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Adjustable frequency PWM, need to further adjust frequency (HELP)

Discussion in 'General Electronics Discussion' started by supak111, Mar 30, 2016.

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  1. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    Apr 29, 2012
    Hey guys I found this cheap SG3525 PWM that has adjustable frequency from 100hz-100khz. I need a PWM with lower frequency (25hz or so) to control a car solenoid for project. On off those is surprisingly hard to find for cheap. You you guys think this module with a different adjustable frequency resistor could go lower then 100hz or 100hz is it?


    [​IMG]

    [​IMG]
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    https://www.fairchildsemi.com/datasheets/KA/KA3525A.pdf
    According to the data sheet... minimum voltage can be as low at 60Hz... but usually typically no less than 120Hz.
    So no... not with this component.
    You can always make one yourself with a 555 timer, but it depends how you want to interface with the PWM driver.
     
    Engineer_Paul likes this.
  3. 73's de Edd

    73's de Edd

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    Aug 21, 2015
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    Sir supak111 . . . . . . .


    Velly . . . velly . . . . strange . . . . they must have made a production change, as they are using a BLUE 2 megglingohms trim pot . . . .err . . . .rheostat on the board,
    whilst the schematic proper was showing a 1 meg trim pot with an adjunct 1K series range limiter resistor.
    Any whoooo lets go for it and adjust that pot to its fastest speed (least resistance) and then shunt / solder tack the existing companion timing capacitor
    between the blue pots with a .1 ufd cap. ( Marked on board as C1)
    This way we will know that the unit will then be operating faster than that desired low end freq that you specified, then you s l o w l y increase the
    adjusted resistance of the pot to show how . . . l o w . . . that the unit will REALLY operate at.

    Thasssit




    73's de Edd
     
    Last edited: Mar 31, 2016
  4. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    See that little thing between the two big blue things. Change it to 473
     
  5. Gryd3

    Gryd3

    4,098
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    Jun 25, 2014
    Good catch on the schematic and board being different.

    How would a 47nF resolve the OPs question when the datasheet suggests using twice that rating to achieve 60Hz which is still double what the op is requesting?

    https://www.fairchildsemi.com/datasheets/KA/KA3525A.pdf
    60Hz typical lowest Freq, but may be as high as 120Hz.
    You may be able to bend the circuit to go lower, but it's not in the datasheet so you may get funny results.
     
  6. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    Apr 29, 2012
    Don't want to build anything, if I like this gadget I might sell them so of the shelf* would be nice.
    Ok so I accidentally damaged the one I had but I ordered another one.

    So basically just change C1 to 0.1uf and then see where that gets me. Would going 0.2uf or 0.3uf possibly drop the Hz even lower? Or what if I just do more then 200kohm resistor?
     
    Last edited: Apr 3, 2016
  7. 73's de Edd

    73's de Edd

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    Aug 21, 2015
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    Sir supak111 . . . . . . .


    The specs tell us that the unit can work on down to 60 cycles with .1 ufd and 200k used as the RC time constant values.
    You probably don't have the instrumentation capability of reading that C1 chip capacitor that is now present in the unit.
    So initially, use a .1 cap shunted across it and start at the highest frequency . . least trim pot resistance . . . since that is going to be probably close to a frequency that the unmodified unit was capable of operating at before.
    Then you increase the trim pot resistance, by slowly adjusting it . . . . . as that output frequency then drops, you can catch any "dodgy" action . . . . . or complete drop out . . . . . .as you work on down towards your desired 25 cycles.
    Realizing that you have far greater range, using that units 2 meg pots capability, than the specified mere 200K ohms.
    HOWEVER . . . . that oscillator circuit just might not like a high R and low C combination, and is better suited to using a low R and high C circuit.
    If by chance you still can't tune on down to 25 HZ . . . . yet the unit is still reliably oscillating . . . .increase the added shunted capacitance to .2 ufd as you mentioned.
    I think that it would not be needing more than that.


    (As your cat presently has its Rubik's cube . . . . I can complete it in 67 moves.)


    73's de Edd

    .
     
  8. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    334
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    Apr 29, 2012
    Thank you for your advice, I will try this as soon as the new module is in. Also Im not really sure that I need to go down to 25Hz, I might be able to control my air solenoid with 60Hz but I definitely know that 100Hz is too fast so I can't reliably control how much air goes through it.

    Few questions.
    Will adjusting the frequency mess with the output voltage at all up or down?

    And one other thing, I'll be driving this by 12v, my solenoid right now it 12v, this module outputs 5v, its that easily adjustable to output 12v? I'm sure my solenoid will work on 5v too or I can get a 5v solenoid but if I could output 12v it would make my life easier.
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  9. 73's de Edd

    73's de Edd

    3,121
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    Aug 21, 2015
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    .

    Sir supak111 . . . . . . .


    Will adjusting the frequency mess with the output voltage at all up or down?

    This unit should be giving you a full logic level LO-HI from its supply . . .at all frequencies.

    And one other thing, I'll be driving this by 12v, my solenoid right now it 12v, this module outputs 5v, its that easily adjustable to output 12v? I'm sure my solenoid will work on 5v too or I can get a 5v solenoid but if

    I could output 12v it would make my life easier.



    The OOOOMPH power required for the drive of a solenoid coil would be requiring the feeding of the pulsed signal into a transistor base . . . .preferably a darlington type . . . .or into the gate of a FET. . . . . . . . . preferably with a low Rds on spec.

    That has full 12VDC then being your solenoid POWER drive level and the 5v feeds into to the driver semiconductors base or gate..
    The solenoid should have a damper diode across it for protecting your connected semis from the the coils back EMF "kickback"voltage . . . . that is developed at turn off time of each pulse fed in..

    • Typical transistor utilization:
    [​IMG]


    • Typical FET utilization:

    [​IMG]








    73's de Edd



    .
     
    Last edited: Apr 4, 2016
  10. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    334
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    Apr 29, 2012
    Awesome, I can't wait to try this. I'll focus on getting the frequency down and if it turns out that this can go low enough I'll then add a transistor.
     
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