H-Bridge Problem

Discussion in 'General Electronics Chat' started by Molimo, Nov 21, 2008.

  1. Molimo

    Thread Starter New Member

    Nov 20, 2008
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    Ok here goes, am trying to build an H-Bridge (2 actually) for a small robot for a class project. I am using 2 TIP125 and 2 TIP121 as Darlington Gates, now here is the main problem. When i try to drive a motor with a load (gearbox) the H-bridge just crashes my Microcontroller (meaning it resets it) and if I try it with the motor on its own it works perfectly.

    Here is how it is Wired. I have the Tip125 on top row, 3Volt VCC coming from a source going to the Emitter of the Transistors. Base goes to a 1K resistor then to the PWM from the Microcontroller and then the Collector pin goes to the next transistor which is a TIP121. His base goes to a Resistor (1K) then to the same PWM. and its Emitter goes to the ground.

    Now that would be ONE side since the other side I insert a Inverted PWM (compared to the other one).

    Now like I sais it works perfectly when I use a motor without load, it seems that when I try using a Gearbox motor that the Startup current is too high and tries pulling it from the MCU for some reason. I just dont get why my MCU Resets (the wheels turn for half a second) and then I have to power down the MCU and power it back up.


    Thanks for the Advanced Help

    Any thoughts will be appreciated

    Marc
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    I would check for switching spikes on your power supply bus. See previous thread about decoupling capacitors. You might need to use a separate power supply for your motors and your CPU

    eric
     
  3. Molimo

    Thread Starter New Member

    Nov 20, 2008
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    Ok, THey are on Different Power Supply, I have two different sources for both system, the first one supplies 9V with a 7805 5V Regulator to the MCU and the 2nd Source only suplies the Motor and gives out 3V.

    Will the Decoupling Capacitors still do any difference?

    Thanks

    Marc
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    It would be very helpful if you could sketch your circuit and post it here.

    Schematics are a very helpful tool in communicating your design ideas to others.

    hgmjr
     
  5. Molimo

    Thread Starter New Member

    Nov 20, 2008
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    Ok here is my schematic, Just note that the Transistors I am using are not the same model, the ones I am using (TIP121 and TIP125) already have the Diode Inside. ) but this is How they are wired.

    Any suggestions will help. I have tried lots lol.

    Thanks again

    Marc
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    I think you may have accidentally reversed Q4.

    hgmjr
     
  7. Molimo

    Thread Starter New Member

    Nov 20, 2008
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    Yeah i just Switched it in the Schematic, but in the circuit they are not reversed
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    214
    I believe the weakness in the design stems from the use of darlington transistors. The problem with darlingtons is their Vce saturation is on the order of a 1 or so at the currents you are using. That means that most of the 3 volts you have to power the motors is dropped across the transistors. That leaves very little if any voltage to power the motor.

    hgmjr
     
  9. Molimo

    Thread Starter New Member

    Nov 20, 2008
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    I do agree with that, But I also started out with 2n2222 Transistors and their Compliment, normal BJT Transistors. I switched since they were only rated at .8A MAX. Same thing was happening with the.

    Also i tried raising the VCC with a Voltmeter at the motor so regulate it at 3V.

    Now last thing is, its not the voltage since My bridge works perfectly with the same motor out of the Gearbox, its just when I try using the gearbox that it barely starts.

    I also tried plugging the Gearbox directly to the 3V source alone and it pulls out around 300-500mA, and over 1A to start.

    So i am wondering Why the MCU is crashing. Is the Base Current Getting That HIGH?

    THanks

    Marc
     
  10. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    KL7AJ's suggestion of large filter caps to suppress the voltage spikes created by the motor when it draws the high current at start up is a good one.

    hgmjr
     
  11. Molimo

    Thread Starter New Member

    Nov 20, 2008
    6
    0
    Ok I tried that this morning and nothing happened. It does exactly the same thing, which is resetting my Microcontroller when too much current is demanded for the motor. So does anyone have any suggestions or working H-Bridge with a MCU and motor demanding over 300mA??

    Anything would help

    Thanks

    Marc
     
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