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Resistance of a Transzorb

Discussion in 'General Electronics Discussion' started by dick56, May 2, 2019.

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  1. dick56

    dick56

    18
    2
    Apr 15, 2011
    I encountered a Transzorb in an autopilot circuit. The transzorb was parallel to a coil used to act as a solenoid. My question is what resistance should I read measuring between the two coil leads which have the transzorb connected in parallel, and the coil is supposed to have 170 ohm resistance? I ended up with 3.8 ohms but wasn't sure if the transzorb could be dropping the resistance or the coil is shorted. The problem arose when the autopilot circuit breaker popped whenever power was supplied from the autopilot computer to the servo. The servo motor runs fine in both directions without popping the circuit breaker. The way the autopilot functions is if a signal from the computer goes to the servo, the coil energizes, engages the gearing and then the servo motor starts. The inner workings of the servo are proprietary so I cannot furnish a schematic.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,543
    2,348
    Nov 17, 2011
    A unipolar transzorb diode (the type probably used here) measures as open circuit in the reverse direction and as a diode (~0.7 V) in the forward direction.
    A weird reading. A diode usually drops 0.6 V ... 0.7 V in the forward direction which shows as 600 Ω ... 700 Ω on a multimeter.
    In the reverse direction the diode should not have an influence on your measurement. you should "se" the 170 Ω. Your reading suggests a short circuit, possibly within the coil.
    On the other hand, measuring components in-circuit is always a tricky matter. To ensure proper readings you should always lift at least one end of the component out of the circuit. Only that way you can be sure to measure only the coil, only the diode etc. without having to fight with the compound effects of components being connected.
     
    Cannonball likes this.
  3. dick56

    dick56

    18
    2
    Apr 15, 2011
    Harald, thanks for the reply. I will send it back to the manufacture for a repair. I was hoping it was something I could fix, but I cannot find the proprietary coil anywhere.
    Dick Welsh
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,543
    2,348
    Nov 17, 2011
    Before you do that, check coil and diode separately. The issue may well be a defect of the diode, too. That one is easily fixable by replacing the diode.
     
  5. dick56

    dick56

    18
    2
    Apr 15, 2011
    Thanks Harald, I unsoldered one end, and the coil was a dead short. It would have been nice to replace the transzorb for a few bucks versus a $535 manufacture repair.
    Dick W
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    10,543
    2,348
    Nov 17, 2011
    You could re-wind the coil yourself. Get a spool of magnet wire of the same diameter as the one currently on the coil, unwind the coil, if possible count the number of turns, then wind the same number of turns onto the core using the new wire. If you can't count the number of turns or lose track of it while disassembling the coil, wind as many turns as are required to achieve the original 170 Ω. An online calculator (example ) can tell you how many meters or inches are required.
    If you are lucky you find the short circuit while removing the first turns and you can simply repair that part using a bit of shrink tubing.
    Even if you should fail in your attempt that is going to cost you only a few bucks, definitely less than your stated repair cost.
    But only if that coil is not a safety critical device. In that case the risk of a DIY repair may be too high. It's up to your judgement.
     
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