DC supply

While secondary coil or 'winding' is commonly identified as output voltage supply AC. If this transformer is step voltage then the secondary voltage can be ranged 20v, 50v, 80v or 120v. The output expected to be lower than primary side.

Identify transformer terminal for measurements

Primary coil/winding

Firstly identify the primary winding following label:

Vp - mean primary voltage or 'input voltage' commonly 240vAC

Np - number of turns on primary winding, unit is 'T' means turns.

Ip - primary current, unit is ampere or 'A'.

Secondary coil/winding

Secondly identify the secondary winding of the transformer with the following label:\

Vs - means voltage secondary or 'voltage output'.

Ns - number of turns on secondary winding, unit is 'T' means turns.

Is - secondary current, unit is ampere or 'A'.

Measuring transformer winding on both primary and secondary winding.

When your multi-meter's probe is place on the t1 and t2 terminal. The multi-meter will have reading taken from the Np value in ohm.

Expected the primary side would be higher resistance, example 85Ω.

Measuring secondary side winding.

Secondary coil are basically use to lower the voltage. Place the multi-meter again on the secondary coil, the meter will reads lower ohm, example 3.5Ω.

Conclusion:

This transformer is a step-down transformer.

Measuring Transformer sequence

Position and place the multi-meter probe on the following terminal.

measure between t1 and t2, result should have some value or 85Ω .
measure between t1 and t3, result should be infinity .
measure between t1 and t4, result should be infinity .
measure between t3 and t4, result should have some value or 3.5Ω .

Transformer fault condition

When using multi-meter to measure the winding, if the transformer having some fault, the meter reading shall be infinity or ' ∞ '

Figure on the right showing example of fault transformer can be identified using a test known as

CONTINUITY test

When measure between t3 and t1 or between t2 and t4, there is no connections between them therefore the meter reads infinity or 'O.L' .

Transformer can be wrongly configure

Example in the figure on the right, the polarity of the transformer is been reversed.

Outcome of this transformer

When load are connected to the terminal t1 and t2, the voltage is greater, this known as step transformer. Both winding resistances is same as measured earlier.

Load can be damaged due to high voltage.

Rectifier

Rectifier basically consist of 4 diodes configure into wheat-stone or 'diamond' figure.

Each of these diode is just a simple semiconductor, which main purpose is to allowing current to flow one direction.

diode + anode- cathode

Forward Bias

When measuring a diode, it is almost important to identify the diode polarity.

The term forward bias means that the meter can reads some ohm Ω value.

Reverse Bias

This means that the meter reading shows infinity or 'O.L' . The current cannot flow through the diode.

Identify rectifier terminal

Basically rectifier consist of 4 diode labeled as shown on the right figure.

The terminal label t2, t4 is the connections from secondary transformer from side ac out from transformer.

While the t1 is connected to a negative terminal (ve-) at output of rectifier.

The t3 is connected to (ve+) at output rectifier.

How to measure a rectifier ?

To measure the rectifier starts with the sequence measurements:

Step 1:

Measure from point t1 to t2 according the figure above. The result expected Ω value because is forward bias.

Step 2:

Measure from point t2 and t3 the readings should be about 50Ω.

Step 3:

Measure from point t1 to t3, the meter readings should be total adding from step 1 and step 2.

Step 4:

Measure from t1 and t4, reading should be same 50Ω.

Step 5:

Measure at point t4 and t3, the result should be same 50Ω.

|Measure | Reading |

| t1 - t2 | 50Ω |

| t2 - t3 | 50Ω |

| t1 - t3 | 100Ω |

| t1 - t4 | 50Ω |

| t4 - t3 | 50Ω | all forward bias

| t2 - t4 | ∞ | <- infinity

| Measure | Reading | infinity

| t2 - t1 | ∞ |

| t3 - t2 | ∞ |

| t3 - t1 | ∞ |

| t4 - t1 | ∞ |

| t3 - t4 | ∞ | all reverse bias

| t4 - t2 | ∞ |

How to identity +ve and -ve on rectifier ?

Correct polarity of rectifier can identify by looking at the test lead/probe of an analog multi-meter.

the highest ohm value is between t1 and t3 which is 100Ω, (ve+) would be the black probe. The (ve-) would be on the red probe.
the no contact on both of the t2-t4, or t4-t2 the result reading would be infinity, ∞ .

Complete DC supply connections

Refer to figure on the right, the once the terminal is identified the connection to AC will able to produce DC voltage. This part still filtering circuit to smoothen the DC wave form into straight line.

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