Low-pass Filter?

[audioguru2]

If you use a single pair of output transistors in the modified 500W inverter project then it will have an output of 100W.
It won't work without a transformer.

View attachment 39495

 

[hericlj]

Hi...I 've just built the 500W inverter circuit as part of my final year project...

Without connecting the circuit to a transformer (floating the circuit output), am I supposed to get 24VAC measuring between the collectors and the 12VDC? 'cause I couldn't get anything from that...

TNX

 

[audioguru2]

From the collector of one output transistor to +12V is a 24V square-wave. Depending on the meter, it will measure 24VAC or 8.4VAC.

Which schematic did you use? The original one has errors.

 

[hericlj]

I used the 500W one...Well, I guess maybe I have some problems in connection or something. Thank you

 

[audioguru2]

Hi Hericlj,
Sorry, I didn't read the part where you did not connect a transformer.

All the transistors are powered by the transformer. Therefore without the transformer, the collectors of the transistors do nothing.

 

[hericlj]

oh...Thank you very much...

Now I know why I can't get anything from the circuit...hehe~~~

One more thing? Just wondering how does the opamp work? Is it functioning as a voltage follower? I thought the input to the opamp is 12V square wave and then the opamp should not be operating at its linear region...right? or is it like a schmitt trigger but instead of positive feedback, it is functioning in negative feedback?

Thank you...really appreciate for your help...

 

[audioguru2]

Now I know why I can't get anything from the circuit...hehe~~~

Transistors work better when they have a power supply, don't they?

Just wondering how does the opamp work? Is it functioning as a voltage follower? I thought the input to the opamp is 12V square wave and then the opamp should not be operating at its linear region...right?

Yes, the opamp is a voltage follower. With a square-wave input it has a square-wave output. A square-wave is not linear and the level is so high that the output of the opamp saturates with it.

 

[hericlj]

sorry, still no output...When I connected the transformer to the collectors of those transistors. They are shor-circuited.

 

[audioguru2]

There are two 500W inverter schematics in the project. The original one has a shorted CD4047, its RC values are wrong, it doesn't have enough 2N3055 transistors and they are shorted.

 

[hericlj]

OK.  This is the one I build my circuit. Do I common ground the two 0V terminals of the transformer and connect it to 12VDC? The collectors are connected to the 12V terminals of the transformer, am I right?

Thank you....

View attachment 39625

 

[audioguru2]

Hi Hericlj,
Good, you built the fixed circuit.
The transformer doesn't have 0V terminals. Its primary winding has a center-tap that is connected to the fused and switched positive 12V, and it has two terminals for the collectors of each side's output transistors.
The emitter resistors of the transistors are connected to 0V and everything with the ground triangle is connected to 0V. The negative terminal of the 12V car battery is also connected to 0V in the circuit.

The circuit doesn't need to be earthed. We say its negative wire is its "ground".

 

[hericlj]

Hi Audioguru:

Thank you for your help...Sorry, I am a begginer in power electronics, so actually i don't know what a center-tap step up transformer looks like. I bought one with 240VAC primary  and two 12V, two 9V, two 0V output terminals at secondary, and thought I could use it to step up the voltage....

I actually connect the collectors to the 12V terminals on the seconday, and 12V DC to the 0V terminals (I tied the two 0V terminals togeter), and the primary side of the transformer is conncted to the load....

So when I switch the power on, the whole thing is current shorted.....

Am I seriously wrong?  Great Thanks....

 

[audioguru2]

I actually connect the collectors to the 12V terminals on the seconday, and 12V DC to the 0V terminals (I tied the two 0V terminals together), and the primary side of the transformer is conncted to the load....

You wired the transformer so that the 12V windings cancel, instead of adding.
Connect one 12V wire to the other winding's 0V wire. Then you will have a 24V center-tapped transformer. Look at my sketch.

So when I switch the power on, the whole thing is current shorted.....

It won't be shorted the way you wired it. It just won't work.
What does it do that makes you think it is shorted? With a 500W load, the current from the battery will be 50A which is nearly a short.
View attachment 39656

 

[hericlj]

Thank you so much....

Actually when I did that experiment last week, I didn't connect any load to the transformer, and I was using those 3A(max) DC Power Source to power this circuit....So when I switched it on, the power source reaches its limited output current and wouln't give 12V out....I thought that was current-shorted....

 

[audioguru2]

I think the power supply current for an unloaded 500W inverter would be around 4A or even higher if your transformer doesn't have enough inductance. Your little 3A supply was too weak.

 

[farhad-ff]

hi
could this inverters output be used for supplying an AC motor like fans or waterpomps ? if no what should be done to make it appropriate for connecting ac motors to it?/
thanks

 

[audioguru2]

Hi Farhad,
A square-wave or modified sine-wave inverter should power an AC electric motor fine at its full speed. I have read on the web that an electronic motor speed control won't work properly unless the waveform is a true sine-wave.

 

[farhad-ff]

hi audioguru
i heared from one of my friends that connecting a motor to inverter may cause to change the shape of its output signal
for example change it from its square shape to something else/Is it true? how?

 

[farhad-ff]

by the way    i meant a simple electric motor

 

[audioguru2]

An electric motor is an inductor. AC current through an inductor is filtered, so a square-wave will have some of its edges smoothed off.