DC Motor Power Amplifier issue

Discussion in 'General Electronics Chat' started by BershaM, Jul 23, 2016.

  1. BershaM

    Thread Starter Member

    Aug 22, 2011
    17
    2
    i built this DC motor power amplifier to drive the motor in both directions using an analog signal (from -1 volt to 1 volt via the potentiometer ) , the motor i use is rated for 24 volts and draws around 400mA at no load , however, when i tested it only one direction works fine ( the NPN TIP142 side) with no significant heat , the other side however doesn't drive the motor all the way to 10 volts and the PNP transistor becomes hot real fast, i replaced it with another tip147 but still got the same results, any idea what maybe causing this?

    Update 1: I inserted a resistor in series with the PNP gate (5.1k) and it enhanced the motor's performance , however the oscillations still exist , should i also consider an RC snubber ??

    [​IMG]
     
  2. AlbertHall

    Distinguished Member

    Jun 4, 2014
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    Is C42 an electrolytic?

    What does 'enhanced the motor's performance' mean?
    What oscillations? These are mentioned previously?
     
  3. tcmtech

    Distinguished Member

    Nov 4, 2013
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    Why are there two Op-Amps in series when one would sufice?
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Agree, I have done just about the same with a single LM311.
    Max.
     
  5. AlbertHall

    Distinguished Member

    Jun 4, 2014
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    Agreed, but that's not the cause of the overheating problem.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I assumed it to be an incidental comment on the apparent over-engineering and not intended as a reason for the original OP's complaint?;)
    Max.
     
  7. Ylli

    Member

    Nov 13, 2015
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    I'm sure the circuit does not like driving that capacitive load. Try adding a 10 ohm resistor in series with the 1 uF.
     
  8. tcmtech

    Distinguished Member

    Nov 4, 2013
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    I would have just used basic 5 pin power Op-Amp like an LM1875 or TDA2003 (<$2) to drive the little motor.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Yes, in my case it was a 240vdc generator field.
    Max.
     
  10. tcmtech

    Distinguished Member

    Nov 4, 2013
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    The problem is from over complicating/engineering a simple system. :p

    One pot, one power Op-Amp IC, and two resistors should be all that's required a .1 uf capacitor on each power lead and across the motor.
     
  11. BershaM

    Thread Starter Member

    Aug 22, 2011
    17
    2
    @AlbertHall 1- C42 is ceramic.
    2- enhanced its performance means it increased the motor's speed, should've explained it better sorry about that.
    3- a semi sinusoidal wave of around 5Vpp above any dc signal i apply to the circuit, measured it at the emitter and the base of the transistors.

    @tcmtech i thought the first stage would act as a buffer ( obviously not :/ ) , i tried a TDA2003 at first but i always ended up overheating it and i couldn't know why , that's why i chose to do it with darlingtons.

    @Ylli thanks for your feedback will try it and see if it works.

    @MaxHeadRoom i know now it may have been made alot simpler , i just wanted to experiment with darlingtons :v

    Should adding a cap from Pin 14 to Ground adjust the phase shift and remove the oscillations ?
     
  12. tcmtech

    Distinguished Member

    Nov 4, 2013
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    Did you have a properly sized heatsink on it?

    A TDA2003 Power Op-Amp should have had no problems running your motor being it's rated for 3.5 amps continuous and 20 watts thermal dissipation with a properly sized heatsink which means that with your +- 15 volt power supply rail voltages it would take a solid 1.33 amps into a stalled motor to reach its thermal dissipation rating limit.

    So given that, if you overheated it with a 400 ma motor load somethings not adding up here.
     
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