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ZFF Self-Modulating PWM

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by skeeterb, May 9, 2008.

  1. skeeterb

    skeeterb Member

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    In one of his videos on Youtube, ZeroFossilfuel, a member of one of the boards I'm a member of, mentioned creating a PWM that would self modulate the levels of frequency and duty cycle of the output of his PWM based on the loading of the output. It's been a while since I watched that video, but it has really created a stir within my mind. How would one do that without using the switches or pots that most PWMs use? I'm not asking for any design schematics for myself, but for anyone who wants to contribute to this Idea. Feel free to post your ideas here and see if they could be incorporated into a PWM that could used for multiple tasks.
     
  2. blueroomelectronics

    blueroomelectronics Well-Known Member

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    Post a link to the video.
     
  3. skeeterb

    skeeterb Member

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    Ok, I'm trying to find the video now, but he only mentions it in passing as the video runs
     
  4. dave

    Dave New Member

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  5. skeeterb

    skeeterb Member

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    here's one link to one of his videos but it's not the one im looking for. but he kind of describes what he is gonna do

    http://www.youtube.com/watch?v=4Ci5OlPlJ68
     
  6. Ubergeek63

    Ubergeek63 Well-Known Member

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    Well there are two "self modulating" regulator types. Hysteretic, which controls the switch directly from the output voltage, and peak current constant off time, which is kind of self explanatory.

    There are chips for both of them. A good one to try would be the LM3485
     
  7. skeeterb

    skeeterb Member

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    How would one go about incorporating the LM3485 into the PWM circuit to replace the pots that change frequency and duty cycle? I'm going to try designing a circuit using the basics of a dual 555 timer IC PWM. Am I going to have to any of the other parts of the circuit if I incorporate the LM3485 into the circuit?
     
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Perhaps you might try explaining exactly what you're trying to do?, I don't know about anyone else, but I'm pretty confused!.

    I suspect you're confusing yourself (and us) with what you mean by PWM?.

    Very basically there are two types:

    1) Speed control - you have a pot controlling either frequency or duty, this adjusts the amount of power to the motor, and thus the speed.

    2) Power supply - there is feedback from the output that regulates the PWM (again either frequency or duty), to maintain a constant output.
     
  9. skeeterb

    skeeterb Member

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    @Nigel

    I didn't mean to confuse you, but it is the second one, where feedback from the output regulates the frequency or duty cycle to maintain a constant output. ZFF mentions that he wants it to start at about 37 amps and when the Hydrogen booster cell gets up to a certain temp keep the input of the booster to a certain stable current level.
     
  10. gramo

    gramo New Member

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    I'm just having a stab in the dark here, but consider;


    If you have a current sensing resistor (say 0.1 Ohms) inline with the PWM controlled output, then as current requirements increase, so too does the peak voltage drop over the resistor.

    As this voltage gets higher, a micro controller could sense it and decrease the duty cycle of the PWM signal, effectively reducing the current available.

    A 50% duty cycle would provide one half the current that a 100% duty cycle could.

    The end result would be a constant current source through PWM for a variable load
     
  11. skeeterb

    skeeterb Member

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    would there be a way of setting an upper and lower limit to the frequency and/or duty cycle to keep the output voltage and/or current at a certain range?
     
  12. gramo

    gramo New Member

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    with a micro, for sure


    Code (text):
    PWM.SetFreq(5000, 50)
    While True

        If ADin(0) > 50 Then
            PWMDuty = PWMDuty - 1
            SetDutyPercent(PWMDuty)
        ElseIf ADin(0) < 50 Then
            PWMDuty = PWMDuty + 1
            SetDutyPercent(PWMDuty)
        EndIf

        DelaymS(50)

    Wend
    This program will setup a 5Khz PWM with a 50% duty cycle.

    From there it will continually monitor (once every 50mS) the voltage over the current sensing resistor. If it increases, then it decreases the DutyCycle and vise versa... forever


    Breakdown on ADC if your not familiar;
    As the ADC results for PIC's are 10Bit, a sample of 5 volts will return 1023 (10 Bit binary number = 1111111111, which equals 1023 in decimal), and a sample of 2.5 volts would read 511, therefore a sample of 50 as the program suggest would monitor 0.048 Volts.
     
