Full range PWM (555)

Discussion in 'General Electronics Chat' started by JMD, Dec 9, 2009.

  1. SgtWookie

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    Jul 17, 2007
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    OK, keep in mind that the 7555s are CMOS. You'll be lucky if they will source 10mA with a 15v supply. However, you're on 12v, so expect less. You would need a 1K output resistor (or more) with a 7555.

    You would actually be better off using PNP power transistors with a CMOS 7555. They can sink up to 100mA, and their outputs can go all the way to Vcc(Vdd)

    BJT 555 outputs can't get all the way to Vcc (even lightly loaded) because they have Darlington outputs. But for NPN output transistors, a bjt 555 will work much better at 12v.
     
  2. JMD

    Thread Starter Member

    Dec 9, 2009
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    So the 1k i used in my breadboard, wasnt that far off :) Ive often used a BC547 to drive a larget FET or transistor - would that be a bad idea?
     
  3. SgtWookie

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    Jul 17, 2007
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    Yep - and that's the value that Bill shows in his schematic.

    But, the circuit I posted differs significantly from Bill's version; the output duty cycle % will be completely independent of the load on the output, and only minimally affected by changes in Vcc.
    You could do that if you wished. However, you can drive a power MOSFET gate directly from a 555 timer - just use a resistor of around 20-50 Ohms between the output and the MOSFET to keep it from "ringing" after transitioning states. The resistor limits peak currents, and acts as a snubber.

    You could also drive a Darlington transistor. TIP142, etc - something like that. They have very high gain. A drawback is the Vce; even under fairly light load it'll be around 0.6v, and will go up to perhaps 1.2v or so (even more) as you approach the rating of the transistor. The Vce translates to power consumption, which means heat dissipation.
     
  4. Wendy

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    Mar 24, 2008
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    Actually, my design should be 100% independent of load and of power supply voltage, within the limits of the chippies. How are they not?
     
  5. SgtWookie

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    Jul 17, 2007
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    Bill,
    Give it a try. As the load increases on pin 3, it will take longer and longer for C1 to charge, as the current is divided between the load and the charge path for C1. If the output load is constant (and not too heavy) you won't notice much of a difference. However, if the output load varies, you'll see a difference in the charge and/or discharge time of C1, which affects the timing.

    [eta] Keep in mind that if you're driving a MOSFET gate with the output, you basically want that gate to transition from low to high, and high to low, as quickly as possible. If you're at near 100% or near 0% in duty cycle, the capacitance of the MOSFET gate will be in competition with C1, the timing cap, for charge/discharge current. This is not a good thing. You will find that the MOSFET will run quite a bit hotter near the PWM limits when it has to share charge/discharge current with the timing cap.

    As far as circuit complexity, they're nearly a wash - except using the output pin to charge/discharge C1 requires two diodes, a pot, and a fixed resistor, where the circuit I posted in a minimal configuration would require one less diode.

    But really, you do need a 0.1uF and a 220uF cap across the Vcc/GND lines to take care of transients created by the timer itself.

    And if you want better stability from the timer, you really do need a small cap on CNTL to ground.

    [eta]As far as efficiency - that's not a wash. The biggest downfall of the circuit I posted is the power consumed in R1 when pin 7 is discharging C1 through R2. That could be improved considerably with the addition of more components - but as long as R1 is kept at least 100 Ohms per volt of Vcc, it's not too bad. If it were a battery powered circuit, I'd increase it to 1k per volt of Vcc - or more. To compensate, the value of R2 could be increased to 1 MEG, and C1 could be reduced to 10nF. That would make the timing somewhat more susceptible to noise, and parasitic current usage in the timer itself would start to come into play, but the timing would basically be unchanged, and current use in the timing circuit would be reduced.

    [eta]
    Just for clarification, Bill never claimed authorship of the 555 timer circuit he posted; he'd previously stated that he'd seen it elsewhere before.
    I don't claim any original authorship of the circuit I posted, either. It is a combination of a wide duty cycle circuit I found in Schematicas' 555 Timer demo program (they seem to be offline at the moment), along with input from AudioGuru about the 220uF/.1uF caps on Vcc and various datasheet info, adapted to the stated requirements.
     
    Last edited: Dec 12, 2009
  6. Wendy

    Moderator

    Mar 24, 2008
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    Yeah, as old as the 555 there aren't many new ideas to be "discovered". I may start substituting my single edge ramp version since a lot of the critiques/comments don't apply with it. It does still short pin7, you can boost the resistor so large it isn't a problem either.

    BTW, with my bush goblins, the ones I made 30 years ago (I didn't have a deep resistor kit back then) used 10MΩ resistors with electrolytic capacitors with no problems. These units sit out in the weather a couple of weeks every year, and they still work per spec. I suspect the idea of bad leaky electrolytics needs stated occasionally, but is over emphasized.

    I think we'll disagree on the need for 220µF cap, since the current spike from the 555 is measured in less than 10's of nanoseconds. The 555 has a very nice edge on it's output. A simple 0.1µF across the power supply is all that is needed, and then not very often. Same thing with Pin 5, it is useful, but a capacitor will usually never be missed if you don't use it. I will be pointing out the exceptions as I find them.

    I have found an application where the interference effect is noticeable, to the point where a 556 is not an option. It will wind up as one of my experiments under "555 Heterodyne LED Fader". This one is deceptively simple, but covers some major concepts in electronics, which makes it a really good one.

    I haven't built it yet, but I think I've figured out a 555 circuit that uses a LR time constant instead of the RC time constant. It requires an extra op amp, but it will be worth building. With the proper R, the period will be a direct measurement of L.

    My current project/article is the 555 PWM circuit. I've misplaced the motor (I need to draw it) and I'm not going to buy a new one.
     
  7. SgtWookie

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    Jul 17, 2007
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    The 555 might be an oldie, but it's still a great goodie. ;)
    Yep, that's a good example all right.

    I very much do remember that 555 het fader, and I tried to explain to Millwood in a long thread about it exactly why he'd run into problems with the frequency lock thing - but he was convinced that his simulation was correct; even though he was using an "idealized" 555 timer, not simulating a "real world" timer. You can easily get "bitten" with simulations, particularly with idealized models. Ask me how I know this. :rolleyes:

    That'll be interesting.
    Need one? I have one or two spares around here...
     
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