Sharing of detection methods for inductor transformers

　　1. Detection of color-coded inductors.
　　Set the multimeter to R×1, and connect the red and black test leads to any lead-out terminal of the color-coded inductor. At this time, the pointer should swing to the right. According to the measured resistance value, it can be identified in the following three situations:
　　A. The resistance value of the color-coded inductor under test is zero, and there is a short-circuit fault inside it. B. The DC resistance value of the color-coded inductor under test is directly related to the diameter of the enameled wire used to wind the inductor coil and the number of windings. As long as the resistance value can be measured, it can be considered that the color-coded inductor under test is normal.

　　2. Testing of the intermediate-frequency transformer
　　A. Set the multimeter to R×1 and check the on/off status of each winding one by one according to the arrangement of the winding pins of the intermediate-frequency transformer to determine whether it is normal. B. Testing insulation performance

　　Set the multimeter to the R×10k position and perform the following tests:
　　(1) Resistance between the primary winding and the secondary winding;
　　(2) Resistance between the primary winding and the housing;
　　(3) Resistance between the secondary winding and the housing.

　　The above test results are divided into three situations:
　　(1) The resistance value is infinite: normal;
　　(2) The resistance value is zero: there is a short circuit fault;
　　(3) The resistance value is less than infinity but greater than zero: there is a leakage fault.

　　3 Inspection of power transformer
　　A Check the transformer for obvious abnormalities by observing its appearance, such as whether the coil lead is broken, desoldering, whether the insulation material has burnt marks, whether the core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.

　　B. Insulation test. Use the multimeter R×10k to 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 each secondary winding. The multimeter pointer should point to the infinite position and not move. Otherwise, it means that the insulation performance of the transformer is poor.

　　C. Detection of the continuity of the coil. Set the multimeter to the R×1 position. During the test, if the resistance value of a winding is infinite, it means that the winding has a circuit breaker fault.

　　D. Identify 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 often marked with 220V, while the secondary winding is marked with the rated voltage value, 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, set the multimeter to the AC current range (500mA, and connect the primary winding in series. When the plug of the primary winding is inserted into the 220V AC mains, the multimeter indicates the no-load current value. This value should not be greater than 10% to 20% of the full-load current of the transformer. The normal no-load current of the power transformer of common electronic equipment should be around 100mA. If it exceeds too much, it means that the transformer has a short-circuit fault.
　　(b) Indirect measurement method. Connect a 10/5W resistor in series in the primary winding of the transformer, and the secondary is still completely unloaded. Set the multimeter to the AC voltage range. After power on, use two test leads to measure the voltage drop U across the resistor R, and then use Ohm's law to calculate the no-load current Iempty, that is, Iempty = U/R.

　　F. Detection of no-load voltage. Connect the primary of the power transformer to 220V mains, and use a multimeter to measure the no-load voltage of each winding (U21, U22, U23, U24) in turn. The no-load voltage should meet the required value. The allowable error range is generally: high-voltage winding ≤±10%, low-voltage winding ≤±5%, and the voltage difference between two sets of symmetrical windings with center taps should be ≤±2%. G? Generally, the allowable temperature rise of small-power power transformers is 40℃～50℃. If the quality of the insulation material used is good, the allowable temperature rise can be increased.

　　H Detect and identify the same-named ends of each winding. When using a power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be connected in series. When using a power transformer in series, the same-named ends of each winding participating in the series connection must be connected correctly and cannot be mistaken. Otherwise, the transformer cannot work properly.

　　I. Comprehensive detection and identification of short-circuit faults in power transformers. The main symptoms of a short-circuit fault in a power transformer are severe heating and abnormal output voltage of the secondary winding. Generally, the more short-circuit points there are between turns in the coil, the greater the short-circuit current and the more severe the transformer heating. A simple way to detect and determine whether a power transformer has a short-circuit fault is to measure the no-load current (the test method has been introduced above). For a transformer with a short-circuit fault, 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 up rapidly within tens of seconds after power is applied at no load, and the core will feel hot when touched by hand. At this time, it can be determined that there is a short-circuit point in the transformer without measuring the no-load current.