Monday, November 30, 2020

Voltage transformer

According to the device and principle of operation, measuring a voltage transformer is slightly different from a power step-down transformer. The only difference is that the VTs are counted on very low power: The voltage transformer's normal operating mode is the no-load mode. Here you can know how to test a transformer using multimeter.

The VT consists of a steel core assembled from electrical steel plates, a primary winding, and one or two secondary windings. As a result of manufacturing, the required accuracy class must be achieved: in amplitude and angle. Three-phase voltage transformers with zero leads are made on a five-rod magnetic core so that in case of a short circuit on the high voltage side, the total magnetic flux is short-circuited along with the core steel (in case of a short circuit in the air, there is a high current, which leads to overheating of the transformer). Three-phase transformers with a three-strand magnetic core, based on the above reasons, do not have any external zero leads and are not used to record "short circuits on the ground." The less the secondary coil of a voltage transformer is loaded (in other words, the greater the resistance in the secondary circuit), the closer the actual Kt transformation factor is to the nominal value. This is especially important when connecting measuring instruments to the secondary circuit, as the transformer ratio affects the measurement's accuracy. Depending on the load, the same voltage transformer can operate in different accuracy classes: 0.5; 1; 3.

Types of voltage transformers

Voltage transformer to be grounded - single-phase voltage transformer, one end of the primary winding of which must be tightly grounded, or three-phase voltage transformer, the neutral of the primary winding must be tightly grounded (transformer with weakened insulation of one of the leads - single-phase VT type SNF or three-phase VT type NTMI and NAMI).

Non-earthed voltage transformer - a voltage transformer in which all parts of the primary winding, including clamps, are isolated from the ground to a level corresponding to the voltage class.

Cascade voltage transformer - voltage transformer, the primary winding divided into several sections connected in series, power transmission from which to the secondary windings is carried out by connecting and leveling windings. It is used in ultra-high voltage units (110kV and above).

Capacitive voltage transformer - voltage transformer containing capacitive divider.

Two-winding transformer - voltage transformer having one secondary winding.

Three-winding voltage transformer - voltage transformer having two secondary windings: the main one and the additional one. Most VTs are three-winding.

The optoelectronic voltage transformer is an experimental type of VT; unlike a conventional (electromagnetic) voltage transformer, it operates on a different physical principle. It is intended for measurements in ultra-high voltage settings (750 kV and more) because electromagnetic VTs for such high voltages are too cumbersome.

How to Check a Transistor

When repairing and constructing electronics, it is often necessary to check the transistor for serviceability.

Let's consider the method of checking bipolar transistors with an ordinary digital multimeter, which is available for almost every beginner radio amateur.

Despite the fact that the method of bipolar transistor testing is quite simple, beginner radio amateurs can sometimes face some difficulties.

About the peculiarities of testing bipolar transistors will be told a little later, but for now let's consider the simplest technology of testing an ordinary digital multimeter.

To begin with, we need to understand that the bipolar transistor can be conventionally represented as two diodes, since it consists of two p-n transitions. And the diode, as we know, is nothing else but an ordinary p-n transient.

Here is a conventional bipolar transistor circuit, which will help to understand the principle of verification. In the figure p-n, the transistor transitions are represented as semiconductor diodes.

A bipolar transistor device of p-n-p structure using diodes is represented as follows.

As is known, bipolar transistors are of two types of conductivity: n-p-n and p-n-p. This fact has to be taken into account when checking. Therefore, let us show the conventional equivalent of the n-p-n structure transistor composed of diodes. We will need this figure for the next check.

A transistor with n-p-n structure as two diodes.

The essence of the method is to check the integrity of these very p-n transitions, which are conventionally represented in the form of diodes in the figure. And, as we know, the diode transmits current only in one direction. If plus (+) is connected to the diode anode output, and minus (-) to the cathode, the p-n transition will open and the diode will start transmitting current. If you do the opposite, connect plus (+) to the diode cathode and minus (-) to the anode, the p-n transition will be closed and the diode will not transmit current.

If suddenly, when checking, we find out that the p-n junction passes current in both directions, it means it is "broken". But if p-n junction does not transmit current in either direction, it means that it is "broken". Naturally, if at least one of the p-n junctions is broken or broken, the transistor will not work.

Please note, that the conditional diode circuit is necessary only for a more clear idea of the transistor checking method. In reality, the transistor has a more sophisticated device.

The functionality of almost any multimeter supports diode testing. On the panel of the multimeter mode of checking the diode is depicted as a conditional image, which looks like this.

I think it is already clear that we will check the transistor with this function.

A little explanation. A digital multimeter has several sockets for probes. Three, or even more. When checking the transistor it is necessary to connect a minus stylus (black) to the socket COM (from the English word common - "common"), and a plus stylus (red) to the socket with the designation of the letter omega Ω, the letter V and possibly other letters. It all depends on the functionality of the device.

Why do I tell you so much about how to connect probes to a multimeter? Yes, because the probes can be easily confused and you can connect a black stylus, which is conditionally considered "minus" to the socket to which you want to connect a red, "plus" stylus. As a result, this will cause confusion, and as a result, errors. Be more attentive!

Now that the dry theory is set out, let's move on to practice.

What kind of multimeter will we use?

The Victor VC9805+ multifunction tester was used as a multimeter, although any digital tester like the familiar DT-83x or MAS-83x will do the job. Such multimeters can be purchased not only at radio markets, stores of radio parts, but also in stores of auto parts. You can buy a suitable multimeter on the Internet, for example, at Aliexpress.

First we will check the silicon bipolar transistor of domestic production KT503. It has the structure n-p-n. Here is its socle.

For those who do not know what this incomprehensible word plinth means, I explain. The plinth is the location of the functional pins on the body of the radio element. For the transistor, the functional outputs are, respectively, the collector (K or C), the emitter (E or E), the base (B or C).

At first, connect the red (+) dipstick to the KT503 transistor base and the black (-) dipstick to the collector output. This is how we check the operation of the p-n junction in the live mode (i.e. when the junction carries current). The breakdown voltage value appears on the display. In this case it is 687 millivolts (687 mV).

As we can see, the p-n junction between the base and emitter also carries a current. The breakdown voltage of 691 mV is shown on the display again. Thus, we have checked the B-K and B-E transitions at direct switching.

To make sure that the p-n transitions of the KT503 transistor are in good working order, we will also check them in the so-called reverse switching on. In this mode p-n transitions do not carry current, and the display should not show anything but "1". If there is a unit "1" on the display then it means

Voltage transformer

According to the device and principle of operation, measuring a voltage transformer is slightly different from a power step-down transformer...