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Stabilising a pulse voltage

  1. Jul 19, 2012 #1
    ok how would i stabilise a voltage pulse of 6 V pulse to a straight forward DC voltage im under the impression that using a resistor in series with with the voltage source of 1K ohm and a capacitor in parallel, (how do you calculate the capacitance value).

    thanks Dan.
     
  2. jcsd
  3. Jul 19, 2012 #2

    vk6kro

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    Science Advisor

    You have a series of pulses (ie steep sided pulses varying between zero and + 6 volts) and you want an output which is the same as the peak value of the pulses.
    Is this right?


    The setup you describe would give an output which would depend on the duty cycle of the pulses.
    If the pulse was high for 3 seconds and low for 7 seconds, then the output would be 3/10ths of 6 volts or 1.8 volts.

    This is because the capacitor could discharge back into the supply when the input voltage was zero.

    You could fix this to some extent by putting a diode in series with the resistor, however, diodes have a forward voltage drop which would mean the output might be something like 5.5 volts.
    Better (Schottky) diodes might increase this to 5.9 volts.

    There is a function called a "time constant" for circuits like this.

    Baically it is the time needed for a capacitor to charge to 63.2 % of the input voltage. It is given by the product of the resistance (in ohms) and the capacitance (in Farads).
    For example a resistor of 100000 ohms (100 K) and a 10 μF capacitor would have a time constant of 1 second. (ie (100000 * 0.00001 = 1 ).

    So, if you knew how long your pulses lasted, you could work out how big a capacitor and resistor you needed. You need about 5 time constants to approximate the peak value of the input pulses.

    You can also build up the voltage on the capacitor over a large number of pulses if they are quite short pulses and they cannot escape from the capacitor.
     
  4. Jul 19, 2012 #3

    berkeman

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    Staff: Mentor

    That would depend on the application. Is this another simulation question to help you understand the basics of electrical circuits?

    If the application is power electronics, you would use a rectifier feeding into a storage capacitor.

    If the application is a precision circuit to hold the full 6V peak value of the pulse, you would use an opamp and rectifier circuit as a "peak detector".

    If you can say more about the context of the question, that will help others to offer suggestions to you.


    EDIT -- Edged out again by vk6kro! :biggrin:
     
  5. Jul 19, 2012 #4
    What do you mean?

    Tell us what are you trying to do. In order to answer your question, you have to be very specific.
    1) Are you trying to program the peak pulse height?
    2) What kind of pulse width?
    3) What frequency of the pulse?
    4) What is the required rise and fall time.

    Answer all these questions first and we can talk. Depend on what the requirements are, there are so many different designs for really simple to really complicate.

    I am venturing to guess that you want either a variable pulse voltage or constant 6V pulse from 0V to 6V. With that, you just use a P MOSFET to pull to the upper rail and use a N MOSFET to pull down to 0V. This is real easy if you pulse width is short and rep rate is low. The upper rail can be just an adjustable supply so you can get vary voltage pulse.
     
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