Analysis of two different 555 PWM circuits

Discussion in 'General Electronics Chat' started by iONic, Nov 17, 2010.

  1. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    I've seen the first circuit before in various similar forms, but not the second one.
    Is one circuit better than the other, more efficient...etc?

    Thanks

    (1)
    [​IMG]


    (2)
    [​IMG]
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    I never use the first circuit but alway use the second circuit instead.
    In the second circuit, the Mosfet can be driven from pin 3 or from pin 7 with almost the same performance (R2 turns on the Mosfet slowly).
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    The 2nd circuit really needs a 1k resistor between pin 3 and the pot's wiper, or else you may burn up the pot if you try for 0% or 100% duty cycle.
     
  4. Wendy

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    Mar 24, 2008
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    Also, the second circuit uses pin 7 as an output. Not really a good idea, though it works.

    A different variation of the same circuit...

    555 PWM Oscillator
     
  5. iONic

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    @ Sgt: Good point about the 1K resistor and the wiper pot for circuit 2. How come Bill's circuit does not use one?

    @ Bill: Of all three circuits, I am leaning towards your circuit, thanks for the reference.
    Q: Why do some 555 PWM designs use 250K - 500K pots whereas your's uses only a 10K pot? Advantages...disadvantages?

    Q: I am using a regulated 12V source to controll a 12VDC motor rated at 400mA. Do I need a MOSFET or is a transistor OK?

    Q: C2, 220uF... can you explain this cap as I have not seen this in other circuits. Circuit 1 that I posted shows a 1uF cap from +12V to GND. Are they acting in the same capacity? I ask because a 220uF 16V cap starts to take up some realestate!


    @AG: Your response was important, but due to a majority decession I have opted for what I feel to me the most common use of the 555 and PWM.
     
    Last edited: Nov 18, 2010
  6. SgtWookie

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    It does; but his is 10 Ohms instead of 1k Ohms.

    My preference is to use smaller value caps and larger value resistors; as caps get physically large with an increase in capacitance. Using 10nF-100nF (0.01uF-0.1uF) caps keeps 'em pretty small, and you don't have to go to aluminum electrolytic caps. It also keeps the current flow via the diodes relatively small, so their Vf is lower.

    <snip>
    You could use a 2N2222/PN2222 if you wished. Base current would need to be 40mA to saturate the transistor. The maximum Vout of a 555 timer is Vcc-1.3 even lightly loaded due to the Darlington configuration. Vbe for a 2n/pn2222 will be around 0.8-0.9v when Ic=400mA. So:

    Rbase = (Vcc-(1.3v+0.9v))/40mA = (12-2.2)/0.04 = 9.8/0.04 = 245 Ohms. 240 Ohms is the closest standard E24 value. 9.8v*.04A*2 = 784mW, so use a 1 Watt resistor.

    If your circuit is battery powered, a capacitor will extend the battery life considerably. If powered by a supply, the 555 timer requires a minimum of an 0.1uF/100nF and a 1.0uF cap across it's Vcc/GND pins for reliable operation. The motor being on the same supply increases that requirement.
     
  7. iONic

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    Nov 16, 2007
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    As usual, Sgt., great information. Thanks! I don't think I have a 240 ohm 1W resistor lying around anywhere. Don't want to order 1 and pay 20X the part price for shippng! Got a ton of 120 ohm, 2W, but 2 of these will take up as much realestate as a Power MOSFET! I may have to place my Mouser order sooner than planned!
     
    Last edited: Nov 18, 2010
  8. SgtWookie

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    Then use five 47 Ohm 1/4W resistors in series for 435 Ohms. Don't pack them tightly together or they won't dissipate the power well.
     
  9. iONic

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    5(47) = 235 ohms. I could, but that would take up twice the space of the 2 120/1W resistors! I'll order them, eventually.
     
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