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555 timer ?

Discussion in 'Datasheets, Manuals and Component Identification' started by bob.steel, Mar 1, 2020.

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  1. bob.steel


    Feb 27, 2020
    Most 555 timers have an input voltage DC limitation of say 5V.-12V

    Is it possible to place it in a 300 volt circuit in any way by limiting its voltage with resistors and the amount of current flowing to its sense triggers?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    What do you mean by "input voltage"? Supply voltage or as your question suggests the input to the trigger pin?

    For the supply voltage:
    Resistors alone will not work properly. Due to changes in the operating current the voltage drop will vary. You'll need some kind of stabilization. A simple zener diode stabilizer can do the job. Add a capacitor of 10 µF or more at the output of the stabilizer to smooth the voltage and supply impulse current.
    This is a linear type of regulator and as all these regulators it is very inefficient. The series resistor has to drop ~290 V (assuming 300 V input, 10 V output). At a moderately small current of 10 mA this means a power dissipation of 2.9 W in the series resistor.
    A more efficient way would be using a switched mode step down regulator, but you'll be hard pressed to find a small and affordable one for this application.
    The most cost effective solution would be to use a small wall wart with 5 V to 12 V output voltage.

    For the trigger input:
    A voltage divider can be used to scale down the 300 V input voltage to a level compatible with the supply voltage of the 555. As an additional protection a zener diode across the trigger input can be used to limit voltage spikes.
    bob.steel likes this.
  3. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    It would help us immensely in suggesting a solution to your "300 volt" problem if you would tell us what you are trying to DO. Do you have a circuit diagram that would do what you want to do IF a "555-type" timer were available in a 300 volt version?

    Sometimes all it takes is an LED-driven optical isolator to separate a low voltage circuit, such as a timer, from a controlling signal that happens to be a much higher voltage. This would be similar to what Harald suggested with the zener diode and its current-limiting resistor with regard to inefficiency, but you wouldn't have to worry about exploding the lid off a 555 timer IC if something goes wrong. OTOH, exploding optical couplers are cheap and easy to replace, but don't ask me how I know this.
    bob.steel likes this.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
  5. Kiwi


    Jan 28, 2013
    Speaking from experience, I have to say that experience is a great teacher.
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