Gradual increase of resistance

Discussion in 'The Projects Forum' started by llawwehttam, Mar 29, 2010.

  1. llawwehttam

    Thread Starter New Member

    Mar 29, 2010
    At the moment I am trying to build a circuit to slowly make a household lamp dim over about 10 minutes.

    I have used logic gates to build the timing elements and am using mosphets to vary the resistance of the output for the dimming part but I am not sure how to gradually decrease the current to the mosphet to make it let less current through thus dimming the output lamp.

    I would rather keep away from microcontrollers if possible.

    I have been trying to work this out for a while now and have got no-where so I am resorting here.

    Any help or pointers in the right direction are extremely appreciated.
  2. SgtWookie


    Jul 17, 2007
    Are you talking about AC mains powered lamps?

    If so, you don't use MOSFETs for that; you use SCRs or TRIACs, which are thyristors.

    If you are talking about DC, then you can control the current using PWM and MOSFETs.
  3. llawwehttam

    Thread Starter New Member

    Mar 29, 2010
    I will be making this circuit aimed at 130~240AC mains voltage so probably going towards the TRIAC direction. I have seen various circuit diagrams for this so will do some experimenting tomorrow.

    I was going down the route of using mosphets as voltage controlled variable resistors to try to decrease the current from the mains slowly over 10 mins but I see this is very inefficient.

    Thanks for the guidence, that put me back on track.
    Last edited: Mar 31, 2010