driving the stepper motor unipolar way

[banh]

i am trying to build up a simple circuit for driving stepper motor:
- unipolar.
- using power transistor

have been searching around.. a few circuits use a AND gate between the microcontroller pin and the transistor B pin. Why dun we connect the microcontroller pin directly to the transistor?

 

[nicleo]

The output current from microcontroller might not be enough to drive the stepper motor. Is the logic gate you mentioned CMOS or TTL?

 

[banh]

its normal TTL gate...

providing more current may be the reason.. any protection for the microcontroller when using the gate?

possible to use NOT gate?

 

[nicleo]

It may give some protection.

NOT gate will invert your switching signals. You can use TWO NOT gate in series.

 

[banh]

yuh, what kind of protection is it?
Say, if i do not use gate but connect the pin directly to the transistor..any problem?

of course, if using NOT gate-> just invert the signal from the microcontroller (change the code)

 

[nicleo]

Sorrie, I did not read your first post carefully. In my opinion, it should be OK to connect the I/O pin of microcontroller to the base of the transistor through a resistor that sets the base current.

Can you give an example of such circuit?

 

[yego]

Actually a gate or some additional transistor inserted between the uC and the transistor driver serves two purposes.
Primarily it offloads the uC pin from sinking or sourcing excessive currents to/from the load,
secondly it protects the uC pin against the Back EMF voltage usually induced in the driven coil when the current is cut off.
This voltage should be clamped to the ground potential with some Schottky diode, but sometimes both the diode and the transistor get damaged, and then you have to buy a new uC too.
So if it doesn't cost you too much, install some protection.
Most microcontrollers are protected with parasitic diodes on the die, but many people find this protection insufficient.
/yego

 

[banh]

Sorrie, I did not read your first post carefully. In my opinion, it should be OK to connect the I/O pin of microcontroller to the base of the transistor through a resistor that sets the base current.

Can you give an example of such circuit?

i lost the link using AND gate...
however, found another link **broken link removed**
using NOT gate together with resistor.

Added after 2 minutes:

Actually a gate or some additional transistor inserted between the uC and the transistor driver serves two purposes.
Primarily it offloads the uC pin from sinking or sourcing excessive currents to/from the load,
secondly it protects the uC pin against the Back EMF voltage usually induced in the driven coil when the current is cut off.
This voltage should be clamped to the ground potential with some Schottky diode, but sometimes both the diode and the transistor get damaged, and then you have to buy a new uC too.
So if it doesn't cost you too much, install some protection.
Most microcontrollers are protected with parasitic diodes on the die, but many people find this protection insufficient.
/yego

thanks
it clears my doubts.