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Kickback diodes — Parallax Forums

Kickback diodes

NewzedNewzed Posts: 2,503
edited 2005-06-27 19:12 in BASIC Stamp
External kickback diodes are connected·cathode to relay positive and·anode to relay negative.· Correct?

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Comments

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-06-27 18:24
    They're inserted reverse of normal circuit current flow (so they don't conduct under normal conditions).· See·the attached diagram.

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    Jon Williams
    Applications Engineer, Parallax
    Relay_Driver.gif
    970 x 848 - 10K
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-06-27 19:12
    I think you're both saying the same thing -- Jon's is less ambiguous, as a relay typically doesn't HAVE an identified 'relay positive' and 'relay negative' terminal.

    The relay coil, when power is removed, has a back EMF from the collapsing magnetic field -- in other words, it took voltage and current to build up the magnetic field, which 'pulls-in' the contactor. Now current through an inductor (or coil) can't change instantaneously. If you remove the driving voltage, the current continues as the built-up magnetic field collapses.

    The diode gives someplace for this current to go, which doesn't disturb other parts of the circuit. Note this means the diode is 'reverse biased' for normal operation, and 'forward biased' only when conducting this 'kick-back' current.

    Without the diode, the current discharges through the 'off' transistor, with destructive effects.
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