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Which kind of diode to handle flyback voltage?

  1. Aug 14, 2015 #1
    Lately I've been rediscovering my old interest in electronics and in my own time I've been looking through some tutorials on various things I decided to go ahead and build one, an H bridge circuit for a 5 volt DC motor. My question is I'm not clear on which diode I should use as flyback diodes, 1N4002 or 1N5817? And I'm also not clear on something else, for a relatively low power 5 volt circuit how high can the flyback voltage get?
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  3. Aug 14, 2015 #2


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    Can you post a schematic and the specs you want to meet? Why are you using the flyback topology instead of a forward converter?

    And what do you know about building an off-line power supply in terms of safety? Where do the fuse, switch, and ground connections go? And why? :smile:
  4. Aug 14, 2015 #3


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    If the diodes are across the H-bridge drivers then a 1N400x series diode should be fine. The 1N5817 is a Schottky barrier type so it will be faster to turn on faster from reverse voltage and back but at low voltage/power levels I don't think there will be much difference.

    How high the voltage can be? Well it depends: https://www.physicsforums.com/threads/flyback-diodes.705218/#post-4470396
    Last edited: Aug 15, 2015
  5. Aug 14, 2015 #4
    It's not critical. Whatever you find in your junk drawer will work fine for a puny little 5V motor. And don't worry about how high a flyback voltage can get. The point of flyback diodes is to ensure that it doesn't.
  6. Aug 15, 2015 #5


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    If you have schottky then use it because it will not be a temporary short circuit when you go back to reverse bias. Read about reverse recovery time.
  7. Aug 18, 2015 #6
    I'm powering it with a 9 volt battery run through a 7805 voltage regulator. It will be built on a breadboard, the ground connections go to the common ground. The fuse is me ripping out the power connections when it starts smoking. The schematic is basically this.

    The post you linked is very informative, thank you. I think I'll go with the 1N4002.

    An excellent point, thank you.

    I'm not going with the schottky diode, but it sounds like something interesting to read about anyway. Thanks.
  8. Aug 18, 2015 #7


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    Ah, I misunderstood your post. I thought you were building an off-line AC-DC power supply (when I saw "flyback"). But you're building a low-voltage motor controller. Never mind my comment then. o0)
  9. Aug 19, 2015 #8
    I think really the term you are looking for is Freewheeling diode vs Flyback - if the driver ckt is Mosfet the body diode may be able to handle the Free Wheeling current - otherwise a basic small diode will probably do the trick - it would depend on if you are doing PWM speed control or just using the H bridge for directional (on off) as well - but using an H bridge I am assuming PWM ..
  10. Aug 20, 2015 #9

    As far as I understand it the chip I'm using isn't mosfet. I'm using it for direction and not speed control.
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