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Faults in Centre Tap Full Wave Rectifier

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Faults in Centre Tap Full Wave Rectifier

The faults in a center tap full-wave rectifier may occur in the transformer or rectifier diodes. Fig.6.36 shows the circuit of a center-tap full-wave rectifier. A fuse is connected in the primary of the transformer for protection purposes.

We can divide the rectifier faults into two classes viz.

1. Faults in transformer 2. Faults in rectifier diodes

Faults in the Transformer.

The transformer in a rectifier circuit can develop the following faults :

(i) A shorted primary or secondary winding.
(ii) An open primary or secondary winding.
(iii) A short between the primary or secondary winding and the transformer frame.

(i) In most cases, a shorted primary or shorted secondary will cause the fuse in the primary to blow. If the fuse does not blow, the d.c. the output from the rectifier will be extremely low and the transformer itself will be very hot.

(ii) When the primary or secondary winding of the transformer opens, the output from the rectifier will drop to zero. In this case, the primary fuse will not blow. If you believe that either the transformer winding is open, a simple resistance check will verify your doubt. If either winding reads a very high resistance, the winding is open.

(iii) If either winding shorts to the transformer casing, the primary fuse will blow. This fault can be checked by measuring the resistance from the winding leads to the transformer casing. A low resistance measurement indicates that a winding-to-case short exists.

Faults in Rectifier Diodes.

If a fault occurs in a rectifier diode, the circuit conditions will indicate the type of fault.

(i) If one diode in the center tap full-wave rectifier is shorted, the primary fuse will blow. The reason is simple. Suppose diode D2 in Fig. 6.36 is shorted. Then diode D2 will behave as a wire. When diode D1 is forward biased, the transformer secondary will be shorted through D1. This will cause excessive current to flow in the secondary (and hence in the primary), causing the primary fuse to blow.

(ii) If one diode in the center tap full-wave rectifier opens, the output from the rectifier will
resemble the output from a half-wave rectifier. The remedy is to replace the diode.

Bridge Rectifier Faults

The transformer faults and their remedies for bridge rectifier circuits are the same as for center tap full-wave rectifier. Again symptoms for shorted and open diodes in the bridge rectifier are the same as those for the center tap circuit. In the case of bridge circuit, you simply have more diodes that need to be tested.