Collection of testing methods for electronic components and transformers!

Date：2021-04-26 16:36:00Views：1436

Transformer is a device that uses the principle of electromagnetic induction to change AC voltage. The main components are primary coil, secondary coil and iron core (magnetic core). In electrical equipment and wireless circuits, it is often used for voltage rise and fall, matching impedance, safety isolation, etc.

In the generator, no matter whether the coil moves through the magnetic field or the magnetic field moves through the fixed coil, the potential can be induced in the coil. In both cases, the value of magnetic flux remains unchanged, but the number of magnetic flux in the chain intersecting the coil changes, which is the principle of mutual inductance. Transformer is a device that uses electromagnetic mutual inductance to transform voltage, current and impedance. The main functions of the transformer are: voltage conversion; Current conversion, impedance conversion; Isolation; Voltage stabilizing (magnetic saturation transformer), etc.

Identification of transformer

Two coils wound on the same frame or iron core can form a transformer. In electronic and electrical appliances, the transformer uses the mutual coupling coil to realize the step-up or step-down function. If a variable voltage (such as AC voltage) is applied to the coil on one side of the transformer (primary coil), an electric voltage will be obtained in the coil on the other side (secondary coil) by using the principle of mutual inductance. If a higher voltage is applied to the primary coil and a lower voltage is obtained in the secondary, this kind of transformer is called step-down transformer. If a lower voltage is applied to the primary coil and a higher voltage is obtained in the secondary, this kind of transformer is called step-up transformer.

Function of transformer

The main function of transformer in circuit is AC voltage conversion and impedance conversion. The voltage conversion of transformer refers to the increase or decrease of circuit voltage through transformer.

Summary of transformer detection methods:

1. Detection of mid cycle transformer

A. Turn the multimeter to R × In gear 1, check the on-off condition of each winding one by one according to the pin arrangement law of each winding of the mid cycle transformer, and then judge whether it is normal.

B. Test insulation performance

Place the multimeter in R × 10K gear, conduct the following state tests:

(1) Resistance between primary winding and secondary winding;

(2) Resistance between primary winding and shell;

(3) The resistance between the secondary winding and the housing.

The above test results are divided into three cases:

(1) The resistance is infinite: normal;

(2) Resistance value is zero: there is short-circuit fault;

(3) Resistance value is less than infinity but greater than zero: there is leakage fault.

2. Detection of power transformer

A. Check whether there are obvious abnormalities by observing the appearance of the transformer. For example, whether the coil lead is broken, desoldered, whether the insulating material is burnt, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.

B. Insulation performance test. Multimeter R × 10K gear shall measure the resistance between iron core and primary, primary and secondary, iron core and secondary, electrostatic shielding layer and secondary and secondary windings respectively. The multimeter pointer shall point to the infinity position. Otherwise, the insulation performance of the transformer is poor.

C. Coil on-off detection. Place the multimeter in R × Gear 1: in the test, if the resistance value of a winding is infinite, it indicates that the winding has open circuit fault.

D. Distinguish 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 mostly marked with the word 220V, while the secondary winding is marked with the rated voltage, such as 15V, 24V, 35V, etc. Then identify according to these marks.

E. Detection of no-load current.

(a) Direct measurement method.

Open all secondary windings, Put the multimeter in AC current (500mA, in series into the primary winding. When the plug of the primary winding is plugged into 220V AC mains power, the multimeter indicates the no-load current value. This value shall not be greater than 10% ~ 20% of the full load current of the transformer. The normal no-load current of the power transformer of common electronic equipment shall be about 100mA. If it exceeds too much, it indicates that the transformer has short-circuit fault.

(b) Indirect measurement.

A 10 ∧ / 5W resistor is connected in series in the primary winding of the transformer, and all secondary windings are still no-load. Turn the multimeter to AC voltage. After power on, use two probes to measure the voltage drop u at both ends of resistance R, and then use Ohm's law to calculate the no-load current I empty, that is, I empty = u / R.

F. No load voltage detection. Connect the primary of the power transformer to 220V mains power, and connect the AC voltage of the multimeter. The no-load voltage values (U21, u22, U23 and u24) of each winding shall meet the requirements. The allowable error range is generally: high voltage winding ≤± 10%, low voltage winding ≤± 5%, and the voltage difference between two groups of symmetrical windings with center tap shall be ≤± 2%.

G. Generally, the allowable temperature rise of low-power power transformer is 40 ℃ ~ 50 ℃. If the insulating material used is of good quality, the allowable temperature rise can be improved.

H. Detect and judge the same name end of each winding. When using a power transformer, sometimes two or more secondary windings can be used in series in order to obtain the required secondary voltage. When using the power transformer in series, the same name end of each winding in series must be correctly connected without mistake. Otherwise, the transformer will not work properly.

3. Comprehensive detection and discrimination of short-circuit fault of power transformer.

The main symptoms of short-circuit fault of power transformer are serious heating and abnormal output voltage of secondary winding. Generally, the more inter turn short-circuit points in the coil, the greater the short-circuit current, and the more serious the heating of the transformer. A simple way to detect and judge whether there is a short-circuit fault in the power transformer is to measure the no-load current (the test method has been introduced earlier). The no-load current of the transformer with short-circuit fault 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 on. Touching the iron core with your hand will feel hot. At this time, it can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.