How to cut power when given max voltage reached?

  1. Hi all

    I need to disconnect the power once a given voltage drop at some component (a capacitor in this case) is reached.

    For example at first the voltage between the leads is 0 and starts climbing up once there is power. When the voltage reaches 2.0 V, I want the power to be cut off.

    Can a relay do it? What other options are there?
     
  2. jcsd
  3. Simon Bridge

    Simon Bridge 15,282
    Science Advisor
    Homework Helper
    Gold Member

    zener diode.
    if the PD across the diode is less than it's rated "zener voltage" then it does not conduct.

    iirc:
    in series ahead of the cap, in the circuit, then the capacitor charges until the PD across the diode drops below the zener voltage.

    in parallel - the cap charges until the zener voltage is exceeded - then the diode channels the current away.
     
  4. Bobbywhy

    Bobbywhy 1,908
    Gold Member

    You may consider using a voltage comparator to monitor the rising voltage. Once your limit is reached, the comparator switches. This can then be used to control an analog switch, which would open, disconnecting the power.

    Cheers,
    Bobbywhy
     
  5. Diode like Simon says. Try buying a few zeners with a reverse breakdown voltage of 2.0 volts. Run it in parallel with the component you are trying to protect, and put the diode in backwards.
     
  6. Thanks for the replies, guys.

    I cant seem to find zener diodes capable of handling the current I am working with. I have a 55W headlight bulb, and the the cap in series. My power supply outputs 4A in this configuration. So I need a zener capable of handling 4A right?

    Are comparators discrete parts or how can I built one? I have no problem with theory, I just don't know what's possible out of the box for my scenario.
     
  7. Bobbywhy

    Bobbywhy 1,908
    Gold Member

    Cyclix, Oops! Had you included this information in your original opening post you’d not have received the kinds of suggestions that were offered! Zener diodes and analog switches are normally used to control signals and not power electronics! I feel like a blind man touching an elephant…each different part I touch causes me to guess wrong about what the object really is.

    Will you please supply a schematic diagram of your circuit? Be sure to include the power supply voltage and show where the capacitor is in the circuit and what is its purpose? The more information you can supply about your project the more likely you will receive useful suggestions. And vice-versa.

    Cheers,
    Bobbywhy
     
  8. Well I am using a computer power supply to charge an ultra capacitor. I use the light bulb as a current limiter because the short circuit protection of the power supply keeps kicking in when I connect just the capacitor.

    It is working great but I have to stick around while the cap is charging in order to disconnect before the voltage difference between its leads reaches 2.5 V. So I am searching for an automatic way to do it at a way lower value, i.e. 2 V

    The time it takes to get from 0 to 2.5 V in my current setup is ~27 minutes. I don't want to start doing something else and forget to pull the plug on time. The formula I am using to calculate charging times is Tsec = Vcap / ( I / C). I also have a multimeter running to check the voltage periodically.

    [​IMG]
     
  9. Simon Bridge

    Simon Bridge 15,282
    Science Advisor
    Homework Helper
    Gold Member

  10. NascentOxygen

    Staff: Mentor

    You could adapt the idea from the first 3 minutes of this tutorial: http://www.youtube.com/watch?v=y0Q0ERSP24A

    Instead of powering a nuclear detonator, power your capacitor. The IC itself could be powered from your +12v supply (this IC doesn't need a negative supply.) Resistor values will need to be reworked, of course. Inputs might need to be swapped for switching as you want.
     
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