bidirectional diode

Discussion in 'Mechanic and Repair' started by Leaf Jockey, Oct 30, 2009.

  1. Leaf Jockey

    Leaf Jockey LawnSite Senior Member
    Posts: 360

    I'm working on a Husky ZTH 7227 that has a problem blowing the fuse for the electric clutch. Looking at the wiring diagram I see a bidirectional diode that runs between the two wires going to the clutch. What is a bidirectional diode, what does it do and can it go bad and be the cause of my problem?

    Scott
     
  2. madmower

    madmower LawnSite Member
    Posts: 43

    a diode only works one way if you conect a ohm meter to it you should get a reading one way and infinity the other way if this is not the case you have a blown diode and this is what is blowing the fuse a diode can be used as a safety device or to convert ac to dc as currant can only travel in one direction

    bill
     
  3. ericg

    ericg LawnSite Silver Member
    Posts: 2,783

    There is not that much out on the web describing what a bidirectional diode is but here is what I found:

    The DIAC, or diode for alternating current, is a trigger diode that conducts current only after its breakdown voltage has been exceeded momentarily. When this occurs, the resistance of the diode abruptly decreases, leading to a sharp decrease in the voltage drop across the diode and, usually, a sharp increase in current flow through the diode. The diode remains "in conduction" until the current flow through it drops below a value characteristic for the device, called the holding current. Below this value, the diode switches back to its high-resistance (non-conducting) state. This behavior is bidirectional, meaning typically the same for both directions of current flow. I attached a diagram of the diode.

    Have you checked the clutch to see if the coil was grounded which could cause the fuse to blow from excessive current? If the diode was bad, it looks like you could be putting ac on a dc clutch which could cause excessive current and a blown fuse.

    Eric

    diac-symbol.gif
     
  4. Leaf Jockey

    Leaf Jockey LawnSite Senior Member
    Posts: 360

    I know the problem isn't with the clutch because it blows the fuse with the harness unplugged from the clutch.
    The machine has a harness plug at the switch console. I was able to unplug and remove the rear section of the harness. After inspection I reinstalled the harness and it worked. Now I need to pull it back out, remove the outer sheath and inspect it again. There must be a worn spot in it somewhere.
    I'm trying to understand how that diode works with the clutch.

    Scott
     
  5. madmower

    madmower LawnSite Member
    Posts: 43

    sorry i did not see it was a bidrecional diode this can be used insted of a relay to switch a load posably the surge to start the clutch
     
  6. ericg

    ericg LawnSite Silver Member
    Posts: 2,783

    Leaf Jockey, the diode converts AC voltage that the the stator produces to DC for the clutch to operate. It seems as if you have found the problem. Now you have to see where the wire is grounded.
     
  7. Leaf Jockey

    Leaf Jockey LawnSite Senior Member
    Posts: 360

  8. unkownfl

    unkownfl LawnSite Gold Member
    Posts: 3,838

    A transient voltage suppression (TVS) diode is an electronic component used to protect sensitive electronics from voltage spikes induced on connected wires. It is also commonly referred to as a transorb, after the brand name TransZorb registered by General Semiconductor (now part of Vishay). STMicroelectronics sells them under the name Transil. The name Tranzil can also be seen.

    The device operates by shunting excess current when the induced voltage exceeds the avalanche breakdown potential. It is a clamping device, suppressing all overvoltages above its breakdown voltage. Like all clamping devices, it automatically resets when the overvoltage goes away, but absorbs much more of the transient energy internally than a similarly rated crowbar device.

    A transient voltage suppression diode may be either unidirectional or bidirectional. A unidirectional device operates as a rectifier in the forward direction like any other avalanche diode, but is made and tested to handle very large peak currents. (The popular 1.5KE series allows 1500 W of peak power, for a short time.)

    A bidirectional transient voltage suppression diode can be represented by two mutually opposing avalanche diodes in series with one another and connected in parallel with the circuit to be protected. While this representation is schematically accurate, physically the devices are now manufactured as a single component.

    A transient voltage suppression diode can respond to over-voltages faster than other common over-voltage protection components such as varistors or gas discharge tubes. The actual clamping occurs in roughly one picosecond, but in a practical circuit the inductance of the wires leading to the device imposes a higher limit. This makes transient voltage suppression diodes useful for protection against very fast and often damaging voltage transients. These fast over-voltage transients are present on all distribution networks and can be caused by either internal or external events, such as lightning or motor arcing.

    I copied this from wiki. It is correct I read it.
     
  9. ericg

    ericg LawnSite Silver Member
    Posts: 2,783

    Unknownfl, thanks for clearing this up. So because there is so much of a voltage spike when you disconnect an electrical coil as in the pto clutch, this bi directional diode bleeds off the excess voltage spike to prevent damage to other electrical components? I can't recall seeing bidirectional diodes in other machines. Is this a common way to correct this problem or is there another way electrically to perform the same function without a diode?

    Thanks
    Eric
     
  10. unkownfl

    unkownfl LawnSite Gold Member
    Posts: 3,838

    regulator. Leaf jockey pm jim q or something like that he is a Husqvarna engineer. Instead of taking everything apart just use a ohm meter to confirm that you have a leak. You may need to hi-pot the cables to find the leak using a megohmeter.
     
    Last edited: Oct 31, 2009

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