Does this soft-start look OK?

Discussion in 'The Projects Forum' started by Jack_K, Jul 9, 2013.

  1. Jack_K

    Thread Starter Active Member

    May 13, 2009
    115
    0
    I need to soft-start a DC motor that runs on 12- 15 VDC and draws about 50 amps. Does this circuit look like it will work?

    Jack
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,969
    744
    I think as soon as the gate gets between 2 and 4 volts it will turn the motor on hard, it may be better to use a pwm slow ramp circuit using an op amp,

    charge the capacitor and let the pwm control the speed, if you use this circuit, remove all items on pin 6, put the 100uF capacitor to pin 6 (Negative) and +12V(Posative), and a 100K preset from pin 6 ,to ground then when the cap starts to charge the op amp lets the pwm through and ramps up,ramp speed can be varied between 1 Second to 10 seconds.
     
    Last edited: Jul 9, 2013
  3. Jack_K

    Thread Starter Active Member

    May 13, 2009
    115
    0
    Oh no! What should I do?
     
  4. wayneh

    Expert

    Sep 9, 2010
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    3,027
    Agreed. The "linear" portion of the MOSFET is a narrow voltage range. You could tweak a soft start circuit like your's to spend most of its time in that range, but I think then you'd have to worry about the MOSFET overheating.

    Perhaps you could ramp up a PWM duty cycle. There have been a few voltage-to-duty cycle circuits discussed here lately
     
  5. Jack_K

    Thread Starter Active Member

    May 13, 2009
    115
    0
    So what you're saying is I have almost designed a solid-state relay? Just need an input on the gate?
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    Well let's back up and define what you need. The turn-on time happens in a smaller voltage range than you expected, so is faster, but you could change that time by tweaking the resistors. You could add diodes on the low side of the voltage divider so that the gate is held up off the lower rail, so that turn-on starts sooner when the switch is thrown. If the transition is fast enough, maybe the heat issue I raised can be dealt with.

    So you may be OK, or not. The devil is in the details.
     
  7. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    If you're good at this, you can tweak that circuit into working, maybe. Doing the PWM method is a lot more circuitry, but it will work with no tweaking and never risk smoking the Mosfet.
     
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