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Resistor values for modified schematic

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andrewlapham:
Hi guys. While I wait for my L293D Stepper driver IC's to arrive from China I wanted to have a go at making my own Full H-Bridge driver. I have searched around and found a suitable schematic and have decided to replace the transistor in the original plan with the BD139 and BD140's I have available. the only problem I face is I have no idea how to replace the resistor values to fit with the new schematic. I have uploaded the original design and my modification if you want to have a look, cheers

Jwillis:
Rb=(Vin-Vbe)/Ib   taking into account Ib =Ic /beta . What are the current demands of the motor.

RoGeorge:
That schematic is not good because if any of the terminals 1, 2 3 or 4 remains disconnected, that will open both the transistors and thus short-circuit the battery.  During a reset, or if by accident if the microcontroller's I/O pins are set as input instead of output.

Also, during switching, too, that schematic will short-circuit the battery.

Always use a fuse in series with the battery.  Fusible is a must because a good battery can easily start a fire if shorted, and a short circuit can happen at any time (even with a good schematic) if transistors fail.

andrewlapham:
Sorry, I forgot to mention the usage. I'm going to be using it to drive the steppers used inside CD-ROM drives. I'm building one of those small drawing CNC's

MarkF:
http://www.bristolwatch.com/ele/h_bridge.htm

http://www.bristolwatch.com/ele/tr5.htm

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