Diode Rectifiers

Discussion in 'Homework Help' started by timmyg123, Jan 26, 2009.

  1. timmyg123

    Thread Starter New Member

    Jan 26, 2009
    [​IMG]Hey guys. In this diagram I understand how the negative current from AC can pass through the first diode then enter into the resistor. But how does the positive charge pass through the other diode, does it have to be negatively charged (since the diode is facing it)?

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Diodes only conduct one way. So the only path to consider is the one that exists when the diodes are forward biased. The reverse biased diodes are just along for the ride, so to speak.

    Take a look at our Ebook section - http://www.allaboutcircuits.com/vol_3/chpt_3/4.html.
  3. timmyg123

    Thread Starter New Member

    Jan 26, 2009
    Yea but when it goes through the top right diode how does it conduct electricity if there a + charge?
  4. bertus


    Apr 5, 2008
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    If the cathode of the top right diode (the one in your link) is positive, it will be reversed biased and will not conduct.
  6. Cabwood


    Feb 8, 2009
    You are not helping yourself by drawing the flow of electrons in your diagram. Diagrams should show conventional current flow (the direction in which a positive electric charge would move). That's why the diodes are drawn pointing in the direction of positive charge flow, not negative electron charge flow.

    Also, you perhaps have missed the idea that a flow of positive charges in one direction is the same as a flow of negative charges in the opposite direction. Since in our universe the charge carriers in most conductors are negatively charged electrons, we are stuck with that, but the maths still needs us to think, draw and compute in terms of the positive.

    There are no positive charge carriers in your circuit. The idea that somehow negative charges enter the resistor and positive charges come out the other end is just plain wrong, so drop that idea right now.

    Think water - molecules of water that go in one end of a pipe section have to come out the other end eventually. They don't go in one end as pushers and emerge at the other end pullers. They don't enter as matter and come out as antimatter. You get the picture.

    This means that it is electrons flowing anticlockwise around that loop in your circuit at all points. The whole loop. Everywhere in that loop, the charges that are moving around, making what we call current, are negative. Electrons.

    As food for thought, not only do the electrons that go into the resistor come out at some point, but however many electrons went in must come out, otherwise you'd end up with a deficit of electrons somewhere in your circuit. That doesn't happen. If a million electrons go in one end every second, then a million are coming out the other end every second, regardless of the resistance, capacitance, inductance of the device you are pushing them through.

    If you push one litre of water into one end of a pipe, one litre is coming out, unless the pipe implodes or explodes.

    Maybe this will help. Water will flow from the high pressure end of a pipe to the low pressure end. Wind blows from high pressure zones to low pressure. Think of voltage as pressure. The resistor you drew has a + and a - symbol. Think of the + symbol as representing a high pressure, and the - symbol as a low pressure. Think of voltage as electrical pressure. Positive charges always flow from high voltage (pressure) to low. To confuse you, electrons always travel from low pressure to high pressure, but as I said if this is confusing you it's because electronic engineers don't usually consider the electrons. They consider the Amps.

    To finish this, a 9V battery is simply a device that produces a voltage (remember - voltage is electrical pressure) difference of 9V. That is the voltage at one connector is maintained to be always exactly 9V higher than the other one. This voltage difference is the "electrical pressure" that drives current around a circuit. And it drives positive charges one way, and negative charges the other.

    Edit: BTW, because your voltage source is AC, when it changes polarity all that happens is that the pressure difference provided by the voltage source has reversed, driving the charges in the opposite direction. Those charges are still electrons, but now they go clockwise. That's why the other pair of diodes become conductive, and the pair that was conducting before now block current flow.
    Last edited: Feb 8, 2009
  7. mik3

    Senior Member

    Feb 4, 2008
    Just imagine that the voltage source turns around and the negative side goes up. The rest is the same but the other two diodes conduct.
    Note that your current direction assumes that are the electrons which make the current and not positive charges. I mean you are using the real current direction rather the conventional.