AC Neutral DC ground

Discussion in 'The Projects Forum' started by roro36, Dec 7, 2010.

  1. roro36

    Thread Starter Member

    Sep 7, 2010
    I have a thyristor that will be running 24AC through it. I want to turn it on via a little 5V system. The thyristor has 2 terminals for AC in and AC out and then another 2 that are from the GATE and Cathode. If I connect the Gate to the 5V circuit through a switch, can I connect the Cathode wire to the 0V line? As in the Picture.

    Would I have to put the load before the thyristor then so that the Cathode is at zero?
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Four terminal thyristors are uncommon. What is the type number of the device?
  3. roro36

    Thread Starter Member

    Sep 7, 2010
    I'm not sure but it of the PUCK type. I just want to make a small test circuit for it that runs on a 24Vac. They usually come out of high power furnaces, but I wanna test at lower values. They generally have two pins on them that are connected to a firing card and one of these pins is the cathode, which is used for referencing the gate voltage.
  4. SgtWookie


    Jul 17, 2007
    Have a look at our E-book's reference to thyristors:

    Have a look at the attached; a demo of an SCR made of two transistors.

    Note that the SCR latches on when it receives a positive pulse on the gate, and turns off when current through it falls to zero. It doesn't turn on if the voltage across the anode and cathode is reversed.

    I don't have a clue how sensitive your SCR's gate will be. Some turn on with very small gate currents. Some require quite a bit of gate current.
  5. DickCappels


    Aug 21, 2008
    You might note that SCRs are current driven devices. If you put 5 volts between the gate and cathode, you will cook and possibly kill it. Use a current limiting resistor in series with the gate!

    Another thing to take into account are that large SCRs may not remain latched at very low currents, so make sure you are running enough current through the SCR. Perhaps you can glean the minimum holding current (or valley current as it is sometimes called) from the data sheet.