EMC Question of the Week: September 5, 2022

components on a circuit board

The white cylindrical power-input component on the left of this figure has "F.T.6.3A" engraved on its endcap. This component is almost certainly a

  1. gas discharge tube
  2. metal oxide varistor
  3. axial capacitor
  4. fuse


The best answer is “d.” The first clue is the schematic symbol for a fuse printed on the surface of the circuit board. Another bit of evidence is the 6.3 A current rating. While all passive electrical components are likely to have both a maximum current and maximum voltage specification, the value most likely to appear on the body of a fuse is its maximum current. On the other hand, the primary specification for transient protection devices is their breakdown voltage, and capacitors are more likely to have their maximum operating voltage on the label.

It is also worth noting that this component is connected in a manner that has an unusually high connection inductance. This is of little consequence for a slow-acting series component like a fuse. However, it would generally be unacceptable for transient protection devices like gas discharge tubes or varistors that need to fire quickly in order to protect other components connected to the same power bus. It would also be an inappropriate way to mount an axial capacitor.

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