Isolated DC pulse generator help needed.

Discussion in 'The Projects Forum' started by alan-1981, Feb 16, 2010.

  1. alan-1981

    Thread Starter New Member

    Jan 22, 2007
    Hello All,

    Im working on a circuit that is essentially a basic TDR circuit, I need to generate an isolated DC pulse. at the moment im using power supplies to simulate the isolated supplies that will eventally be replaced with separate mains transformers and then rectified etc.

    im using a 555 to generate a pulse and then putting this through an optocoupler. from this then triggering a p channel mosfet. from an isolated supply.

    The reason for the isolated supply is that i need to be able to ground either side of the pulse alternately to geneate +12v and -12v respectively (there is a diode in the circuit im testing) and i compare the difference in the traces (Channels A&B) in excel to check for faults.

    when i ground the one side of the pulse (Y) it works fine makes no difference to the pulse, however when i ground the other side (X) of the pulse i get a very slow turn off from the p channel fet.

    This is confusing me because the supplies are isolated and no current should be flowing in the ground connection.

    If anyone has any ideas then please advise, or if there is any better method than the p channel fet.

    Any help would be greatly appreciated.