We often use transformers in our lives. Transformers can be found in televisions, radios, air conditioners and other electrical appliances. So what are the common transformers in life and how are they used? After reading this article, we have more or less understanding. The following is a detailed introduction to some commonly used voltage regulators:
1. Medium frequency transformer
Open the back cover of the radio cassette player, and you will see several silver-white cube metal shells with a "1" notch on the circuit board. You can use a non-inductive screwdriver to gently rotate them. They come in red, white, green and other colors. This is Medium frequency transformer. There are two types of intermediate frequency transformers used in transistor radios: single tuning loop and double tuning loop.
Intermediate frequency transformers are mainly used in radios for frequency selection and inter-stage coupling. Frequency selection refers to selecting useful signal frequencies among many signals and transmitting useful signals. In an AM radio, the useful signal of 465kHz is selected through an intermediate frequency transformer. And coupled to the next stage for amplification. At the same time, signals other than 465kHz are suppressed so that they cannot be transmitted to the next stage.
2. Power transformer
In daily life, various household appliances use different voltages. Household appliances all use low-voltage DC power supply to work. They need to use a power transformer to convert 220 V AC mains power into low-voltage AC power, which is then rectified by diodes and filtered by capacitors to form a DC power supply. Moreover, during this transformation process, its own energy loss is small, thus achieving the purpose of convenience and economy. The TV picture tube requires tens of thousands of volts to operate, which is supplied by the "row output transformer".
In view of the wide variety of electronic devices, the required voltage volts and the power required to be transmitted, as well as the installation location and the small space inside the whole machine, the types and shapes of transformers are also different accordingly.
As can be seen from Figure 2, power transformers have several shapes such as "口" shape, "日" shape, and "C" shape. Even with the same appearance, their sizes are different. For example, the power transformer in a radio is compared with the transformer in a television or power amplifier. Much smaller in size.
3. Switching power transformer
The switching power supply transformer is an important component in the switching power supply of color TVs. It is a pulse transformer. Its function is to transmit power, provide the required power voltage for the color TV set, and achieve reliable electrical isolation between input and output.
Inductor and transformer testing methods and experience
1. To test the color-coded inductor, place the multimeter in the R×1 position, and connect the red and black test leads to any terminal of the color-coded inductor. At this time, the pointer should swing to the right. According to the measured resistance value, the identification can be divided into the following three situations:
A. The resistance value of the color code inductor under test is zero, and there is a short circuit fault inside it.
B. The DC resistance value of the color code inductor under test is directly related to the diameter of the enameled wire used to wind the inductor coil and the number of winding turns. As long as the resistance value can be measured, the color code inductor under test can be considered normal. of.
2. Detection of mid-cycle transformer
A. Set the multimeter to the R×1 position, check the on-off status of each winding one by one according to the pin arrangement of each winding of the mid-circuit transformer, and then judge whether it is normal.
B. Check the insulation. Place the multimeter in the R×10k block and perform the following state tests:
(1) The resistance value between the primary winding and the secondary winding;
(2) The resistance value between the primary winding and the casing;
(3) The resistance value between the secondary winding and the casing.
There are three situations in the above test results:
(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. Detection of power transformer
A. Check whether there are any obvious abnormalities by observing the appearance of the transformer. For example, whether the coil leads are broken or desoldered, whether the insulation material has scorch marks, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.
B. Insulation test. Use a multimeter with an R×10k scale to measure the resistance values between the core and the primary, the primary and each secondary, the core and each secondary, the electrostatic shielding layer and the secondary, and the secondary windings. The multimeter pointer should all point at the infinity position. Move. Otherwise, the insulation performance of the transformer is poor.
C. Detection of coil continuity. Place the multimeter in the R×1 gear. During the test, if the resistance value of a certain winding is infinite, it means that this winding has an open circuit fault.
D. Distinguish the primary and secondary coils. The primary pin and secondary pin of the power transformer are generally drawn from both sides, and the primary winding is often marked with 220V, while the secondary winding is marked with a rated voltage value, such as 15V, 24V, 35V, etc. Then identify based on these marks.
E. Detection of no-load current.
(a) Direct measurement method. Open all secondary windings, put the multimeter in the AC current setting (500mA), and connect it in series to the primary winding. 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 It is greater than 10% to 20% of the full load current of the transformer. The normal no-load current of a power transformer for common electronic devices should be around 100mA. If it exceeds too much, it means that the transformer has a short-circuit fault.
(b) Indirect measurement method. A 10/5W resistor is connected in series to the primary winding of the transformer, and the secondary is still completely unloaded. Set the multimeter to AC voltage. After powering 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 I, that is, I = U/R.
F. Detection of no-load voltage. Connect the primary of the power transformer to the 220V mains power, and use a multimeter to connect the AC voltage to measure the no-load voltage values of each winding (U21, U22, U23, U24) in sequence, which should meet the required values. The allowable error range is generally: high voltage winding ≤ ±10 %, low-voltage winding ≤ ±5%, the voltage difference of two sets of symmetrical windings with center tap should be ≤ ±2%.
G. Generally, the allowable temperature rise of small power power transformers is 40℃~50℃. If the insulation material used is of good quality, the allowable temperature rise can be increased.
H. Detect and identify the identical ends of each winding. When using a power transformer, sometimes two or more secondary windings can be connected in series in order to obtain the required secondary voltage. When using the power transformer in series, the same terminals of each winding participating in the series must be connected correctly and cannot be mistaken. Otherwise, the transformer will not work properly.
I. Comprehensive detection and identification of short-circuit faults in power transformers. The main symptoms after a short-circuit fault occurs in a power transformer are severe heating and abnormal secondary winding output voltage. Generally, the more inter-turn short-circuit points inside the coil, the greater the short-circuit current and the more serious the transformer heating. A simple way to detect and determine whether the power transformer has a short-circuit fault is to measure the no-load current (the test method has been introduced previously). The no-load current value of a transformer with a short-circuit fault will be much greater than 10% of the full-load current. When the short circuit is serious, the transformer will quickly heat up within tens of seconds after no-load power-up, and the iron core will feel hot when touching it with your hands. At this time, it can be concluded that there is a short circuit point in the transformer without measuring the no-load current.
