PWM, MOSFET, circuit build help

Discussion in 'The Projects Forum' started by Tom_001, Apr 20, 2012.

  1. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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    details in nextpost
     
  2. Tom_001

    Thread Starter New Member

    Mar 9, 2011
    13
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    Hello,
    I'm hoping for some help and feedback, the goal is to build a motor controller 24-48v 100a or so, I have had a hard time trying to find any complete circuit diagrams to just use and build.

    After looking at many separate simple circuits some designs and components repeated and seamed more common, so i mocked up this circuit using what was found.

    The reason for me using using 5 mosfets instead of 1-2 high current fets is due to package limitations to transfer heat and to even conduct close the rated current in the cheaper packaging,
    It is my understanding that the gate resistor is to help stop inductive noise and control the charge/discharge of the mosfets, I'm hoping using a resistor on each gate will do this and ballance the gate charge of the 5 parallel mosfets.

    There is no current limiter circuit yet but if this is usable I'll try to build on it.

    Working backwards I think the mosfet, mosfet driver, boosting transistor set up is maybe fine when values are worked out? but the 555 pwm generator is 37hz, (i confused 37hz for 37khz at first) so it cant be used, can a 555 be used at 30+mhz? can the circuit be adapted? is there another pwm generator ic that I can use?

    Thanks for any help and input Tom.
     
    Last edited: Apr 20, 2012
  3. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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  4. wayneh

    Expert

    Sep 9, 2010
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    Nope. Maybe 1MHz max. But why in the world would you need such high frequency for a motor?

    Why the comparator and transistor? I think you can drive the MOSFET driver directly with your clock.
     
    Last edited: Apr 20, 2012
  5. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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    Dam, sorry 30khz+ not 30mhz typo, It wont let me edit it now, i'll try later, but if it can be pushed to 1mhz, 37khz is well in limits then. thanks, do you know what values to play with i take it the c1 is one?

    well my thinking was the driver needs square wave in, but i need to adjust the duty cycle (3-98% in this one not tested) so i need the comparator-pot, but the comparator output is low so can not directly drive the mosfet driver, its one of the largest ic drivers that you can get. but i may well be very rong :)
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    What application is this?

    What is teh motor rating? Is it a common brushed DC motor?
    Depending on the application, can the motor turn backwards or faster than the driving signal commands?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Read the datasheet. I'm pretty sure it needs only a few µA at the input.

    You should look for 555-based PWM circuits here. I think they usually use adjustment of the 555 to change the duty cycle. But I do see how your arrangement would also work.

    [oops, nevermind that last bit. comparator is right]
     
    Last edited: Apr 20, 2012
  8. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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    Hello,

    good question, its a relatively low power electric motorbike, lots of fun building but i dont want to just buy a plug and play controler,
    However the motor is a small, older forklift drive motor, its been striped and fields rewound.
    Its a dc motor the field windings are series-parallel, its 4 pole, two series sets of two parallel field coils then in series with the armature, it acts like a normal series motor.
    in this set up it will run 24v 120a continuously, however it dose normaly pull 60a on a flat, but low rpm or stall 500a? or as much as the battrys will put out. it can be set up for higher votage and less current but i know the 25v from testing.

    Tom
     
  9. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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    thanks the comparator out is 20ma, driver in is 50ma max, so no transistor just a matched resistor, do you think its worth me trying to work out ltsplice to see if i can get 37khz and that the needed duty cycle?, or give up and use another generator?
    Tom,
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    No need to use 37kHz, most store bought motor controllers use ~18-20kHz frequency. Just to keep it around the upper audio frequency.

    Here is a PWM motor driver circuit that I've used before - http://wzmicro.virtualave.net/3525.htm Use a gate driver and mosfet to suit your motor.
     
  11. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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    Thanks, sorry for my ignorance but is that not a variable frequency drive, 2 pots being used for amplitude and 1 for frequency not pwm, I'm not familiar, what is pin 8 for and if the output was used for a driver ic what to do with pin 8.

    I'v been playing with ltspice and got a 555 to do what I think I need, but its 250hz not 20khz+..
     
  12. panic mode

    Senior Member

    Oct 10, 2011
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    if it works for you, just reduce C1 value. for example try 1nF instead of 100nF, that should bring frequency 100x up (100*250=25000Hz=25kHz)
     
  13. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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    thanks,

    I'v ordered the parts including 50k switch pot as it never is quite fully off, so power and pwm in 1,
    also if you change r3 for 100 ohm and c1 to 10nf you can use a 5k pot so you can use normal electric bike-scooter throttle, however you'll need a off switch.
    have something up for fryday I hope.
     
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