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wpk5008

Wed Dec 29 2010, 09:59PM

Registered Member #3546 Joined: Tue Dec 28 2010, 08:24AM
Location: Pittsburgh, PA
Posts: 18

The plasma speakers I have found on this forum have inspired me to try to build my own. Tobias's in particular looks fantastic, but I don't want to just steal a circuit and rebuild it, I would rather understand it and build my own. But I am having some difficulty in understanding how the TL494 works or the SG3525. I am used to dealing with PICs where the datasheets are pretty explicit on the operation of the device, but the datasheet for these IC don't give much more than a simplified function block diagram.

Honestly, I have been having so much trouble trying to figure out how thTL494 works that I have been thinking about just building my own variation of the circuit using a PIC, but because music waveforms have negative voltages it makes a PIC unsuitable without having to split the signal, rectify and invert the negative portion, and then either add or subtract the signal from the PWM output, which would just be a pain in the ass to code. Could anybody maybe help explain how these PWM ICs work or point me in the right direct to get me started?

kerver

Wed Dec 29 2010, 10:24PM

Registered Member #522 Joined: Thu Feb 15 2007, 01:04AM
Location: The Netherlands
Posts: 33

Hi
From what i saw mostle is that the feed some line level amplified audio trough a cap on the freqentie resistor.
This way the audio voltage level, chance the timing value as a fm modulation pwm singaal.
Thought when the am modulate it.
The chance the death-time.
My experiments with an fm modulation tl494 can get you after good trimming and design a good to listing audio sourge's.
Greetings from the lowlands.

IamSmooth

Thu Dec 30 2010, 06:21PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I've been working with the TL494 recently. If you pull up the datasheet and look at the test circuit you can use it to design a simple pwm circuit. You can tie the error amplifiers as shown to ground. Tie the DTC to ground. Use the FEEDBACK to control your pwm. I found that about 0.8v gives near 97% duty; 4v gives zero duty.

tobias

Fri Dec 31 2010, 02:16AM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

To learn about the TL494 the best datasheet I found was Motorola's @

If you still want to use the PIC instead I sugest doing a reference change on the voltage. Just change the -3V to 3V voltage range to 0 ~ 6 V. Read more about the audio output voltages here

I think a capacitor in series with the positive sound output pin followed with a resistor voltage divider would do it: see the simulation I did at

wpk5008

Fri Dec 31 2010, 07:29AM

Registered Member #3546 Joined: Tue Dec 28 2010, 08:24AM
Location: Pittsburgh, PA
Posts: 18

Thank you everybody. I hadn't even thought to check out data sheets from other companies. I guess I have a lot to learn when it comes to researching ICs.

@ tobias
I was looking at your schematic wondering what you were doing with that cap and voltage divider and I actually guessed that is what it was for. You sort of made a dc restorer, minus the diode. When I get back to school in a week and have access to an oscilloscope I'm going to have a lot of testing to do. By the way, nice website. I'm amazed though that I can still read Spanish considering it has been four years since my last class, I never was able to speak or listen to it though.

I'll post updates here as I make progress.

tobias

Mon Jan 03 2011, 01:48AM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

Walter

Thanks for the compliment!

And the language I spoke and write is portuguese. hehe

But I'm glad that people from USA and other countries are finding my website and are able to understand it.

If you start the project with the PIC please let me know. I have interest on this.

Th3_uN1Qu3

Tue Jan 04 2011, 02:21PM

Registered Member #2614 Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26

wpk5008 wrote ...
I'm amazed though that I can still read Spanish

You don't have to if you don't want to.

Anyway, i thought the TL494 was complicated too when i first started with it, but now i have a decent understanding of how it works. Here's a few pointers:

It has low drive capability. 200mA is very low when you're driving MOSFETs. It also tends to get quite a bit of DC offset at high load which makes it pretty much unable to drive a MOSFET gate by itself. Totem pole drivers are absolutely necessary.
You can tie the collectors to VCC and drive the totem pole via the emitters. This spares the transistor required to invert the signal.
12 volts really isn't enough. Most power supplies i've seen implementing it used between 18 and 24v to power the 494.

wpk5008

Tue Jan 04 2011, 04:10PM

Registered Member #3546 Joined: Tue Dec 28 2010, 08:24AM
Location: Pittsburgh, PA
Posts: 18

Thank you Th3 uN1QuS, that is an excellent guide to the Tl494, and tobias, as far as programming a PIC to do this, with the dc restorer, I think it would be a lot easier to try to build. I've just been very lazy lately. I've got a couple PIC projects floating around unfinished right now. When I finish them I'll try working on this with the PIC

Th3_uN1Qu3

Tue Jan 04 2011, 08:00PM

Registered Member #2614 Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26

If you do it using the PIC be sure to drop some code my way, i've long wanted to get started on PICs but never got the kickstart if you know what i mean. I'm still unsure if a device running code can be relied upon for running an off-line switching power supply.

kerver

Sat Jan 08 2011, 10:26AM

Registered Member #522 Joined: Thu Feb 15 2007, 01:04AM
Location: The Netherlands
Posts: 33

Hi,
i sometime's used the Ref. volt output. in serie a pot resister about 10k to earth. With a cap's' ofcourse .!n,100nf,220 like that of so_o
To controlle the cycle lengte from the Ref Vout 5v. To 0v. to. 2.9v max is 0/45%max in puss-pull.
This way you probably have good with pot range.
Works always on copper bord->fild with good smal cab's try with film, sybiet, or other hf-dump.

Greetings

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