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Diode and Capacitor Question for motor connection

Discussion in 'Sensors and Actuators' started by Daniel.Bry, Aug 19, 2017.

  1. Daniel.Bry

    Daniel.Bry

    3
    0
    Aug 19, 2017
    Dear All,

    I have some doubts and I would like seek advice.
    Recently, I am trying to connect a motor to a motor shield, and I saw this picture online.
    l298.png

    I would like to ask about the 100 uF capacitor and 1 uF capacitor in the circuit.
    Would it be all right for me to connect both the capacitor right after pin 4 before the diode, instead of connecting before ground? I had drawn what I am thinking below. Is it better and Does it make any difference to the circuit?
    l298n.png

    For the motor, I realised that each motor is connected with 4 diode. 2 diode to each output.
    Example the first motor is connected to pin 2 (Output1) and then another to ground. Does anyone know why is there a need to connect 2 diode for each output, is it possible for me to connect a diode from to pin 2(Output1) and then directly to ground? Example:
    l2981.png

    Thank You!
     
  2. Bluejets

    Bluejets

    3,510
    709
    Oct 5, 2014
    Capacitor location is the same in both. Diodes cannot be left out/ bridged as you have done otherwise a short to ground will arise in certain circumstances.
     
  3. Daniel.Bry

    Daniel.Bry

    3
    0
    Aug 19, 2017
    Hi Bluejets, in terms of using the capacitor as bypass capacitor, is placing the capacitor infront better? Or is it still the same. I have some confusion.

    Thank You!
     
  4. AnalogKid

    AnalogKid

    2,305
    648
    Jun 10, 2015
    The primary job of a schematic is to show the electrical connections, not the placement of components.

    The two decoupling capacitors should be as close as possible to the IC power and ground pins.

    ak
     
  5. Minder

    Minder

    2,871
    594
    Apr 24, 2015
    Maybe just a misprint, but the small cap should be 0.1μf, it is a H.F. decoupler for the supply.
    M.
     
  6. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    1. As mentioned, the capacitors should be placed near the IC and definitely not after the load. There will be high current surges rushing in and out of those capacitors at the switching edges, that can cause high frequency disturbance.
    If you have added conductors (by putting the capacitors after the load), those conductors will:
    a: work as transmitting antennas for noise,
    b: due to inductance and resistance, delay/reduce the function of the capacitors (to smooth the voltage)

    2. If the load would be purely resistive, you would not need those diodes at all (just remove).
    They are free wheeling diodes that you need if the load is inductive (motors, solenoids etc).
    They are connected "backwards" as in flyback converters or the diodes connected parallel to relay coils and will only be active in the switching moments till the stored energy in the coil has been removed.

    If the L298 would run in regulating PWM mode (not normal DC mode), the diodes would be an essential part of the function. Together with the inductive load they do basically make a switched power supply where the output not necessarily has to be voltage, but can be toque in a motor, force in a solenoid or whatever you want. In such case the diodes preferably would be fast diodes or schottky diodes.
     
  7. Daniel.Bry

    Daniel.Bry

    3
    0
    Aug 19, 2017
    Dear All,

    Thanks for all your valuable input! I have one more question, do you think it is necessary to add two capacitor (ceramic and electrolysis) in-between the respective motors?
     
  8. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    Where exactly do you want to add capacitors?
    I do not think you need more than those two existing caps. Rather try to gather all external power strands (from the diodes) to two distinct spots near the IC (ground and positive supply).
    Place the capacitors between those spots.
     
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