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h-bridge will not stop motor

e14 Contributor

image

 

I built most of this circuit, but before I finished it I connected the motor to just one tip125 then to the tip120 on the other side

of the bridge.  I was glad I did because I could never get the tip120 to turn off.  It is an H-bridge that is controlled by

(hopefully) an arduino.  I am hoping to use an op amp to amp the voltage from the arduino to then control the two 2n4403

transistors.  My problem is the signal from the small 2n4403 to the h-bridge transistors never goes to ground.  I tried controlling

it with potentiometers and whatever I tried I could not turn off the tip120 s with the 2n4403.  I can control the tip125 s well enough

but that doesn't make a working h-bridge.  Any help with this would be appreciated.

 

schematic made with easyeda.com software

Parents

  • We shouldn't ignore the diodes that are already come with the Darlingtons when considering this circuit.  Two ways to avoid using the parasitic diode is to install a blocking diode or shunt them with a lower-drop diode such as a Shottky.  These diodes typically have half the current-carrying capacity as the primary conduction path.

    Base-emmiter shut-off resistors should be considered.

    One fairly easy way to avoid the negative effects of(simultaneous activation of the PU and the PD [for John: pull-up, pull down]) punch-through (and solve some other problems as well) is to put a constant current sink between the H-drive and ground.

    Another way to avoid punch-through would be to use four-wire control.

    I'd probably use something like an HC'49 to drive the power Q's for the push-pull.

    The clamping scheme shown here is first-order and will cause the motor to have low torque toward the end-of-step.

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  • We shouldn't ignore the diodes that are already come with the Darlingtons when considering this circuit.  Two ways to avoid using the parasitic diode is to install a blocking diode or shunt them with a lower-drop diode such as a Shottky.  These diodes typically have half the current-carrying capacity as the primary conduction path.

    Base-emmiter shut-off resistors should be considered.

    One fairly easy way to avoid the negative effects of(simultaneous activation of the PU and the PD [for John: pull-up, pull down]) punch-through (and solve some other problems as well) is to put a constant current sink between the H-drive and ground.

    Another way to avoid punch-through would be to use four-wire control.

    I'd probably use something like an HC'49 to drive the power Q's for the push-pull.

    The clamping scheme shown here is first-order and will cause the motor to have low torque toward the end-of-step.

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