In this article we will focus on conventional, "iron" transformer (not to be confused with pulse devices on ferrite cores).
Despite the large size and weight, such transformers are still used quite widely, especially in high-quality audio equipment. They do not give high frequency interference, reliable and less "picky" in operation.
How to determine the transformer?
If you have a voltage transformer and its windings are not known (in particular - it is not known where the primary coil), that define it is not difficult to using an ordinary tester.
Sequentially measuring the resistance of the windings. For this purpose, connect one probe tester (ohmmeter) to any wire, and a second touch probe alternately all other wires. Thus we will be able to find a single coil, it may contain two or more pins (Figure 1). Write (memorize) the resistance of this winding, and continue to search the same manner the remaining windings.
As a rule, the power winding (~ 220) has the highest resistance of all the others available on the transformer (if it goes down, not up transformer).
To verify the correctness of the winding network, you need to include it in a network of 220 volts through fuse with a current of 1 ampere and leave for some time, controlling the temperature transformer. With proper identification and connection of the transformer if it is OK, it should not be heated without load.
You can then determine the voltage remaining windings. Turning tester AC measurement mode (initially set at the maximum limit of the measurement, e.g. 1000 volts) and successively are measuring the voltage between all the remaining wires (or terminals) transformer.
How to determine the maximum current of the transformer
The maximum output current of the transformer can be determined by connecting it to the secondary winding load different values. If you do not have a device that measures the AC value should be connected through a rectifier load.
In the simplest case, this may be any rectifier diode, calculated on the operating voltage is not lower than the voltage on the coil, e.g. KD226, 1N4001 - 1N4007 and the like (Fig.2). By changing the load resistance and controlling the voltage across the winding, determine the maximum current at which the voltage starts to "squander".
The load is convenient to take a strong wire variable resistor, or use a separate set of resistors of different resistances, connecting them one by one. The resistors should have resistance in the range of several to several tens of ohms and be designed for a power of 10 watts or more. In any case, the measurement should be carried out quickly, not allowing the resistors to heat up too (if they are not a great power).
How to determine the number of turns of the transformer?
The number of winding turns of wire can determine with sufficient accuracy by the simplified formula: n = 50 / S. Where «n» - the number of turns, "50" - mains frequency (220 volts, 50 Hz), «S» - the transformer core cross-sectional area (in square centimeters).
To determine the value of «S», need to measure the length and width of the diameter of the middle portion of the core, which coil is fitted with windings. Be measured in inches and then multiply these values, receiving sectional area as well. in square centimeters.
To determine the desired number of windings and can be empirically.
To do this, the windings wound on top of, e.g., 10 turns of any wire (if it allows the transformer design) and measure the voltage across this "time" winding. Based on the values obtained, it will be easy to calculate the number of windings to achieve the desired winding tension.