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Detection of low power transformer
Sep 08, 2018

Power transformer detection

A. Observe the obvious appearance of the transformer by observing the appearance of the transformer: if the coil lead is broken, whether it is de-soldered, whether the insulating material has burnt marks, whether the iron-tightening screw is loose, whether the silicon steel sheet is rusted, and whether the winding coil is There are exposed and so on.

B. Insulation test: measure the resistance between the core and the primary, the primary and each secondary, the core and each secondary, the electrostatic shielding layer and the secondary and secondary windings with a multimeter R×10k block, and the multimeter pointers are Should be referred to in the infinity position does not move. Otherwise, the transformer insulation performance is poor.

 

C. Detection of coil on/off: Place the multimeter in R×1 gear. During the test, if the resistance value of a certain winding is infinite, it indicates that the winding has a faulty fault.

 

D. Discriminate between the primary and secondary coils: the primary and secondary pins of the power transformer are generally led out from both sides, and the primary winding is marked with 220V, and the secondary winding is labeled with the rated voltage, such as 15V. , 24V, 35V, etc. Then identify them based on these markers.

 

E. Detection of no-load current:

 

(a) Direct measurement method: Open all the secondary windings and place the multimeter in the AC current block (500 mA, stringed into the primary winding. When the plug of the primary winding is inserted into 220V AC mains, the multimeter indicates no load. Current value: This value should not be greater than 10% to 20% of the full load current of the transformer. Generally, the normal no-load current of the power transformer of common electronic equipment should be about 100mA. If it exceeds too much, the transformer has short-circuit fault.

 

(b) Indirect measurement method: A 10 /5W resistor is connected in series in the primary winding of the transformer, and the secondary is still completely idling. Turn the multimeter to the AC voltage block. After power-on, the voltage drop U across the resistor R is measured with two meter pens, and then the no-load current I is calculated by Ohm's law, that is, I null = U/R.

 

F. Detection of no-load voltage: connect the primary of the power transformer to 220V mains, and use the multimeter AC voltage to sequentially measure the no-load voltage value (U21, U22, U23, U24) of each winding to meet the required value, and allow the error range. Generally: high voltage winding ≤ ± 10%, low voltage winding ≤ ± 5%, the voltage difference between two sets of symmetrical windings with center tap should be ≤ ± 2%.

 

G. Generally, the low-power power transformer allows the temperature rise to be 40 ° C ~ 50 ° C. If the quality of the insulating material used is good, the temperature rise can be increased.


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