555 PWM Circuit Frequency

Discussion in 'The Projects Forum' started by HiProfile, Mar 16, 2009.

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  1. HiProfile

    Thread Starter Active Member

    Mar 15, 2009
    I HATE to ask what will seem an obvious, n00b, annoying question that has been asked in other ways before, but seems as simple as 1+1 to you guys. To me, however, its like speaking Chinese. I have zero formal education in electronics. :(

    I would like to use the 555 timer PWM circuit (http://www.dprg.org/tutorials/2005-11a/index.html) for controlling a +12vdc solenoid valve. It has to be either a 25Hz or 31Hz frequency (to match other hardware controls), but I can't seem to get the frequency formula correct. How do I work it??

    For that link, they give the formula as Frequency = 1.44 / (R1 * C1). When I plug in 1.44 / (100000 * .1), I get .000144; 1/.000144 is 6944Hz, or am I seeing that wrong?? Do electrical engineers have a different definition of frequency, or do I plug the farads in differently? I've seen absolutely no websites show the values in the formula, just the raw formula, frequency, and schematic. For me, "monkey see monkey do" is usually the way I learn.

    Again, I apologize for such a base question. I just can't find an answer I understand, and I've searched for a while. Although all I need are the values to make 25Hz & 31Hz, I'd LOVE to see the working formulas showing numbers, not letters. I would like to use different component values since I have some scrap pots & caps I could save money with.

    PS: An IRF510 would work in place of the IRFZ46N, right? My solenoid draws 5.4 watts, and radioshack shows it capible of dissipating 20 watts.
  2. bertus


    Apr 5, 2008

    The calculations on that website are correct:
    1/(100 X 10^3 * 0.1 X 10^-6) = 144 hz.
    The resistor is in ohms and the capacitor in Farads.

  3. HiProfile

    Thread Starter Active Member

    Mar 15, 2009
    Thanks a lot, it makes sense now! They conveniently mixed microfarads in the schematic with farads in the formula w/o noting that. I'd guess that's how its typically done, but very confusing for a n00b like me. I was always taught to not to mix digits up like that, but obviously I never entered an electronics class.

    "Assumptions is the mother of all [screw]-ups" sums up my feelings. ;)
  4. HiProfile

    Thread Starter Active Member

    Mar 15, 2009
    One more question. I can't get the 1N5818 schottky diodes for a week, would a normal 1N4001 work for a quick test? Based on how schottky diodes have far less voltage drop, I'm sure the PWM % would be messed up vs the final circuit, but I don't need the other feature, which is fast recovery. Its only 31Hz max.
  5. jpanhalt


    Jan 18, 2008
    In similar versions of that circuit, I use the 1N4148. Based on that, I suspect the 1N4001 would work for a quick test.

  6. HiProfile

    Thread Starter Active Member

    Mar 15, 2009
    Thanks for all the help guys, my little test works. Even with subbing in a 1n4001 and IRF510, I get 0% to 100% duty cycle for my solenoid valve. I don't have a 'scope to test it, but it clicks like it should.

    Time to order parts! :D
  7. Tahmid

    Active Member

    Jul 2, 2008

    Or you could put R in kOhms and C in uF and the frequency comes in kHz.

    e.g. 1.44/(100*0.1) = 0.144 kHz = 144 Hz
  8. prashanthb


    May 17, 2015
    Thank you for clearing this. I had a similar doubt.
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