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tsikscar
Apprentece
United Kingdom
23 Posts |
 Posted - Mar 08 2011 : 6:50:41 PM
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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.
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Mar 08 2011 : 9:56:32 PM
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A Wien bridge oscillator needs something to stabilize its output amplitude so it doesn't keep rising until it saturates and produces a square-wave instead of a sine-wave. Many years ago an incandescent light bulb was used because its resistance changes with the AC voltage level that drives it but a Jfet is also used.
With the stabilizing circuit then the gain of the circuit can be high so that it gets up to normal amplitude wquickly. Without the stabilizing circuit then the gain is turned down so it takes a long time to reach max level but then the clipping distortion is low.
You are making a linear inverter with an oscillator, a power amplifier and a transformer. They are never made because they waste a lot of power making heat. Manufactured pure sine-wave inverters use pulse-width-modulation at a high frequency so that the power devices switch comletely on and off without making much heat but the duty-cycle of their switching produces a pure sine-wave.
Yout output level drops when loaded because you need an amplifier that has enough power. |
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tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Mar 09 2011 : 08:50:40 AM
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Thanks very much Audioguru. I have followed your advise to others on various forums and i hope you will also help me.I have attached the wien-bridge oscillator circuit and the multisim simulation results i got. Could you help me with the next stages of the circuit in order to get an ac voltage output of less than 60v and a current not exceeding 2A. The idea is to build a perfectly working low power inverter circuit.
Please may you also point out the mistakes i have made so far?
Thank you. |
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tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Mar 09 2011 : 09:31:04 AM
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Here are the Circuits Audioguru. I thought it would be an easy attachment. Thank you once again.
Download Attachment:  sinewave2.jpg
25.19 KB |
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tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Mar 09 2011 : 09:50:48 AM
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This is the sine wave i was getting before connecting and loading the transformer.
Download Attachment: sine wave 1.jpg
47.46 KB |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Mar 09 2011 : 1:04:25 PM
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I already replied to you today on another website.
Your simulation results here are too small to be seen. Also, you saved them as a fuzzy JPG file type instead of as a very clear PNG file type.
You have a very weak oscillator that does not have enough output current to drive a transformer. Its max output current is only about 20mA. You need it to feed a power amplifier that drives the transformer. If the power amplifier is analog then it will waste a lot of power making heat. A class-D switching power amplifier uses PWM and will create much less heat and the battery charge will last longer.
Your circuit will be missing a voltage regulator so the voltage will be too high when the battery is fully charged and/or when the load is light. The voltage will be too low when the battery is running down and/or when the load is heavy. Maybe the Jfet stabilizing parts in the oscillator can be used for voltage regulation by sensing attenuated output voltage.
What do you have that will run from only 60VAC? Isn't your mains 230VAC?
I think somewhere you said that 60V is safe. No it is not. If you are damp or sweating then 60VAC will kill you. Its peak voltage is 85V.
In Canada, PA systems in schools are limited to only 25V so that they don't kill kids who are crawling around in the ceiling. |
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tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Mar 09 2011 : 5:51:30 PM
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| Thank you very much Audioguru. I will now work on the power amplifier. Any ideas of implementing the pwm in the amplifier? |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Mar 10 2011 : 2:47:08 PM
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Usually a PWM controller IC like a TL494 makes the PWM in an inverter. Or a microcontroller IC.
The "amplifier" is two or more power Mosfets used as on-off switches. The Mosfets drive a stepup transformer. |
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tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Mar 11 2011 : 03:41:09 AM
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Audioguru, sorry for sounding so silly. If i use the tl494 do i just feed the output from the oscillator into the input of the ic and the output of the ic into the transformer? Is it possible to use a pulse transformer instead of the Tl494? If so do i connect it as i described? I am relly stuck thats why i am asking questions you may find silly or trivial. Will i need a filter circuit for this?
Thanks for your help so far. |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Mar 11 2011 : 08:37:46 AM
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The TL494 is a high frequency oscillator and it produces the PWM waveform. Your sine-wave generator feeds the TL494 as modulation. The TL494 feeds power Mosfets that drive the transformer. The transformer must have a good enough high frequency response to pass the high frequency PWM signal. The high voltage output of the transformer has the high frequencies removed with a small LC filter.
Simply buy a cheap Chinese pure sine-wave inverter and look at the circuit inside. |
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tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Mar 11 2011 : 1:47:54 PM
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| Thanks Audioguru for the valuable information. I think i know the way now. You have been a massive help. |
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tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Mar 13 2011 : 12:16:17 PM
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| Audioguru, i was looking at the TL494 pins. What is now confusing me is that it has 2 input test pins and i have 1 output from my oscillator. Can you help me with a circuit showing how i should connect my oscillator output to the TL494 and the outputs to the power mosfets. Does it make sense to use a pair of darlingtons as the amplifier driver for the mosfets in the output stage? Thank you once again for your help so far. |
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tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Mar 14 2011 : 5:26:57 PM
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Audioguru, can you please help me with my request?
Thank you. |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Mar 14 2011 : 11:04:20 PM
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quote:
Originally posted by tsikscar
Audioguru, i was looking at the TL494 pins. What is now confusing me is that it has 2 input test pins and i have 1 output from my oscillator. Can you help me with a circuit showing how i should connect my oscillator output to the TL494 and the outputs to the power mosfets.
I don't know and I do not need to know because my electricity is reliable, I don't go hunting and fishing and I don't go to school anymore. If I want a pure sine-wave inverter then I would buy one because I probably cannot find the special parts.
I have never seen a TL494 IC.
quote:
Does it make sense to use a pair of darlingtons as the amplifier driver for the mosfets in the output stage?
No.
I guess you know nothing about darlington transistors and Mosfets.
A darlington transistor can provide a high output current when it has a low input current. But how much current does the gate of a Mosfet draw?? |
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pebe
Nobel Prize Winner
United Kingdom
1078 Posts |
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tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Mar 15 2011 : 3:46:53 PM
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| Thank you very much Pebe. |
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pure sine wave inverter circuit design
sinewave2.jpg
sine wave 1.jpg