To use a power transformer in stock, you need to know its key characteristics as accurately as possible. There are almost never any difficulties in solving this problem if the markings are preserved on the product. The required parameters can be easily found on the Internet by simply entering the letters and numbers stamped on the transformer into the search bar.
However, quite often there are no markings - the inscriptions are erased, destroyed by corrosion, and so on. Many modern products (especially cheap ones) are not marked at all. In such cases, of course, there is no point in throwing away the transformer. After all, its price on the market can be quite decent.
The most important parameters of power transformers
What do you need to know about a transformer in order to use it correctly and, most importantly, safely for your purposes? Most often this involves repairing some household appliances or making your own crafts powered by low voltage.And you need to know the following about the transformer lying in front of us:
- Which terminals should mains power be supplied to (230 volts)?
- From which terminals should the undervoltage be removed?
- What will it be (12 volts, 24 or others)?
- How much power can the transformer produce?
- How not to get confused if there are several windings, and, accordingly, paired terminals?
It is quite possible to calculate all these characteristics even when there is absolutely no information about the brand and model of the power transformer.
To complete the work you will need the simplest tools and consumables:
- multimeter with ohmmeter and voltmeter functions;
- soldering iron;
- electrical tape or heat shrink tubing;
- mains plug with wire;
- a pair of ordinary wires;
- incandescent lamp;
- calipers;
- calculator.
You will also need some kind of wire stripping tool and a minimum soldering kit - solder and rosin.
Definition of primary and secondary windings
The primary winding of the step-down transformer is designed to supply mains power. That is, it is to it that you need to connect 230 volts, which are in a regular household outlet. In the simplest versions, the primary winding may have only two terminals. However, there are also those in which there are, for example, four conclusions. This means that the product is designed to operate on both 230 V and 110 V. We will consider a simpler option.
So, how to determine the terminals of the primary winding of a transformer? To solve this problem you will need multimeter with ohmmeter function. With its help you need to measure the resistance between all available terminals. Where it will be the most, there is the primary winding. It is advisable to immediately mark the findings found, for example, with a marker.
The primary winding can be determined in another way. To do this, the wound wire inside the transformer must be clearly visible. In modern versions this is most often the case. In old products, the insides may be filled with paint, which precludes the use of the described method. The winding whose wire diameter is smaller is visually highlighted. It is primary. It needs to be supplied with mains power.
It remains to calculate the secondary winding from which the reduced voltage is removed. Many have already guessed how to do this. Firstly, the resistance of the secondary winding will be much less than that of the primary. Secondly, the diameter of the wire with which it is wound will be larger.
The task becomes a little more complicated if the transformer has several windings. This option is especially scary for beginners. However, the method for identifying them is also very simple, and is similar to that described above. First of all, you need to find the primary winding. Her resistance will be many times greater than that of the others.
To conclude the topic on transformer windings, it is worth saying a few words about why the resistance of the primary winding is greater than that of the secondary, but with the diameter of the wire everything is exactly the opposite. This will help beginners understand the issue in more detail, which is very important when working with high voltage.
A mains voltage of 220 V is supplied to the primary winding of the transformer. This means that with a power of, for example, 50 W, a current of about 0.2 A will flow through it (we divide the power by the voltage). Accordingly, a large cross-section of wire is not needed here. This is, of course, a very simplified explanation, but for beginners (and the solution to the problem posed above) it will be enough.
More significant currents flow in the secondary winding. Let's take the most common transformer, which produces 12 V. With the same power of 50 W, the current flowing through the secondary winding will be about 4 A. This is already quite a large value, because the conductor through which such current will pass must be thicker. Accordingly, the larger the cross-section of the wire, the less its resistance will be.
Using this theory and a simple ohmmeter, you can easily calculate which winding is on a step-down transformer without markings.
Determination of secondary winding voltage
The next step in identifying the “unnamed” transformer will be to determine the voltage on its secondary winding. This will allow us to determine whether the product is suitable for our purposes. For example, you are assembling a 24 V power supply, but the transformer produces only 12 V. Accordingly, you will have to look for another option.
To determine the voltage that can be removed from the secondary winding, the transformer will have to be supplied with mains power. This is already a rather dangerous operation. Through carelessness or ignorance, you can get a strong electric shock, get burned, damage the wiring in the house, or burn the transformer itself. Therefore, it would be a good idea to stock up on a few safety recommendations.
Firstly, when testing, the transformer should be connected to the network through an incandescent lamp. It is connected in series, into the break of one of the wires going to the plug. The light bulb will serve as a fuse in case you do something wrong, or the transformer under test is faulty (shorted, burnt out, wet, etc.). If it glows, something has gone wrong.There is a short circuit in the transformer, so it is better to immediately pull out the plug from the socket. If the lamp does not glow, nothing stinks or smokes, work can continue.
Secondly, all connections between the outputs and the plug must be carefully insulated. Do not neglect this recommendation. You won’t even notice how looking at the readings multimeter, for example, if you try to straighten twisted wires, you will get a good electric shock. This is dangerous not only for health, but also for life. For insulation, use electrical tape or heat-shrink tubing of the appropriate diameter.
Now the process itself. A regular plug with wires is soldered to the terminals of the primary winding. As stated above, an incandescent lamp is added to the circuit. All connections are isolated. Connected to the terminals of the secondary winding multimeter in voltmeter mode. Make sure that it is turned on to measure AC voltage. Beginners often make a mistake here. Installing the handle multimeter to measure DC voltage, you won’t burn anything, however, you won’t get any sane and useful readings on the display.
Now you can insert the plug into the socket. If everything is in working order, the device will show you the reduced voltage generated by the transformer. Similarly, you can measure the voltage on other windings, if there are several of them.
Simple ways to calculate the power of a power transformer
With the power of a step-down transformer, things are a little more complicated, but there are still some simple techniques. The most accessible way to determine this characteristic is to measure the diameter of the wire in the secondary winding. To do this you will need a caliper, a calculator and the information below.
First, the diameter of the wire is measured.For example, let's take a value of 1.5 mm. Now you need to calculate the cross-section of the wire. To do this, you need to square half the diameter (radius) and multiply by the number “pi”. For our example, the cross-section will be about 1.76 square millimeters.
Next, for the calculation you will need the generally accepted value of current density per square millimeter of conductor. For household step-down transformers, this is 2.5 amperes per square millimeter. Accordingly, a current of about 4.3 A can “painlessly” flow through the second winding of our sample.
Now we take the previously calculated voltage of the secondary winding and multiply it by the resulting current. As a result, we get the approximate value of the power of our transformer. At 12 V and 4.3 A, this parameter will be around 50 W.
The power of an “unnamed” transformer can be determined in several other ways, however, they are more complex. Those interested can find information about them on the Internet. Power is determined by the cross-section of the transformer windows, using calculation programs, as well as by the nominal operating temperature.
Conclusion
From all of the above, we can conclude that determining the characteristics of a transformer without markings is a fairly simple task. The main thing is to follow safety rules and be extremely careful when working with high voltage.
