What do you do with un-used taps on a transformer?

Discussion in 'General Electronics Chat' started by rustypigeon, Apr 16, 2011.

  1. rustypigeon

    Thread Starter New Member

    Jul 25, 2010
    As the title says, what do you do with un-used taps on a transformer. I have another thread going in the projects forum about making a 500v circuit for a Geiger Counter. I was able to get 500v using a transformer from a disposable camera.

    My question now is what to do with the un-used center tap. I was originally not going to do anything with it since it seems to be working fine. I did however notice that in the disposable camera circuit board the center tap was connected to batt + with a 1.5k resistor.

    I also noticed another Geiger Counter schematic where the center tap of the transformer went directly to batt + with no resistor at all. Here is a copy of that schematic... http://uzzors2k.4hv.org/index.php?page=geigercounter

    Is there a problem with having a floating transformer tap? I can't think of a reason it would need to be connected to anything. But then again I am new at this stuff.
  2. K7GUH


    Jan 28, 2011
    If you're a neat freak, solder it to an old fashioned terminal strip. Otherwise, insulate the lead and let it dangle.
  3. rustypigeon

    Thread Starter New Member

    Jul 25, 2010
    What is the purpose of having the center tap going to positive voltage like I am seeing in the schematics?
  4. SgtWookie


    Jul 17, 2007
    The center tap in the schematic you linked to is the current source for the transformer in that schematic.
    The BD139 transistor is switched by current from the left side of the transformer.
    It sinks current from pin 6 of the transformer until the transformer is saturated, which causes the BD139 transistor to turn off, causing a voltage spike on pin 5 of the transformer, which is rectified by the UF4007.

    Apparently, the schematic you used is different. If it works, then don't worry to much about it.