Author Topic: New PWM Circuit Question (Read 8376 times)

Farlander · « **on:** September 15, 2008, 11:54:40 PM »

Hey guys,

I'm currently building a new PWM, as shown below. Does anybody have any ideas why this design would not work? It's simpler... KISS
(2)Quad op amps-- first, one high gain amp is paralleled with a capacitor. This produces the up and down sawtooth wave, adjustable via a potentiometer (though still not sure how).
The second amp acts like a trigger. When Vin > Vref, the chip fires it's output. WaLA you get a square wave

students.umw.edu/wfc%20site/lm324 Function Generator.bmp

pese · « **Reply #1 on:** September 16, 2008, 12:06:31 AM »

Error 404
students.umw.edu
Monday, September 15, 2008 18:05:01
Apache/2.0.59 (NETWARE) mod_perl/1.99_12 Perl/v5.8.4 mod_nsn/1.1_0 mod_jk/1.2.15

GP

vdubdipr · « **Reply #2 on:** September 16, 2008, 01:34:22 AM »

do you go to uwm pese?

pese · « **Reply #3 on:** September 16, 2008, 01:53:57 AM »

Quote from: vdubdipr on September 16, 2008, 01:34:22 AM

do you go to uwm pese?

if i try to open above link
i receive this error message... (linux)

Pese

Farlander · « **Reply #4 on:** September 16, 2008, 03:43:16 AM »

sorry here's the schematic

(http://students.umw.edu/~jsera9mu/wfc%20site/index.26.bmp)

Farlander · « **Reply #5 on:** September 16, 2008, 03:45:11 AM »

The circuit calls for a Vref of 1/2 Vcc

If I use a 10 megohm resistor on the positive rail, that splits 12v to 6v... should I use the same positive rail for both amps, in parallel? or was this circuit designed to be a dual rail power supply?

insane4evr · « **Reply #6 on:** September 16, 2008, 07:29:34 AM »

Quote from: Farlander on September 16, 2008, 03:45:11 AM

The circuit calls for a Vref of 1/2 Vcc

If I use a 10 megohm resistor on the positive rail, that splits 12v to 6v... should I use the same positive rail for both amps, in parallel? or was this circuit designed to be a dual rail power supply?

This circuit is designed either for single positive rail or dual rail. For single positive rail, I would use two resistors of equal value (two 47K). Connect them in series, the top end of the top resistor goes to the positive rail, the bottom end of the bottom resistor goes to ground. Connect the two Vrefs to the junction of these two resistors. My 2 cents.

Note that datasheet from national semiconductor shows schematics for single positive rail application. there is one called adjustable vco.

Farlander · « **Reply #7 on:** September 16, 2008, 10:46:21 PM »

Jackpot baby! I totally feel you on that. It's just like this more sophisticated PWM shown here http://www.cpemma.co.uk/pwm.html What resistor should I replace with the pot?

With all of us trying different frequencies and coil combinations, one of us is bound to get it right. Keep it up!

Anybody having any luck with a pickup coil/PLL circuit to determine the resonance frequency? Here's my two cents on resonance after much deliberation...
The resonant frequency is a frequency at which dielectric breakdown of the water molecule occurs over and over again but just for an instant. Basically it's like an electrolytic capacitor. The fluid and water are dielectrics, meaning they resist current but pass voltage. Eventually, the fluid can no longer handle the volts, it blows up, and an arc short circuits causing massive electron leakage. At this instant, the power is cut and turned back on, restricting amp flow, and step charging of fluid occurs again. In the case of a capacitor, they literally do blow up, but in a tank of water, electrolysis occurs... the pickup coil can detect this short circuit and somehow alerts the driver circuit to cut power, varying the duty cycle. Any thoughts?

fritz · « **Reply #8 on:** September 16, 2008, 11:12:12 PM »

Dear All,

I can highly suggest using AVR TINY (25..) controllers for building up pwm blocks, vcos, both, with or without control loop.
The small ones come in an DIL8 package, have internal oscillator up to 16MHz, can drive lots of load on the output ports, are superfast (internal pwm can work with up to 64MHz), offer differential A/D input and/or a comparator and cost only 2 bucks.

For a vco/pwm generator you need only the chip, 2 pots and 4 caps, thats it. (and very simple program written in assembler or gcc with avr studio. Programmers (for example ponyprog) are for free and can be build up easily without any effort)

Maybe its quite a change in technology - but it would make even replications easier.
Just send the code, play around with parameters, no messing around with opamps or 555
anymore.

IÃƒâ€šÃ‚Â´ve done a lot with opamps, analog vco and plls - but believe me - we have the year 2008 - and embedded controller can do that very well.

If somebodies interested, I can make an example ciruit+program. Just let me know your needs.

rgds.

Farlander · « **Reply #9 on:** September 17, 2008, 03:58:21 AM »

Beautiful information dude. Thanks. I have some specific needs. You can actually here the resonation in this video of a pwm wave.
http://hydrogencollective.com/PacketPulser1.mpg

A thought came just now about resonance. In an LC circuit (inductor (coil) & capacitor) there is a formula for caluclating resonance frequecy. Since both inductance and capacitive resistance are functions of the frequency, supply, and wires, it is possible to build a coil to match the needs of the wfc.
Checkout the 'inductance calculator' http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=util_inductance_calculator

Farlander · « **Reply #10 on:** September 20, 2008, 01:16:01 AM »

I checked out the AVR microprocessor. It looks a lot like this setup
http://feynmanslab.blogspot.com/2008/05/second-i2c.html

Can you help me with the programming to make the square wave shown in the video above? Adjustable frequency, pulse width, and duty cycle. Thanks

Farlander · « **Reply #11 on:** September 20, 2008, 01:59:37 AM »

On another note, I built the LM324 circuit, shown here
(http://students.umw.edu/~jsera9mu/wfc%20site/index.26.bmp)
but it is a fairly narrow range of frequencies. I'm going to try replacing the center, 75R with a pot and see what happens. Right now I"m not sure how to read the scope, it looks like the pulse width is shortening causing higher frequency, not very useful.

Farlander · « **Reply #12 on:** September 22, 2008, 07:43:03 PM »

Ok that lm324 setup didn't work very well, very narrow range of frequency, not very clean, and does not satisfy the criteria of adjustable packet width AND frequency. Now I'm looking into some integrated chips like the 74LS123, the MAX038, but I'm leaning once again towards the 555 for experimental purposes.

Trying to avoid computer integration.

students.umw.edu/~jsera9mu/wfc%20site
http://hydrogencollective.com/PacketPulser1.mpg

Joe

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