Hi all,
I need help with my design. I am designing a pure sine wave inverter to produce 60 volts ac from 12 volts dc. I used a wien-bridge for the oscillator section and calculated the capacitance and resistor values to give me a pure sine wave output @ 51 hz. My problem now is how to proceed from here. I tried connecting a transformer directly to the output of the oscillator circuit and connected a resistor as load for simulation purposes. The amplitude and voltages have gone down.
Also when i was running the oscillator simulation on it's own, it takes some time before it gradually builds up the amplitude until it gets to a pure sine wave, why is this?
Thanks.

 

What power do you expect? For higher loads 48-96V battery voltage is more usual.
To make a sinewave inverter you need to use high current PWM modulated by the sine wave and feed it to a transformer. There are other approaches too like using two alternating modlated SMPS, which is much more complex but doesn´t use a huge transformer.

 

You need a high power output stage... but we need more information:
- how much output current? (peak and continuous)
- operating input range - you say 12V, is it regulated or from a 12V SLA? (11.5V - 13.8V typically.)
- what transformer are you using?
- how efficient?

Although maybe it's better to think carefully about your application. What will you use the AC for? Many devices will be happy with a square wave, which is considerably easier to generate.

 

He has an oscillator and is shorting its output with a transformer. There is no power amplifier to drive the transformer so of course the output level drops when the oscillator is loaded.

But he is making a heater, not an inverter. A linear power occillator makes almost as much heat as its output power. An inverter is different because it is efficient. A pure sine-wave inverter uses efficient pulse-width-modulation to build its sine-wave.

It takes time for the output level to reach maximum because the oscillator gain is turned down instead of using an amplitude stabilizing circuit.

 

Waoh! Just to add a bit more information. The input supply will be a 12 volt battery. The outputs have been constrained to low power, an ac voltage not exceeding 50v @ a current not exceeding 2 amps. This was all done for safety reasons. The idea is to build a perfectly working inverter within these constraints.
I did not think i was making a heater and i was really knocked for 6 to learn that, but i suppose that's how you learn.
I have posted my wien-bridge circuit and the multisim simulation results i was getting.
Please assist me with the circuit stages so as to get my desired outcomes, not a heater, lol.
Thank you.

 

Whoa again.
You have a nice Wien bridge oscillator powered by two 12V batteries, one for +12V and the other for -12V.

It is NOT an inverter because its max output voltage is about only 7.6V RMS into a 4k ohms load. Its voltage drops when loaded to a max output current of about 0.02A.

You did not post simulation results.

 

Thanks again Audioguru for being helpful. I have attached some of the simulation results i was getting. Please also point out where i have gone wrong and the next stages i need to implement this task successfully. This will also allow me to learn more about such circuits.
Thanks.

 

The oscillator has a max output current of only 20mA peak. (Multisim shows much more current so Multisim is wrong.) If the oscillator drives a 1:20 stepup transformer then its output voltage will be 20V peak but then its output current will be only 1mA peak.

You used a 5 ohm load. If the transformer's output voltage was 20V peak then the current would be 4A peak at the output and would be 80A peak from the opamp which is impossible.

Where is your power amplifier?

 

There is no power amplifier as i initially thought the transformer will increase the voltage from the relationship: N1/N2 =V1/V2. But now i have realised that i need a power amplifier stage. But now how do i implement PWM from the sine wave i have to drive the transformer? Will it not end up distorting the output sine wave? Will i need a filter too?

 

If you don't understand PWM then ask your teacher to explain it or look in Google for a tutorial.

Many audio amplifiers today operate in class-D and use PWM so they do not waste a lot of power making heat. Buy one and use it to amplify the power from your oscillator.

 

Pulse Width Modulation

 

Thank you very much for your valuable help. I now have an idea of the direction to take. I will come back, hopefully with a working inverter.

 

Hello again Audioguru, could you assist me with a circuit i can use to input from the oscillator so as to drive the transformer. I apologise for being such a bother but i need help.
Thank you.

 

You never answered what you want to power with only 50VAC or 60VAC.
If you don't know how to design a PWM "amplifier" for an inverter then you have a lot more learning to do than I want to bother teaching you (I am not a teacher). Aren't you learning about electronics in school or university? Then ask your teacher to teach you or look at tutorials in Google. There might be some good tutorials on this website.

As I said earlier today on the other website (I hate these echoes), commercial pure sinewave inverters do it with a PWM controller IC like a TL494 driving Mosfets that drive a stepup transformer. But cheaper inverters (from you know where, I am not allowed to say where) use a voltage stepup circuit running at a high frequency so their stepup transformer has a ferrite core, is lightweight and is inexpensive. Then the PWM sinewave is made with a TL494 driving high voltage Mosfets.

 

Hi Audioguru, please could you help me with a circuit showing the connection from my wien-bridge oscillator output to the input/s of a TL494 and outputs of the TL494 to the power mosfets driving transformer? Do i need a special transformer for this inverter circuit? What is confusing me is that i have a single-ended output from my oscillator but i noticed there are 2 inputs for the TL494 identified as test inputs.
Is there any equivalent of TL494 as my multisim library does not seem to have this?
Thanks once again

 

My electricity is very reliable and I don't go hunting nor camping so I have never seen an inverter. I have also never used a PWM controller IC. If I need an inverter then I will simply buy an inexpensive pure sine-wave one.

Many semiconductor manufacturers make PWM controller ICs that are used to make pure sine-wave inverters. Many people have posted the schematics on the internet and some people have taken apart a purchased inverter to see how it was made. So instead of re-inventing one I would simply copy one if I wanted to search for the parts and build one.

It sounds like you are designing an inverter as a school project and will simulate (but not build it) using your school's old Multisim software that does not list the parts. Look for applications notes of the TL494 on the Texas Instruments website.
Good luck.

 

Thank you Audioguru for taking time to reply. I want to build the inverter but i want to construct the circuit first and simulate it before i buy the components so as to avoid buying more components after blowing them up. I wont afford it that's why i am asking for help and some guidance from the likes of yourself. The multisim is mine but it's an old version. Could you at least point me in the direction of some sites i can find some circuits showing me how i can feed my single-ended wien-bridge output into an pwm controller and then to the power output stage. Please do not get frustrated Audioguru.

Thank you.

 

I don't need an inverter and they are inexpensively sold so I am not interested in their design. I don't know where you would buy a suitable transformer for a pure-sine-wave inverter. Buy an inexpensive pure sine-wave inverter to see how it is designed.

I don't know any sites that have a good design. Look in Google.

 

Hello people, i am back again with the same problem. My wien-bridge oscillator is taking hours to build it's oscillations. i have connected the output of the wien-bridge to the non-inverting pin of a comparator and an output of a triangular generator to the inverting output of the comparator. The problem is with the wien-bridge taking long although i have used a JFET for AGC. Please help me speed up the oscillator. Thanks.

 

Hello people, i am back again with the same problem. My wien-bridge oscillator is taking hours to build it's oscillations.
.....
Please help me speed up the oscillator.

Simply increase the gain of the opamp more above 3 so that its output level increases quicker. Then the AGC circuit quickly reduces the gain. To do it change the 10k resistor R1 to 12k or to 15k.
The output will have more distortion than before but should not be too bad.