  13. skeeterb

    skeeterb Member

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    hrrm interesting, where would I put the micro? and what micro or PIC would you suggest that I use for this gramo? and could I incorporate a section that would also control the frequency of the PWM? I have several different PWM schematic drawings here on my computer, so I don't need the main PWM drawings. I just need to know where i can drop the changes in so I can modulate the frequency and/or the duty cycle of the PWM using the changes in the loading by the Hydrogen on Demand booster cell. I know in most of the PWM drawings that I have seen the frequency and duty cycles controlled by 2 or more pots. Would I have to remove those pots and drop the PIC or Micro there and wire it into the inputs and outputs of the IC?
     
  14. gramo

    gramo New Member

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    There's a lot of fundamentals your missing straight up here, such as the hardware peripherals of PIC micros and simple programming :eek:

    But to further answer your questions,

    The PIC micro would be in your application circuit, and should be isolated with an optocoupler to high power devices. I'd use an 18F PIC Micro, and a program development tool called Swordfish



    I think it best that you make some simple programs to begin with. Save a copy of Davids PWM module found here and then start experimenting

    You can connect the POT's to the IC for both frequency and duty cycle control. By using the Analogue to Digital Converting peripherals built in to almost every PIC micro, you can have a direct interface with the devices
     
  15. skeeterb

    skeeterb Member

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    (WHOOSH) that's the sound of your last post going over my head. :) That sounds way too complex for me. I watched one of your tutorial videos on your site and it sounds pretty easy, but the stuff you mentioned in your post is way over my head. I was just trying to figure out a way of controlling frequency and duty cycle of a PWM, and PICs sound like a way to go, but its out of my league as a newbie like myself. I know my idea is an advanced project, but I want to find a way that removes the manual controls for a PWM and allows it to regulate itself depending on the loading of the output.
     
  16. gramo

    gramo New Member

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    Give me some variables, I'll see what I can come up with

    Target Application Voltage and Current Requirements:
    Target Application Default PWM Frequency:
    Target Application Default PWM Duty Cycle:

    Maximum PWM Frequency:
    Minimum PWM Frequency:

    Maximum PWM Duty Cycle:
    Minimum PWM Duty Cycle:
     
  17. Ubergeek63

    Ubergeek63 Well-Known Member

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    Actually it is the nature of the regulator that changes the frequency and duty cycle. So you would not need any of that.

    The video you link to seems to have nothing to do with your question other than some odd little comment about a gimmicky piecemeal regulator based on a dinosaur of a part.

    The LM3485 is strictly a voltage regulator. He is looking to regulate current and you would be much better off with an HV9910 set up for constant off time regulation with and external FET driver.

    Nowhere is the power source mentioned or what exactly is going on. Near as I can surmise, it appears that he is trying to run an engine off of hydrogen and the "cells" are generating hydrogen from water. Unfortunately the last thing I heard was that it takes more power to extract the hydrogen than you get from burning it.
     
  18. skeeterb

    skeeterb Member

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    Right now ZFF is using it as a booster to improve the MPG of his gasoline burning Toyota Tacoma. Like I said, he only mentioned it in passing when he posted one of his videos, I can't remember which one.
     
  19. skeeterb

    skeeterb Member

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    I'm just interested in controlling the frequency and duty cycle using solid-state components, not by using a bunch of pots and switches that could malfunction after a period of use and/or the environment in which it is situated. Looking again at the schematic of a PWM that I have, i know I would have to put it somewhere in the section where the controls for frequency and duty cycle are.



    Edit:

    I have found a PWM that uses Dip Switches to control the frequency and uses a pot to control the duty cycle. That would simplify my search for a method to self regulate the PWM. I have attached the PWM drawing that I found.
     

    Attached Files:

    Last edited: May 12, 2008
  20. Ubergeek63

    Ubergeek63 Well-Known Member

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  21. skeeterb

    skeeterb Member

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    I looked at that chip and its interesting that a PWM can be achieved using that IC. How would one adjust the output of the PWM using the HV9910 IC?
     

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