Last Week's Question


Which of the following transient protection components is a "crowbar" device (so called, because it acts like a short circuit when the voltage across its terminals exceeds the threshold).

  1. diode
  2. metal oxide varistor
  3. thyristor
  4. fuse


The correct answer is "c". Diodes and metal oxide varistors (MOVs) hold the voltage across their terminals to a maximum constant voltage. Thyristors on the other hand, "turn on" when the maximum voltage is exceeded and effectively look like a short circuit. This is like dropping a crowbar across the bare wires of the circuit causing a short. Gas discharge tubes are also crowbar devices. Fuses are current-limiting devices.

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