DC motor current drawn

Discussion in 'The Projects Forum' started by jcm2112, Feb 6, 2011.

  1. jcm2112

    Thread Starter New Member

    Jan 11, 2011
    6
    0
    Hello, I apologize if this question is trivial

    My robot uses 2 DC motors as wheels and is controlled by a PIC18. I am using the L298 Dual Full-Bridge driver. My voltage supply is 10x1.5 V alkaline batteries in series and I then produce the 12V input to the L298 using a 7812 regulator. The L298 is capable of providing 1A (more if properly sinked) per bridge, however the maximum current the 7812 can provide is 1 A.

    My concern therefore is how to limit the current drawn by the motors. Do I do this with PWM? I researched the datasheet of the L298 and there is a current sensing resistor, however this only allows me to measure the current supplied to the load, not to limit it. 500 mA per motor should be enough, but how do I limit my load currents to this value?

    Thank you for your help
     
  2. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    To controlle the current use transistors simple.You can use current mirrors circuits ?

    Good Luck
     
    Last edited: Feb 6, 2011
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Well, if you can measure it then you can control it and keep it at a desired level by controlling the PWM duty cycle.
     
  4. jcm2112

    Thread Starter New Member

    Jan 11, 2011
    6
    0
    Thank you for your help. It turns out the motors will actually draw 600 mA each when loaded, which is fine for the L298 but a problem for my 7812, as it can supply 1A max. Can this be fixed by PWM? E.g. if I have a 50% duty cycle, to the 7812 will it seem as if each motor draws only 300 mA?

    Alternatively, do you know any 12 V regulators rated slightly above 1A? (2A is likely enough; I was looking at other threads and someone suggested a Toshiba 7.5 A, but I prefer something economical if possible and 7.5 A is more than I will need).

    Thank you
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    No, it won't be fixed by PWM because it is the average values that matters. However, you can control the current by using PWM as not to exceed 500mA. The drawback of this is a reduction in the output torque.

    There are regulators rated for more than 1A.
    Have a look here: www.national.com

    If you want, you can use a 1A regulator per motor.

    Have a look at switching regulators too.
     
    jcm2112 likes this.
Loading...