Resistor values for modified schematic

[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

[WattsThat]

The L293 is an ancient device that isn’t usable at 5 volts due to its very voltage drop (>2 v). Modern devices with mosfets are far superior in performance to the old bipolar parts.

Beta only matters when using a transistor in its linear region. The general rule of thumb for driving a transistor in saturation is to use 1/10 the collector current. The required base current is usually specified for switching applications when the device is considered a good switch with low c-e voltage when saturated. This is not the case for the BD1xx parts as the are considered amplifiers, not switches. You can paint an apple orange but it’s still an apple.

[andrewlapham]

Can anyone suggest a schematic to use for my purpose?

[RoGeorge]

MarkF did suggest a few schematics in those links posted a few messages ago.

Another idea will be to use the circuit already existing on the CD-ROMs.  Since you want to use the mechanical parts from older CD-ROM units, I guess you have the electronic parts, too.  Follow the PCB traces and see what IC was controlling each motor.  Find the datasheet for those IC, and you have the control bridges.  That's what I'll try first, but it's OK to build your own H bridges too.

Back to the attached schematic, note that most of the online schematics are using 4 I/O pins, one for each transistor.  The attached schematic is trying to use only 2 I/O pins.  What is very bad in the OP schematic, is the fact that you connected together the 2 base resistors from the transistors on the same side.  Another thing is it does not have protection diodes.  For very small motors it might work without diodes, but it's better to have them if you plan to use PWM.

Here is a random example of controlling the 4 transistors with only 2 inputs.  Not the best, not the worst either, at a first look seems OK:  http://www.learningaboutelectronics.com/Articles/H-bridge-circuit-with-transistors.php  It is mostly to show how to control the upper side without making a short circuit with the transistors.  Still needs diodes to be OK.

Keep going, and tell how it all went!   

[schmitt trigger]

What RoGeorge posted is a good start, and if you add the protection diodes it will be better.

What I would add is a series current sampling resistor from the two bottom emitters to ground. Calculate its ohmic value for a small voltage drop at the motor's full rated current. let's say, 0.2 volts.
Sense that voltage with your micro, and disable the H-bridge drive if the voltage drop becomes too large. Let's say 3X the maximum rated current.

An RC filter to smooth out spikes would also be helpful.

Please note that I am not giving you exact values, because that has to be confirmed experimentally by you.

[andrewlapham]

Before I read this I had a lightbulb idea in bed last night, haha. exactly what you suggested. finding the IC doing the job from the original board. cheers for the suggestions.

[Jwillis]

You can still play around with the circuit you have.It's not perfect and  there are issues you need to be aware of. Here,s a good tutorial on what your trying to acommplish with your circuit. It shows how drivers work in their simplest form. https://www.digikey.com/eewiki/display/Motley/How+to+Drive+a+Stepper+Motor][url]https://www.digikey.com/eewiki/display/Motley/How+to+Drive+a+Stepper+Motor[/url]