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555 - Adjustable duty cycle circuit questions

Discussion in 'Electronic Basics' started by andy, Jul 25, 2004.

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  1. andy

    andy Guest

    duty cycle means the proportion of the period of a square wave that it is
    high rather than low.

    -------- -------- -------- -------- --------
    | | | | | | | | high duty cycle.
    ----- ----- ----- -----



    ----- ----- ----- ----- low duty cycle.
    | | | | | | | |
    -------- -------- -------- -------- --------
     
  2. John Fields

    John Fields Guest

    time on * 100
    Duty cycle = -------------------- %
    time on + time off
     
  3. Rubicon

    Rubicon Guest

    Hello,

    I have found a little circuit with formulas that is a 555 adjustable
    duty cycle circuit. I'm not entirely sure what "duty cycle" fully
    entails despite reading an explanation some time ago now.

    Am I correct that this circuit can be used to turn another cicruit on
    and off either directly or via a transistor/mosfet if need be to save
    power consumption?

    Are there any pitfalls that a rank amateur should know about here?

    Regards,

    Andrew.
     
  4. It certainly will not provide 10 milliamps with zero voltage drop. but
    the drop may be acceptable. This data sheet:
    http://cache.national.com/ds/LM/LMC555.pdf
    specifies no more than a .6 volt drop for pull up with a 5 volt supply
    and a 2 mA load for a pull up resistance of 300 ohms. But with a 12
    volt supply, the pull up voltage drop is no more than 1.5 volts with a
    10 mA load for a pull up resistance of 150 ohms. With a 9 volt
    supply, I would expect a value between these two. Something on the
    order of 200 ohms, perhaps. So a 3.8 mA load would drop about .7 or
    ..8 volts across the LMC555 pull up output.

    If you could operate your 3.8 mA load during the pull down part of the
    cycle, the LMC555 output resistance would be between 75 and 40 ohms.
    If you use a 1/3 duty cycle pulse to drive the 3 volt motor from a 9
    volt source, the average voltage is 3 volts (9 volts 1/3 of the time,
    0 volts 2/3 of the time) so there is no use for the voltage regulator
    at all. The duty cycle replaces the regulator. The difference as far
    as the battery is concerned is that with the regulator, the battery
    current equals the motor current at all times, but with the duty cycle
    version, the battery current equals the motor current only during the
    1/3 on time. During 2/3 of the time, the motor current coasts through
    a flyback diode, and the battery current is zero. So the average
    battery current (over the pulse cycle) is about 1/3 of the motor
    current. A 500 mA motor current is produced with an average battery
    current of about 167 mA.
    When the motor gets going and its current falls to 290 mA, the battery
    current will approach 100 mA. The only reason the battery current
    will not be exactly 1/3 of the motor current is he small power loss in
    the switch and flyback diode.

    And there is no reason to do this switching at 30 kHz, except ot make
    the process quiet. 5 to 10 kHz is probably plenty fast to allow the
    motor inductance to hold the current roughly constant during a cycle
    (though you should verify the current ripple before finalizing the
    design). Remember that each switching process adds a bit if loss to
    the operation.
     
  5. Rubicon

    Rubicon Guest

    John,

    I have a circuit consuming 3.8mA comtrolling another circuit consuming
    up to 500mA both from a 9V battey and I thought that the 555 (P.D.M)
    circuit could increase the batteries "life" by having the first
    circuit on only a part of the time - half a second on/half a second
    off. As it consumes 3.8mA I hoped that a CMOS 555 with its 10mA output
    current source could switch it directly.

    Another problem I have is that the first circuit triggers a CMOS 555
    to switch on a 3VDC/500mA geared motor for about 15 seconds via a
    BD139 NPN transistor. The transistor controls a LM317 regulator set to
    3VDC out which powers the motor (Motors V+ to LM317 3V+out, Motors V-
    to transistors collector with LM317 V-). The motor has the required
    diode and caps and a cap from the LM317 V+out to the transistors
    emitter. The whole thing works from a 9VDC regulated power supply
    (wall wart) but not well from a 9V battery. The startup surge of the
    motor is 500mA then settles to approx 290mA and the geared motor
    rotates but with less torque. I could adjust the LM317 for more
    voltage out I suppose.

    In a previous post when I first started using a 555 and asked
    regarding the running of a 3VDC motor from a 9V battery and triggered
    by a 555 John Popelish wrote:
    *******
    Simplest and most efficient are two different solutions. A series
    resistor that sets the speed to about what you need is probably the
    simplest. The most efficient would be a something like a second 555
    timer set to mid ot high kilohertz frequency , gated on by the slow
    555 described, above, to act as a pulse duty modulator to lower the
    average voltage applied to the motor. If it puts out a pulse width
    that is about 1/3 of the total cycle time, the average voltage to the
    motor will be about 1/3 of the supply voltage, with no intentional
    losses. And the battery current will average a little more than 1/3
    of the motor current, extending the battery life, considerably. But
    you will have ot pay more attention to transistor turn on and turn off
    times and use a fast diode (Schottky) across the motor to keep the
    switching losses low.
    *******

    Not yet figured out the values for the 555 pulse duty modulators or
    the transistor. I think that the output of one 555 should be
    capacitively coupled to the trigger of the next but not sure.
    Not even sure that the LM317 will like being switched by the 555 and
    the transistor and what exactly is mid to high kHz frequency? 30kHz+?

    As always any help is appreciated.

    Andrew.
     
  6. Rubicon

    Rubicon Guest

    John,

    Thankyou for all the information. It's late after an evening of
    experimenting and I hope you'll forgive me if I don't post much of a
    reply.

    I'll make up a circuit diagram of what I've got and understand and
    upload it to some space I have for all to look at and comment upon
    soon.

    Regards,

    Andrew.
     
  7. BobGardner

    BobGardner Guest

    I'm not entirely sure what "duty cycle" fully
    The cap charges thru both resistors, so the time const is (r1+r2)*c. It
    discharges thru the resistor to pin 7 only. So if the resistor values are
    equal, the duty cycle should be about 50% (duty cycle means % of total period
    that wave is on). There are several cool tricky ways of hooking a pot and a
    couple of diodes up to make the duty cycle easily adjustable over a wide range
     
  8. cpemma

    cpemma Guest

    One way shown http://www.cpemma.co.uk/555pwm.html
     
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