What kind of diodes for back EMF?

Discussion in 'General Electronics Chat' started by somlioy, Jul 31, 2010.

  1. somlioy

    Thread Starter Member

    Jul 28, 2010
    22
    3
    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. :)
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    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).
     
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    Be careful with 1N400x diodes, they may be too slow. All of my stuff is small, 5V or 12V so I use 1N4148's.
     
  4. somlioy

    Thread Starter Member

    Jul 28, 2010
    22
    3
    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?
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    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.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    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.
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,764
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    Wookie, did you review his needs?

    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.
     
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