Alternate dimmer

Discussion in 'General Electronics Chat' started by AnalogKid, Sep 10, 2015.

  1. AnalogKid

    Thread Starter Distinguished Member

    Aug 1, 2013
    I have a thought about an alternate approach to lamp dimming, but I want to investigate it first in the isolated, low power DC realm. Here is the basic idea. Vin can be anything from +160 Vdc to -160 Vdc, and shares a common ground with the control circuit. The power MOSFET is oversized to handle the peak voltage and max load current. The 4017 clock has no timing relationship the load or the load power. Note: ignore the MOSFET internal diode for this thought experiment.


    The intent is for the 4017 and dipswitch to form a crude PWM:
    Switch 1 only on = 33%
    Switch 2 only on = 67%
    Switch 3 only on = 100% (ignore reset signal race)

    Again ignoring the MOSFET internal diode, any serious problems with this concept?


  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    The only potential problem I see is that the 4017 can't drive much current so a large FET like that will switch very slow which may be a problem if you plan to run it very fast.
  3. gerty

    AAC Fanatic!

    Aug 30, 2007
    I would add a bias resistor to the gate of the FET to Vcc. I'm curious, what type of lamp are you planning on using that's 160Vdc?
  4. ScottWang


    Aug 23, 2012
    When the Vin = -160V, and the CD4017 always stay in +V, how can it drive the Vgs?
    Or adding a totem pole connecting to +/-9V and the input of totem pole connecting to CD4017?
  5. AnalogKid

    Thread Starter Distinguished Member

    Aug 1, 2013
    Because the 4017 power is referenced to the MOSFET source, so Vgs is positive even when Vds is negative. That bi-directional channel in a MOSFET can be a real problem, but sometimes it is the solution.