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555 timer voltage doubler.

Discussion in 'Hobby Electronics' started by Wayne, Dec 21, 2007.

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  1. Wayne

    Wayne Guest

    I have been seeing a few circuit that incorporate a 555 IC to generate
    pulses at a higher voltage than the input.

    Is this reliable? All I am wanting to do, as an experiment, is to run a
    chain of LEDs.

    It is all hypothetical at this stage as I am only at the planning phase.
  2. I have been seeing a few circuit that incorporate a 555 IC to generate
    Why reinven the wheel!
  3. Wayne

    Wayne Guest

  4. The circuit is reliable but the voltage will drop fast when you load it (as
    stated in the text). I doubt it to be usefull for a chain of LEDS. You'd
    better use the power source as is and put the LEDs (or shorter chain of
    LEDs) in parallel.

    petrus bitbyter
  5. Wayne

    Wayne Guest

    I plugged the circuit into LTSpice. The result for a 12v supply had the
    output voltage at 20v, and with a single LED down to 18v.

    I know that SPICE has a margin for error, but for the cost involved, it
    is easier for me to get a 24+v power supply.
  6. ian field

    ian field Guest

    If you want increased voltage at a decent amount of power, use the 555 in a
    self boosting SMPS configuration.

    Feed the 555's Vcc via an inductor & forward biased diode (cathode to Vcc
    pin - inductor to supply I/P) then drive a MOSFET with the 555's O/P and
    connect the drain to the junction of the diode/inductor. You will need
    voltage sensing and control of the M/S ratio as even with the minimum 4.5
    Vcc, the boosted voltage will easily exceed the 555's Vcc max. A very simple
    way of regulation is burst mode, sense the boosted voltage and switch the
    reset pin when the voltage exceeds a set value - use a large electrolytic on
  7. ian field

    ian field Guest

    Complementary emitter followers are great for beefing up the drive current,
    but don't forget that each B/E junction costs you 0.7V off the drive
    amplitude - certainly significant where the Vcc is 5V and the peak O/P may
    be a bit less impressive even with 12V Vcc.

    The bipolar 555 can source/sink 200mA which is more than plenty for driving
    a diode capacitor charge pump.
  8. ian field

    ian field Guest

    If the OP wants significant power output then my earlier suggestion might be
    useful, make a boost converter by driving a MOSFET to switch the current
    through an inductor, the data sheet suggests the 555 guaranteed down to 4.5V
    but most chips will go down to about 3V - this is a bit low for fully
    efficient gate drive so it may be convenient to supply the 555 from the
    boosted rail, the flyback voltage can easily boost the voltage to more than
    the recommended Vcc max so I'd suggest using a zener and transistor to shunt
    the reset pin as a coarse burst mode regulation - the CV pin can be used to
    vary the duty cycle for more precise regulation.
  9. ian field

    ian field Guest

    In my experience with using a 555 in a self boosting configuration, even a
    low gate drive will invoke enough drain current to get things going, from
    that point onwards the Vcc rises rapidly. The circuit I built used a MOSFET
    liberated from the SMPSU of a scrap monitor - I can't remember the type but
    I think it needed about 6 - 8V for full conduction (the circuit was powered
    by a 4.8V NiCd pack), the first prototype didn't have any regulation and the
    Vcc shot up to about 30V, the spec sheet for the Hitachi 555 I used said Vcc
    max 18V so its a miracle it survived. The boosted Vcc was brought under
    control by tying the reset pin to Vcc via a 4k7 resistor and shunting it
    with a control transistor who's base was fed from the boosted Vcc via a
    zener and current limiting resistor. This crude regulation was adequate for
    my application so I left it at that, but if the OP is driving LEDs there
    might be a noticeable fluctuation in the brightness, so a little extra
    circuitry driving the CV pin might be needed to provide continuous smooth MS
    ratio control. Since LED brightness is directly related to current, I'd
    suggest using an OP-AMP to compare the voltage across a current sensing
    resistor with a reference voltage and directly control the CV pin.
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