Hi

I'm trying to work out a stepper motor driver for my CNC project. I got some 2.2V / 2A unipolar steppers and planning to use the TIP122 darlington transistors.
And I've been reading a bit of back EMF and diodes are the solution for the back EMF(?). So I'm wondering how I calculate or know what kind of diodes I need to use for my circuit.

Also, the TIP122, do I want a resistance to reduce current on the base-side?

Edit: oh, and dont bite me hard if I did anything wrong, said anything wrong or posted in the wrong place. I'm new to these forums.

 

I usually find the 1N4148 small signal diode is fine for most motors; exceptions apply for bigger motors. If you want to be extra safe you can use any of the 1N400x series. Since the diode is shorting out the motor, the Vrrm is only a concern if you are using 50V+ motors, and the current rarely flows for any more than a few milliseconds so most diodes are OK with it.

Base current for the TIP122 can be quite low, because the gain is very high. I suggest a 1k resistor, that will guarantee to put it into saturation, as long as you have a high enough supply (>2V).

 

Be careful with 1N400x diodes, they may be too slow. All of my stuff is small, 5V or 12V so I use 1N4148's.

 

Thanks for answers, both of you. I'll take a look at the 1N4148.
Considering the back EMF, the voltage can be high, but what about the current?

 

I've never had any problem with 1N400X, they work very well, and they will absorb the currents nicely. This is how various myths (like resistors aren't good enough for LEDs) get started.

 

1N400x will work OK if you only will step the motors at low speeds. At higher speeds, the diodes won't turn off quickly enough, and performance will suffer. I suggest fast-recovery diodes instead.

1N4148/1N914 diodes are very fast, but have limited forward current; 100mA being the max. I suggest using fast-recovery diodes that are a bit more robust.
MUR120, SR202, UF400x, 1N5818 would all work and be fast.

If you want your motor current decay times decreased, you could use Zener diodes back-to-back with a fast recovery or Schottky diode. The fast recovery diodes' cathode goes towards the more positive connection, the Zener's cathode towards the more negative connection.

 

Wookie, did you review his needs?

I'm trying to work out a stepper motor driver for my CNC project. I got some 2.2V / 2A unipolar steppers and planning to use the TIP122 darlington transistors.

I tend to like high speed high current schottky diodes, but this is a pretty modest application I would think. Fairly low power and inexpensive darlingtons.