h-bridge for steper motor

[tsibos22]

h-bridge for steper motor[SOLVED]

Dear freaks,
I found this schematic from the internet (https://sv1hag.blogspot.gr/2012/05/stepper-motor.html)
and i want to drive a stepper motor with an avr. The circuit works maximum at 24 volt but i want to drive my stepper at 34-36 volt(at 24 volts the stepper needs 0.7 Amperes). I read the datasheets and the transistors can works at 40 volts can i drive the stepper at 34-36 volts? I think that i need to change r5,r6,r7,r8 resistors. If i use 1.5K resistors will be ok? I use lm7805 to drop down the voltage 24->5 volts for the avr.
Best regards,
Tsibos

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[barry]

It would probably work with 1K or 1.5K resistors . Those resistors determine the base current of the darlingtons. At 36 volts, that would be about 35 mA, which is well within spec of the TIP126, TIP120.

 

[tsibos22]

Thanks for your answer!!! could you please explain me how you calculate 35mA? Because that was my problem.

 

[barry]

Assume the following: you have 36 volt supply; the saturation voltage of Q5 is 0, the Vbe drop of a darlington is about 1.2 V, the Vbe of Q2 is .6V. Thus, the voltage across R5 is:

36-1.2-.6=34.2V

and the current is 34.2/1K=34.2mA