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PWM with SG3525

Discussion in 'Electronic Design' started by parkc23, Aug 13, 2004.

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  1. parkc23

    parkc23 Guest

    Hi, could anyone help me with SG3525?
    I have a PWM circuit with SG3525 and need to modify the circuit so
    that I can get ~0.2 Hz cycle period. This is very slow compared to
    all of the circuits that I can get.
    Probably, I can change the frequency by changing the capacitor or
    resistor at Rt and Ct (pin 5 and 6), but I don't know the equation to
    calculate those.
    Please help me!!!
  2. legg

    legg Guest

    data at

    The lowest frequency range specified for this device, using the
    internal oscillator is 100Hz. The use of timing C values larger than
    100nF isn't documented. I expect larger C values might be acceptible
    if an external device was used to discharge it - a small mosfet driven
    by an external signal to the sync pin might suit.

    Bias current into the C timing pin is between 2 and 20uA, so this
    limits the size of the timing resistor to ~150K, if charging time
    isn't to be severely affected over temperature. This would require
    ~16uF timing capacitor to get down to near 0.2Hz.

    Have you got frequencies and periods mixed up? For a 50Hz (.02uSec)
    operation, with outputs operating out of phase, the oscillator needs
    to run at 100Hz. (.1uF/30K or .02uF/150K)

    The internal oscillator has a fairly simple internal structure,
    allowing external clock drive.

    As I recall, the 3525 comparator wass not latched. The 3525A version
    latched, allowing only one output pulsewidth per timing period. This
    is a usefull feature, so recheck your part number.

    At the low frequency stated, you might get better results with
    non-integrated components.

  3. parkc23

    parkc23 Guest

    Thank you so much!!!

    The load is a solenoid (12V, 250mA) and I just don't know if a 16uF
    capacitor can discharge it. Do you think it is OK?

    As to the latching, does it generate multiple pulse width within a
    period if it does not latch regardless of the circuit?

    I tried the frequency equation on the data sheet
    (f=1/Ct(0.7Rt+3Rd))but got a wrong number. Should I use a different

    Finally, could you let me know the circuit with non-integrated
    components? I am only a beginner in the electronics and don't know

    Thanks again.

  4. legg

    legg Guest

    The load in your circuit will not be involved directly with the timing

    If the SG3525 is used to drive a solenoid, directly, as a load, the
    timing priods required will depend on the magnetic characteristics of
    the solenoid itself and the physical characteristics of the
    mechanically-coupled armature's load.
    Multiple pulse widths are possible if the feedback signal varies
    within the timing period, unless the first pwm comparison is latched
    and disabled until the next clock timing period.
    As long as your units are Farads, seconds and ohms, the result in Hz
    should be pretty unambiguous. Post your numbers if you want your math
    You indicated originally that you were modifying an existing piece of
    hardware. If you have a schematic of this device, it might be quicker
    if you can copy it to ABSE.

  5. parkc23

    parkc23 Guest

    Thanks again.
    I attach the link for the circuit.

    If magnetic properties of the solenoid as a direct load affect the
    operation of PWM, how can I isolate each? (maybe, some kind of
    Also, if latching is necessary, can I directly replace SG3525 with
    I am still confused if I can go upto 16uF to get 0.2Hz or higher Rt.
    What about the circuit with non-integrated components?
    I can choose any circuit since I have not started building any yet.
  6. R.Legg

    R.Legg Guest

    I suggest that you do not have sufficient grasp of the finction of
    the 3525 PWM function, yet, to modify the original circuit.

    I suggest that if you want to learn about the 3525 and pulse-width
    modulation, then you should build the circuit as described, with the
    components suggested in the reference and then use it to turn the
    solenoid on and off by adjusting the rheostats intended originally to
    alter motor speed. You could then figure out how to command the 3525
    to turn the solenoid on and off at your intended low frequency rate by
    manipulating the appropriate analog or digital control line with a
    cmos 555.

    The 555 could be used to control a simple bipolar of mosfet switch to
    do the same thing, without going through the PWM controller, if
    switching the solenoid is your only aim.

  7. parkc23

    parkc23 Guest

    I guess I need more homework before asking questions. Could you
    recommend a good book for the PWM and 555, not too difficult? I am
    reading a book by Malvino (Electronic Principles), but this touches
    those relatively briefly.

    By the way, the reason I need a solenoid control is that I want to
    control a 3-way solenoid valve where fluids A and B coming in and a
    certain ratio of mixture A and B going out by changing switching
    between A and B by simply turning a potentiometer. However, the
    solenoid valves are not designed to operate at that high frequency.
    That's why I need a very slow PWM.

    Would it be possible to do this just by 555 timer without using PWM?
    Can I get a circuit?
    Thank you.
  8. Ken Smith

    Ken Smith Guest

    Ascii art:

    -->|-- ------------
    ! ! ! CMOS 555 !
    \ !--------!thr Q!------- Out
    Out -->/ ! ! !
    \ !--------!trg !
    / ! -----------
    100K ! !
    --- 1U

    Is this what you mean. As you turn the pot from end to end the duty cycle
    of the Out signal varies from about 0% to about 100%. You just need to
    make it so that Out being high turns on A and out being low turns on B.
  9. parkc23

    parkc23 Guest

    Yes, this is exactly what I want!!
    Could you get me a more complete circuit? (if it's better for you to
    attach file, you could email me as an attachment.)
    What's coming in and what's going out? I guess I hook up the
    solenoid(12V, 250mA) directly to the pin out, but not sure how I
    configure the pot in the input side.
    In my humble electronics book, there is an example of astable
    operation, but it only operates the duty cycle of 50~100%. Here is
    the equation for the duty cycle.
    So, theoretically, the shortest duty cycle only goes down to 50%.
    Is it possible to get a range of 0~100% duty cycle in your circuit?
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