De-Spiking Diode - Video Illustrations?

Discussion in 'Homework Help' started by pprendeville, Nov 19, 2009.

  1. pprendeville

    Thread Starter New Member

    Nov 19, 2009

    Hi all,

    My first post on this website. Looks like a great resource.

    Started an Electrical Control Systems course in August and progressing steadily. One of the units we're studying at the moment is about diodes and it's discussing their applications. One such application is to prevent spiking. There is a diagram illustrating the principal of operation of this concept but as of yet I can't grasp it. I'm wondering does anyone know of any java videos or any type of videos that might illustrate this concept graphically that are available online?

    Here's the text to go with:

    It is sometimes necessary to limit the amount of voltage present in an electrical circuit. For example, if the dc voltage to a coil is switched OFF, a back EMF is produced due to electromagnetic induction as we discussed in Unit 1. This back EMF produces an arc at the contacts of the switch used to disconnect the coil.
    If the high-voltage spike generated by switching an inductive device OFF is allowed to reach electronic components like a transistor, the transistor can be destroyed. The de-spiking diode in Diagram (a) protects the circuit from surge currents developed when the current flowing through an electromagnetic device is abruptly interrupted.
    The diode is connected in reverse bias so that current from the supply passes through the coil when the switch is on.

    When the circuit is switched off by, the magnetic field of the coil collapses, inducing a back EMF in the coil. This EMF causes a current to flow that passes through the diode and not through the opening contacts of the switch. No spark is produced, and the sensitive switch contacts switching the coil are protected against damage."

    Hope to get a reply.
  2. pprendeville

    Thread Starter New Member

    Nov 19, 2009
    actually, I've just seen the videos tab up top here so i'm going to give a look at that now.
  3. Paulo540


    Nov 23, 2009
    Hi there,

    I don't have anything diagram-wise to post that isn't proprietary unfortunately. However, I have extensive experience in contactors and relays which many happen to use these or similar components. Basically, what a unidirectional diode does is redirect the burst of voltage (often well over 100V) that occurs from shutting off the power to the coil, and sends it on a loop de loop around the coil until the energy dissipates through the resistance. At least that's my crude way of describing it.

    Not that you asked...but this also has a side effect of delaying the contacts from releasing due to the residual magnetic field circulating in the coil.

    There are other options to navigate around the latter issue while providing voltage attenuation, however, thats definitely another topic.

    I will fish around for some animations but I hope this helps draw at least a rough picture in your mind of what is going on.

    I think they could improve their impact by making a clearer distinction between the two switches they speak of.

    my 2.333 cents