Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Switcing DC on an off repeatedly 'laymans question'

  1. Jul 27, 2009 #1
    I realize that this may be an unlearned question.

    Is switching DC current, on and off, lets say 60x/sec, similar or exactly the same as AC. From a laymans point of view it is not. I may stand to be corrected but, AC indicates that the current flows, or is switched to flow, in opposite directions and DC, be it switched or not, will still flow only in one direction....

    Thanks for your patients
  2. jcsd
  3. Jul 27, 2009 #2


    User Avatar
    Science Advisor

    Your understanding is correct. What you describe is often termed pulsed DC and it can also be thought of as AC with a DC bias--if you put this signal through a capacitor, the DC component is removed and you're left with an AC square wave.
  4. Jul 30, 2009 #3
    I read through Wikipedia's description in regards to 'capacitors and DC bias. Thanks negitron, not that I understand it all or even a small percentage, there is so much for me to learn.

    OK, so I'm looking for a capacitor that will reach equilibrium with the source voltage, at that point the current through the entire circuit will decay. In essence discharging a charged capacitor, returning the entire circuit to zero, hence the square wave....yes?

    Do you have a recomemdation to which type of capacitor I should be looking at?
  5. Jul 30, 2009 #4


    User Avatar
    Science Advisor

    Imagine a capacitor in series with a resistor to ground.

    Apply a positive voltage to the open end of the capacitor. It will charge up via the resistor until there is no voltage across the resistor and all the voltage is across the capacitor.

    Now briefly connect the open end of the capacitor to ground.
    The capacitor doesn't have time to discharge, so the voltage across the resistor goes negative by the amount that the capacitor was charged to.

    Now briefly connect the capacitor to double the previous voltage. Again, the capacitor doesn't get time to charge more and the voltage across the resistor plus the voltage across the capacitor equals the new voltage, so the resistor voltage equals the increase in voltage.

    This is how capacitive coupling works.The capacitor stays charged to the DC voltage, but the changes in voltage are passed through to the other side.

    In the case you describe, a pulsed wave of say 5 volts alternating with 0 volts would charge the series capacitor to 2.5 volts and give square wave output swinging from +2.5 volts to -2.5 volts
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Similar Threads for Switcing repeatedly 'laymans
Max distance of multimode fiber without repeater station