| Author |
 Topic  |
|
tsikscar
Apprentece
United Kingdom
23 Posts |
 Posted - Mar 26 2011 : 06:07:44 AM
|
| Hello people, i am back again with the same problem. My wien-bridge osillator 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. |
 |
|
|
pebe
Nobel Prize Winner
United Kingdom
1078 Posts |
Posted - Mar 26 2011 : 09:54:43 AM
|
You have got two cascaded RC time constants in the feed back loop from D3 to Q1 base - I can read C5 and C3 but cannot read the resistor numbers (the drawing is fuzzy because you saved it as a .jpg file - you should have saved it as a .gif file).
I don't understand why you have the feedback path R?/C5 from Q1 drain to gate. try removing it so the gate will get an immediate response to changes in the voltage across C3. BTW, C3 is shown with wrong polarity.
What are the values of C3 and its shunt resistor? |
|
|
|
audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Mar 26 2011 : 10:33:25 AM
|
Echo, echo.
I answered this thread a few minutes ago on another website where the schematic is very clear.
It was saved an an awful DOC file instead of as a GIF or PNG file.
I think a copy of the Texas Instruments Sine-wave Oscillators tutorial was posted on the other website so I don't know why this circuit didn't copy it. |
|
|
|
tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Apr 12 2011 : 5:16:37 PM
|
Hi people, i have a few questions which i know some of you may find annoyingly simple, but hey that's why i am seeking help on these forums.
I have got to a stage on my design where i have a PWM signal from discrete components. Where i am a bit lost is: i now want to drive a resistive load with a pure sine wave. My initial idea(which was not very successful) was to drive a complimentary pair of BJT's as a class B amplifier, then use an LC low pass filter. What i am getting out is just a negative square wave output. My questions are: Can you drive BJT's with a PWM signal? If so how do you connect them in a circuit? My sine wave reference signal is 50Hz, Triangular wave is 1.92KHz and PWM signal is 1.92KHz. Is this frequency enough? I need help on the amplifier circuit using BJT(they are available to me) and filter stage.Any ideas?
Thanks |
|
|
|
audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Apr 12 2011 : 10:37:31 PM
|
Pure sine-wave inverters use a PWM frequency of 20kHz to 900kHz. Then the LC filter is small and inexpensive.
Complementary Mosfets are used as the switching output transistors.
Besides the very low switching frequency you are using, you didn't post your PWM oscillator circuit nor your transistor output circuit so I have no idea what is wrong. |
|
|
|
tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Apr 19 2011 : 07:37:20 AM
|
Hi Audioguru, Here is my schematic.This is the onlf file format i know how to upload. I have also included anothe circuit i found on these forums which i want to know if it can work on my circuit as an out put stage. Will i need to add or subtract anything. Please help. I want to try and get the prototype up and running by the end of this weeekend.
Thanks.
Download Attachment:  untitled.GIF
71.26 KB
Download Attachment: mosfets.GIF
26.74 KB |
|
|
|
tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Apr 19 2011 : 07:47:58 AM
|
I have resubmitted schematic using a different format now.
Download Attachment: untitled 1.PNG
52.02 KB |
|
|
|
audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Apr 19 2011 : 08:33:04 AM
|
I saw your very fuzzy DOC file on another website.
Did you know that a lousy old 741 opamp is 43 years old and has trouble above only 9kHz? Most opamps made in the past 30 years go perfectly to 100kHz.
In your schematic I cannot see at what DC voltage the inputs of the 741 opamp are biased.
It looks like your PWM carrier is only 2kHz (so it will be very noisy) instead of 20kHz to 200kHz.
Your circuit with Mosfets and an extremly slow LM324 opamp will not work. |
|
|
|
tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Apr 19 2011 : 4:04:27 PM
|
| What can i do Audioguru please?i want to finish this circuit. |
|
|
|
audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Apr 19 2011 : 8:48:05 PM
|
1) Throw away the old 741 opamp and replace it with a proper comparator iC like an LM393 dual comparator.
2) Increase the triangle waveform to 30kHz using a faster opamp like a TL081 or MC34071.
3) The output of the comparator can drive complementary Mosfets that feed the LC filter. |
|
|
|
tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Apr 20 2011 : 5:44:34 PM
|
| Thank you very much Audioguru. |
|
|
|
tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Apr 24 2011 : 3:40:37 PM
|
Hi Audioguru, i have tried the mosfets in the configuration shown but it seems not to work.I calculated the frequency for 30KHz but on simulating can only get about 8KHz. But being low power i suppose this will have to do? Can you please sent me a circuit that works i can use for the output stage( from comparator into the mosfets/amplifier? Also i have at my disposal 2 BUZ11 mosfets and a TIP 2955/3055 complmentary BJT pair. Can i use them in anyway in the output stage.Does this stage require a transformer? Please help me cross the finishing line. Anymore advice is welcome. Thank you very much, you have been a massive help so far.
Download Attachment: mosfets1.PNG
62.42 KB |
|
|
|
audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Apr 24 2011 : 5:56:54 PM
|
You need to read about how a Mosfet works. Both of your Mosfets are upside down (they don't do anything and their internal diode is conducting all the time) and have nothing driving them.
You also need to read about the LM339 quad comparator or LM393 dual comparator because their output is just the collector of an NPN transistor that pulls down a negative voltage and you must add your own pullup resistor.
I don't use an inverter because my electricity is reliable. I have never seen a schematic of a good pure sine-wave PWM inverter. |
|
|
|
wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - Apr 25 2011 : 01:29:09 AM
|
John, don't you get sick of repeating yourself? I mean... Every time I come up here it seems like the same thing is happening over and over again. You have a lot of patience with these noobs.  |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
|
|
|
tsikscar
Apprentece
United Kingdom
23 Posts |
Posted - Apr 25 2011 : 04:03:20 AM
|
It's sad that you feel that way Wassup1990. What Audioguru and a lot of other people are doing on these forums is very much appreciated by many people. We are all not gurus in electronics but through these forums and sometimes making a fool of one's self one will end up understanding much much more of this vast field of electronics.These forums should be for electronics lovers, both experts and novices, i believe.
Audioguru and others, i am not alone in appreciating what you do on these forums. Please don't be discouraged, you are doing a great job. |
|
|
|
Topic |
|
pure sine wave inverter circuit design
untitled.GIF
mosfets.GIF
untitled 1.PNG
mosfets1.PNG