Full Wave Vs. Half Wave Rectifier at Small Voltages

Discussion in 'General Electronics Chat' started by rperea, Jul 24, 2009.

  1. rperea

    Thread Starter Member

    Apr 7, 2009
    Hello, I am creating a rectifier but apparently the full wave rectifier looks less adequate than the half wave one.
    I am trying to convert an AC voltage to DC of 1Hz amplitude of ~0.2V.
    I am using a half wave and a full wave rectifier with some capacitors.. I just got the negative amplitude turned into positive once with the FW rectifier but then I tried again and it just bulges a little bit. The HW rectifier just gets rid of all the negative signal which is similar with the FW since it bulges very very little (around 0.010V).
    What could have happened? I have two questions:

    1. Why is the half wave rectifier better, does the diode have a minimun voltage to operate? Could I've broke the diodes somehow?

    2. Any equations I can look to check the adequate Capacitor value that depends on my resistance?

    Thank you!
  2. mik3

    Senior Member

    Feb 4, 2008
    Neither the half way nor the full way rectifier are suitable for this application. The half way works better in this case because there is only one diode drop subtracted from the rectified signal. With a full wave there are two diode drops. However, you have to use a precision rectifier for this job because your voltage is small. Google precision rectifier.
  3. SgtWookie


    Jul 17, 2007
  4. rperea

    Thread Starter Member

    Apr 7, 2009
    Thank for your reply. But why a precision rectifier? Can you give me an explanation why this will be better? Because the small voltage? What happen if I use a half wave instead?
  5. bertus


    Apr 5, 2008

    A precision rectifier is better as it has a much lower threshold.
    A silicium diode has a threshold of 0.6 - 0.8 volts.
    A scottky diode has a threshold of 0.3 - 0.5 volts.
    A germanium diode has a threshold of 0.2 - 0.4 volts.
    A precision rectifier will have a threshold of several milivolts.

  6. rperea

    Thread Starter Member

    Apr 7, 2009
    Well....by looking at the voltages I am working with, I guess I would get a precision rectfier. I looked at some schematics of precision rectfiers, can I build one? or so they sell them in stores such as digikey or radioshack?

    Now...I know it needs and op-amp and the circuit looks more complicated than just a diode. Is this a IC than they sell or what? I checked out this website:

    Thank you again for all you guys and your replies!!