1. Detection of color code inductors
Place the multimeter in the R × 1 block, and the red and black test leads are connected to any of the terminals of the color code inductor. At this time, the pointer should swing to the right. According to the measured resistance value, the following three cases can be identified:
A. The measured color code inductor has a zero resistance value and has a short circuit fault inside.
B. The magnitude of the DC resistance value of the measured color code inductor is directly related to the enamelled wire diameter and the number of winding turns used for winding the inductor coil. As long as the resistance value can be measured, the measured color code inductor is considered normal. of.
2. Detection of transformers in the middle of the week
A. Dial the multimeter to R×1 block, check the on/off condition of each winding one by one according to the arrangement rules of the windings of the windings of the mid-week transformer, and then judge whether it is normal.
B. Detecting the insulation performance Place the multimeter in the R×10k block and do the following status tests:
(1) a resistance value between the primary winding and the secondary winding;
(2) the resistance value between the primary winding and the outer casing;
(3) The resistance value between the secondary winding and the outer casing.
The above test results are divided into three cases:
(1) The resistance is infinite: normal;
(2) The resistance is zero: there is a short circuit fault;
(3) The resistance is less than infinity, but greater than zero: there is a leakage fault.
3. Detection of power transformer
A. Check the appearance of the transformer to see if it is obviously abnormal. If the coil lead is broken, de-soldering, whether the insulating material has burnt marks, whether the iron-tightening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed or the like.
B. Insulation test. Use the multimeter R×10k block to measure the resistance between the core and the primary, the primary and the secondary, the core and each secondary, the electrostatic shielding layer and the secondary and secondary windings. The multimeter pointer should refer to the infinity position. move. Otherwise, the transformer insulation performance is poor.
C. Detection of coil continuity. Place the multimeter in the R × 1 block. During the test, if the resistance value of a winding is infinite, the winding has a faulty 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 marked with 220V, and the secondary winding is labeled with rated voltage, such as 15V, 24V, 35V. 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 (500mA, stringed into the primary winding. When the plug of the primary winding is plugged into 220V AC mains, the multimeter indicates the no-load current value. This value should not be It is greater than 10%~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, it means that the transformer has short-circuit fault.
(b) Indirect measurement method. A 10/5W resistor is placed in series in the primary winding of the transformer, and the secondary is still completely empty. Turn the multimeter to the AC voltage block. After power-on, the voltage drop U across the resistor R is measured with two test leads, 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 measure the no-load voltage value (U21, U22, U23, U24) of each winding in order to meet the required value. The allowable error range is 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.
H. Detect and identify 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 the power transformer is used in series, the same name of each winding participating in the series must be correctly connected, and no mistake can be made. Otherwise, the transformer will not work properly.
I. Comprehensive detection and discrimination of short-circuit faults of power transformers. The main symptoms after a short-circuit fault in the power transformer are severe heat generation and abnormal secondary winding output voltage. In general, the more turns between the turns inside the coil, the greater the short-circuit current, and the more severe the transformer heats up. A simple way to detect if a power transformer has a short-circuit fault is to measure the no-load current (tested earlier in the test method). A transformer with a short-circuit fault will have a no-load current value that is much greater than 10% of the full-load current. When the short circuit is severe, the transformer will heat up quickly within a few tens of seconds after the no-load power-on, and the iron core will feel hot when touched by the hand. At this time, it is not necessary to measure the no-load current to conclude that the transformer has a short-circuit point.