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A common check item for the quality of a switching power supply transformer.
Mar 28, 2018


     A common check item for the quality of a switching power supply transformer

The switching power supply transformer is a power transformer added with a switch tube. In addition to the voltage conversion function of the ordinary transformer in the circuit, it also has the function of insulation isolation and power transmission, and is generally used in switching power supplies and other applications involving high frequency circuits. The quality of the switching power supply includes the insulation test of the transformer, the inspection of the same name of the transformer windings, the detection of no-load current, the detection of short-circuit faults, the continuity of coils, and the detection of primary and secondary coils. What should be done for these items? What about testing? Please see the specific analysis below.

Switching power transformer insulation test.

Use a multimeter R×10k to measure the resistance between the core and the primary, the primary and each secondary, the core and the secondary, the static shielding layer, and the secondary and secondary windings. The pointer of the multimeter should be in the infinite position. move. Otherwise, the transformer insulation performance is poor.

Detect and distinguish the end of the same name of each winding.

When using a power transformer, two or more secondary windings may be used in series to obtain the desired secondary voltage. When a power transformer is used in a series method, the same name of each winding participating in the series must be correctly connected, and no mistake can be made. Otherwise, the transformer does not work properly.

No-load current detection.

a, direct measurement method. All the secondary windings are all open circuit and the multimeter is placed in the AC current block (500mA, in series with the primary winding. When the plug of the primary winding is inserted into 220V AC commercial power, the multimeter indicates the no-load current value. This value should not be It is greater than 10% to 20% of the full load current of the transformer, and the normal no-load current of the common power transformer of the electronic equipment should be around 100mA.If it exceeds too much, it indicates that the transformer has a short-circuit fault.

b. Indirect measurement method. A 10?/5W resistor is connected in series with the primary winding of the transformer, and the secondary is still completely unloaded. Turn the multimeter to AC voltage block.

After power-up, the voltage drop U across the resistor R is measured with the two table pens, and then the idle current I empty is calculated using Ohm's law, that is, I empty = U/R. F? No-load voltage detection. Connect the 220V mains of the power transformer to the primary and use the multimeter's AC voltage to measure the no-load voltages (U21, U22, U23, U24) of the windings in order to meet the requirements. The allowable error range is generally: high voltage winding ≤±10 %, the low voltage winding is ≤±5%, and the voltage difference between two sets of symmetric windings with a center tap should be ≤±2%.

Comprehensive Detection and Identification of Short Circuit Fault of Power Transformer .

The main symptom of the short-circuit fault of the power transformer is that the heat is severe and the secondary winding output voltage is out of order. In general, the more the inter-turn short circuit point inside the coil, the greater the short-circuit current, and the more severe the transformer heat is.

A simple method of determining if a power transformer has a short-circuit fault is to measure the no-load current (as described earlier in the test method). With a short-circuit fault transformer, the no-load current value will be much greater than 10% of the full-load current. When the short circuit is serious, the transformer will heat rapidly within tens of seconds after no-load power-up. Touching the iron core with your hand will make you feel hot. At this point, it is not necessary to measure the no-load current to determine that the transformer has a short-circuit point.

Coil on-off detection.

Put the multimeter in R×1 block. If the resistance value of a certain winding is infinite during the test, it means that the winding has a trip failure.

Distinguish 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 marked with the rated voltage, such as 15V, 24V, 35V. Then identify these tags.

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