certain gauge.

so the holes would need to be enlarged for a bigger gauge wire which i did latter.  the 10 ohms wire wound resistor ,at 10 watts would need to be increased to a higher wattage. as it got hot dureing testing,    just one only i felt to be hot.
the whole transistor and heatsink on the right got hot but held up well and so the recamended heatsinks could be increased in size more to.

at first test useing only normal wire,  before i changed it to thicker wire after redrilling the holes for  a larger size , i got a voltage of 139.7 volts so i turned the tripot fully to the right and it went up to 143.3 volts ac . and thats from useing only a 12 volts sealed lead acid cell battery and not a car battery .

the transformer made the low whining sound or huming sound .

in the picture is a microwave oven screw type globe i powerd that with the output no problems  it wasent fully bright , but bright enougth,  it was daytime so ill try it at night . since ive chaged the wires to a thicker gauge ill do another test soon , to see if its improved

the power transformer i got  has 2x 12 volts and 2x 15 volts ac outputs and 2x zeros i bridged the zeros and used them both and i used the 12 volts outputs


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:)the latest test with the thicker wire all installed after the last picture was taken gave me the total voltage output of 156 volts ac which dropped to 152 as the one transistor and the whole heatsink got hot so i adjusted it down to 150 volts ac.  which would be ok as it dident heat the transistor up and the heatsink its on , i waited it to cool down and fired it up again and at 150 volts ac it was only warm so unlless i can get the tempiture details for thoses trasistors so i can know how hot they get before they fail if they do, ill leave it at 150 volts ac which is more than enougth for the water gas circuit and the cold fusion reactor , there seems to be only one supplier in this country for the bd234 which i need more of to make a better inverter and they are over in queens land australia , the water gas circuit uses only 120 volts ac to separate hydrogen from water . this circuit if i ran it off a car battery which has more current it may of cooked the transistors i think, so my smaller battery is fine for it and the 150 volts would be more handyer for countrys that use that voltage , so now i have a 12 to 150 volts invertor for expserimenting with and when i can get a decent size houseing for it ill slowly adjust the trimpot and bring the voltage back up steady till its not to hot and settle at that , those microwave oven tempiture switchs sure may be handy to switch the circuit off when it gets to hot , you can get them for different tempitures to , i may latter try a bigger heatsink or use the 2n3055 transistor

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Hi Steven,
It Sort-of works! ;D
How many Watts is the light bulb rated for? You said it wasn't fully bright so it was glowing at less than its rated power.

Your voltmeter is calibrated to read sine-waves unless it is "True RMS", so it probably shows a higher voltage that is actually there because the inverter produces a square-wave.

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:)theres no details on the microwave oven light globe but i have another one around so ill se if it has any details on it but when i l;it it up it was pritty bright but in the daytime you cant be sure but i reckon it would be brighter at night ill remeasure its voltage with my other meter when i get to it

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:)the bd234 transistor is very hard to get here where i am as all the electronics suppliers ive checked dont have it , so the only way to get more is to pay $19 for another kit from jaycar electronics, i want to increase the size of the main board and make a few improvements , id like it if someone can look at the circuit again and make some changes to it so that it can use more comon  easyer to get transistors rarther than the bd234, the bd234 from what i can find dosent have any comon replacements here otherwise i could get them ,2amps 45 volts 25 watts thats all i can find out about the bd234 , so if anyone can make the chnages to the circuit so it can use a better easyer to get part id be happy

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Hi steven,
Hundreds of transistors will replace the BD234 in that circuit. The TIP32 for example costs $.60US.

Why replace the BD234 driver/oscillator transistors? It is the 2N3055 transistors at the outputs that do all the work and if they are paralleled then the output power might be a little more.

This circuit depends on high gain output transistors but you can't buy high gain ones.
2N3055 transistors have a wide range of gain so if you buy some you might be lucky and they will have high gain, or you might be unlucky and they will have low gain.
You said that one output transistor gets hot. It must be a low gain one. 

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:)i dont want to replace them i just want to exspand the size of the circuit and  biuld a more spacey one so i just want these bd234 trannys but if the tip142 is ok ill try it ,even though they are bigger,tonight i rplaced the tesla trannys for 2n3058 trannys which i was told are more poerfull than the 2n3o55, but the voltage went down and the same left tranny got hot so i put the tesla ones back and turned the voltage output down to 100.7 volts ac  if i turn it up more the voltage drops and the tranny gets hot so ive got it set to that , ill charge up my 12 volts battery more as its around 11 volts .


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i just want these bd234 trannys but if the tip142 is ok ill try it ,even though they are bigger

No. The huge TIP142 is completely different. It is an NPN darlington transistor. A TIP32 is a PNP single transistor, is not too big and will replace the BD234.

i rplaced the tesla trannys for 2n3058 trannys which i was told are more poerfull than the 2n3o55, but the voltage went down and the same left tranny got hot.

No. The 2N3058 is a small PNP transistor, not an NPN power transistor like the 2N3055. You need a power transistor with high current gain, not high power. Then the project will produce more power.
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