Help me improve this PWM controller

Discussion in 'General Electronics Chat' started by acheddadi, Oct 25, 2015.

  1. acheddadi

    Thread Starter New Member

    Oct 22, 2015
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    [​IMG]

    How would one go about improving this circuit so that it is able to handle a 15V 5A motor?
     
  2. tom_s

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    Jun 27, 2014
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    there will probably be better options available using mosfets.

    although, for this circuit -

    flyback (snubber/freewheeling/suppressor/clamp) diode on the motor would be nice.

    15v regulator to the 555 power to be safe.

    replace the darlington with something that will handle at least 20amps and fused
     
  3. MikeML

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    Oct 2, 2009
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    Ditch the Darlington. Replace it with a modern NFET that has a Vth ~ 4V, Vds >50V, Id > 20A, Rds < 30mΩ, and mount the NFET on a suitable heatsink...
     
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  4. MikeML

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    Here is how I would drive the 5A 12Vdc motor with the manually adjustable PWM speed control using the 555 as a PWM generator.

    I show the NFET switch, which depending on the specific NFET choosen, which unlike the Darlington, requires only minimal heatsinking. The simulation shows the motor current as a function of the pot position, from 10% to 90% in steps of 20%. As the pot is moved, the average current can be adjusted. Note that at a PWM frequency of ~250Hz, the inductance of the motor does a good job of smoothing the current through it provided that the snubber diode is connected across the motor...

    167b.gif

    The steering diodes around the pot do a good job of keeping the PWM frequency more or less constant as the duty-cycle is adjusted with the pot. Note that the frequency of oscillation stays near 250Hz as the pot is moved.

    167rc.gif
     
    Last edited: Oct 25, 2015
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  5. Roderick Young

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    Feb 22, 2015
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    I agree with Mike, a Darlington is a poor choice for most power applications, because when fully turned on, it will still have about one volt from collector to emitter. If you have 5 amps flowing, that's 5 watts of heat you have to deal with when the transistor is fully on.

    If you use a MOSFET with an on resistance of 10 milliohms at 5 amps, the power will be I^2*R = 25 * .01 = 0.25 watt, again, when the transistor is fully on.

    The transistor will dissipate more during transitions, so two things that help are to keep the switching frequency low (like Mike's 250 Hz), and driving the transistor hard at the gate. The faster you can charge and discharge the gate between positive and ground, the better the transistor will perform. So I would decrease that gate resistor from 100 ohms, if the 555 will bear the load. If you can drive the gate effectively, you may not need a heat sink at all on the MOSFET.
     
  6. MikeML

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    The 100Ω is there to prevent high-frequency oscillation in the NFET.
     
  7. Roderick Young

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    Feb 22, 2015
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    Fair enough. Is the waveform on the gate slowed to reduce ringing?
     
  8. MikeML

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    More like to kill gain at MHz. Not all NFETs oscillate in a circuit like this, but putting between 10Ω and 250Ω there will stop the oscillation if it happens.
     
  9. ian field

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    Oct 27, 2012
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    Not just MOSFETs - they used to be called grid stoppers.
     
  10. tom_s

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    edit: nvm, i just read 'Do u no wot the diodes do, or do u not ?' thread

    /me tips hat at Rod, Mike and Ian for patience

    and here's me thinking a diode was a one way valve. what could have i been thinking....
     
    Last edited: Oct 26, 2015
  11. ian field

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    Most diodes only conduct one way, but there are exceptions - tunnel diodes and zeners spring to mind.

    And you only need enough voltage, to make *ANY* diode conduct in the reverse direction.
     
  12. umphrey

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    Dec 1, 2012
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    How much gate charge can the 555 provide?
     
  13. MikeML

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    More than enough at 250Hz...
     
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