Motor issue

Discussion in 'General Electronics Chat' started by Dritech, Apr 10, 2012.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    5
    Hi,

    I am driving a motor (12v 2A) using an H-bridge. The supply use is from a bench power supply which with the same supply, the PIC, LCD and other ICs are connected using the 5v.

    Now the problem is when the motor starts, the 5v supply rail (which is supplying the PIC, LCD etc.) is increasing to approx 5.25V.

    Is this normal? and how can i eliminate the 0.25v increase in the 5v supply?

    Any help is appreciated.

    Thanks in advance.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you have the complete schematic of the H-bridge with the driver?
    (including all caps for decoupling and diodes to prevent back EMF troubles).
    It can happen that the motor is giving pulses on the powersupply lines, that are giving trouble in the powersupply itself.

    Bertus
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi, thanks for the reply. Attached is a diagram of the H-bridge used. The only cap which i am not using is the .1uF which is connected in parallel with the motor.
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    No it's not normal. What instrument did you use to measure the 5V? Scope or Meter? Are you sure the increase is real?

    I've also seen bench top supplies doing or displaying strange things when drawing a non-DC current.
     
  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    I used both the scope and meter to measure the output voltage
     
  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Another question; i am using a single schmitt trigger from the 40106 IC. The remaining five schmitt triggers are not being used. Is there another IC with a single or dual schmitt (which is not SMT) ??
     
  7. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Ok finally i found out what was causing the problem. When the motor is active, the h-bridge is draining approx 0.2V to ground. This slight increase in ground is causing the 7805 to increase the output voltage.

    Is it normal that when an h-bridge drives a 12v 2A motor, slight voltage drops to ground ?? if yes, how can i provent the 0.2V from going to the 7805 ground pin ??
     
  8. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
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    any suggestions please?
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What kind of wires are you using to connect the H-bridge to the powersupply?
    Can you try to use thicker wires?

    Bertus
     
  10. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi, i think the wires used are not thick enough. The reason for using tin wires is since the circuit is on a breadboard and thick wires won't clip. Can this be the reason for the 0.2V leakage to ground? if so, why?

    Thanks.
     
  11. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The thin wrires have a higher resistance as the thicker wires.
    Ohms law is applying here V=IxR.
    You can use thick wires with a very short piece of thin wire soldrered at the end to adapt it to the breadboard.

    Bertus
     
    Dritech likes this.
  12. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Bertus, If i make the wires thicker, will the circuit work?
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The voltage drop over the wires will be less if you use thicker wires.

    Bertus
     
  14. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Sorry, i didn't see post 11. Can i use 2 thin wires in parallel instead?
     
  15. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    If you run two thin wires parallel, the voltage drop with be half.
    If you run 4 thin wires parellel, the voltage drop will be 1/4.

    Bertus
     
    Dritech likes this.
  16. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks a lot Bertus for your help :) as you said, the thin wire was causing the problem ;)
     
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