Troubleshooting Diodes

Types of failure which could occur in power supply rectifiers include:

  • open diodes
  • shorted diodes
  • open filter capacitors
  • shorted capacitors
  • leaky capacitors
  • open transformer windings
  • shorted transformer windings


Open diodes:

As shown in the following figure, a test for an open-diode in a half-wave rectifier shows 0V at the output. For a center-tapped full-wave rectifier, if the circuit were functioning properly, a normal ripple voltage would be seen at the frequency of 120 Hz. If one of the diodes were open, a ripple voltage would be observed at 60 Hz, with a larger than normal amplitude. This happens because the capacitor discharges more than it would with a normal full-wave voltage, resulting in a larger ripple voltage with a frequency of 60 Hz. An open diode in a bridge rectifier creates the same problems as those just discussed for the center-tapped rectifier. It provides a half-wave output with an increased ripple voltage at 60 Hz.



Shorted diodes:

A shorted diode has failed in such a way that it has very low resistance in both directions. If a diode suddenly becomes shorted in a bridge rectifier, an excessively high current flows during half of the input cycle, possibly burning open one of the diodes. If one of the diodes does not burn open, the transformer can get damaged unless the power supply is properly fused. According to the number of diodes damaged, one observes either a half-wave signal or no voltage at the output.


Open filter capacitors and shorted or leaky filter capacitors

An open filter capacitor results in a full-wave rectified output voltage. A shorted capacitor would most likely cause some or all of the diodes in a full-wave rectifier to open due to excessive current or it would blow the fuse. In any case, there would be no voltage at the output. A leaky capacitor can be represented by a leakage resistance in parallel with the capacitor. The effect of the leakage resistance is to reduce the discharging time constant. This increases the ripple voltage at the output.

Open or shorted transformer:

Though we are not using real transformers for the experiments here, instead only the Uni-Train function generator, it is important to remember that the ac power supplies whose voltage we are trying to rectify are usually transformers. If either the primary or secondary winding of the transformer in a power supply opens, there is no output voltage. If some of the turns in the primary winding are shorted, this effectively increases the transformer's turn ratio, thus raising the dc output voltage impermissibly. If some of the turns in the secondary winding are shorted, this effectively reduces the turns ratio, thus lowering the dc output voltage impermissibly. 

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