H bridge dc motor power problem.

Discussion in 'The Projects Forum' started by kerolah, Oct 14, 2012.

  1. kerolah

    Thread Starter New Member

    Mar 11, 2012
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    Hi, i am new here so please be nice.

    i have to make an h bridge to control a dc motor go forward and reverse for school.

    i basically followed the diagram below and it works.

    Problem is my motor does not get enough power and sometimes needs me to spin it by hand to get it started...

    i took the motor out of an old toy along time ago and dont know what power it needs.

    So basically i need to know how to get more power to my motor. should i lower my resistor values to allow more current through my transitors or will that damage them?


    [​IMG]

    and here is my circuit.

    [​IMG]

    Thanks in advance guys!
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a schematic of your circuit?

    The 9 Volts battery may have to much internal resistance.
    Try the circuit with a battery pack of 6 AA batteries or larger.

    Bertus
     
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  3. kerolah

    Thread Starter New Member

    Mar 11, 2012
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    Thanks for the reply Bertus.

    i dont have a schematic. i will draw one up later tonight.

    i got the whole circuit from here

    Thanks.
     
  4. kerolah

    Thread Starter New Member

    Mar 11, 2012
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    im not sure which way leg is the emitter on the schematic.
    small current is applied to the base when the switch is pressed down. allowing the circuit to work.
    How much current will the 2n2222a transitor take in the base to allow my circuit maximum power.

    i have 9v/22k = .4milli amp ? is this maximum ?

    [​IMG]
     
    Last edited: Oct 14, 2012
  5. nerdegutta

    Moderator

    Dec 15, 2009
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  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    your schematic is incorrect. I will assume the motor reverses when you press the reverse switch.

    If you have access to a multimeter, measure the voltage across the motor.

    If you can measure the voltage on the motor when you connect the battery directly to it. You will see the speed of the motor and you will see what the battery measures under that load.
     
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  7. kerolah

    Thread Starter New Member

    Mar 11, 2012
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    I dont have access to a multimeter. i will try use one when i go to school.

    i know when i connect the motor directly to the battery it is much faster.

    can you please explain why my schematic is wrong? so i can get a better understanding.

    also how what component do i change to allow my motor to run faster. Thanks Alot.
     
  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Discounting the diodes in the schematic on the link that nerdegutta posted, which are needed, you will see the difference between your schematic and one properly drawn.

    What are the motor specifications .... voltage and watts ... or voltage and current?

    attached is the instructable graphic that you used with the transistors annotated and the wire color scheme changed for easier deciphering. It's still an unsat design for a variety of reasons.
     
    Last edited: Oct 14, 2012
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  9. Audioguru

    New Member

    Dec 20, 2007
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    Nearly every Instructable is WRONG. This one is also WRONG.

    I agree that the switches are connected to the WRONG transistors.

    The value of the resistors is also WRONG.
    If the resistors are 2.2k (red, red, red) then the transistors will turn on better like on the other schematic. If the resistors are 680 (blue, gray, brown) then the transistors will turn on MUCH better.

    The upper transistors are emitter-followers that have a voltage loss of about 0.8V to 1V.

    If the battery is six new AA alkaline cells in series to make 9V then the motor will spin much faster.
     
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  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    concur

    They do meet the objectives of their authors though ... their motor turned cw and ccw.
     
    Last edited: Oct 14, 2012
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  11. kerolah

    Thread Starter New Member

    Mar 11, 2012
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    Thanks for the help guys.

    But in this one how does a small current get to the base of the transistor to turn it on?

    and am i right in saying the diodes stop the circuit from shorting if both buttons are pressed simultaneously?




    Why do you say this? doesn't the switch in my circuit allow a small current to the base enabling the transistor to open?

    Ok so what is a formula i can use to calculate the optimal resistor without the transistor blowing up. if the transistors is in my case a 2n2222a ( 2n2222a datasheet) and my battery is a single 9 volt battery.


    Why is this make it spin faster? wouldn't they both have the same power?


    Thanks alot for your response guys you are helping me out tremendously and i really appreciate it.:):):):):):)
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Which one? your instructables or the one on the linked page?

    no you are not correct.

    I'm guessing you didn't read the page that nerdegutta linked:

    Pressing both buttons simultaneously will help the economy as you will be buying four new transistors and possibly a new battery.
     
    Last edited: Oct 15, 2012
  13. kerolah

    Thread Starter New Member

    Mar 11, 2012
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    yes i read this. But my motor is small and has no way of producing a large voltage spike that is why i am confused why you said they are very important for my circuit.

    if my motor was larger than obviously i would have diodes in my circuit.

    and i meant my circuit.

    so do you know the formula for calculating what resistor i need ?
     
    Last edited: Oct 15, 2012
  14. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Small is a relative term. What is the rating on your motor? Are there any numbers on it?
     
  15. nerdegutta

    Moderator

    Dec 15, 2009
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    Code ( (Unknown Language)):
    1. (Supply voltage – voltage drop) / (Collector current / Hfe)
    I think this link explains a bit. Hopefully it's not full of errors, like some of the Instructables... :rolleyes:

    http://kaizerpowerelectronics.dk/calculators/transistor-base-resistor-calculator/
     
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  16. kerolah

    Thread Starter New Member

    Mar 11, 2012
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    nope but the toy is came out of had no more then 6 AA batteries. maybe 4.
     
  17. kerolah

    Thread Starter New Member

    Mar 11, 2012
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    (9-.6)/(.8/100) = 1050 ohm. awesome. i used the min vbe sat value. because wasnt sure what to use. ill get some resistors later today and test it out.

    Thanks for this formula man
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    You and your "CALCULATOR" are making the same error as the Instructable.

    1) The datasheet for any transistor shows that when it is a LINEAR AMPLIFIER (not saturated like a switch) then its hFE is used to calculate the base current needed when its collector to emitter voltage is much higher than when it is saturated switch. (hfe is the AC current gain and hFE is the DC current gain).
    The hFE for a 2N2222A transistor is calculated when the collector to emitter voltage is 10V (then it is an amplifier with a 20V supply).

    2) The datasheet shows that when the transistor is a saturated switch then its maximum collector to emitter voltage is 0.3V to 1V depending on the amount of collector current when its base current is 1/10th the collector current. hFE is not mentioned when a transistor is a saturated switch.

    We don't know how much current the motor uses. It might be 100mA. The little 9V battery (A carbon-zinc one is rated for 5mA and an alkaline one is rated for 25mA) as shown on its datasheet but six AA alkaline cells (they are rated for 5 times the current or for 5 times as long as the 9V battery) as shown on their datasheet.

    Assuming a motor current of 100mA then the base current should be 10mA. The base resistor should be (9V - 0.7V)/10mA= 830 ohms. Use 820 ohms.
     
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