Device for selection of thyristors. Scheme, description

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Device for selection of thyristors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Devices for testing the performance of thyristors have already been described, but they are good when thyristors are used at low supply voltages. When thyristors are needed to operate at the maximum voltages allowed for them, for example, in electronic car ignition systems, it is imperative to measure the highest forward voltage at which the thyristor goes into the open state before installing them. For example, thyristors KU202N and KU202M must withstand (according to the reference book) a direct voltage of 400 V, but according to this indicator they have a spread of 350 to 450 V.

Therefore, the selection of thyristors can be made according to the scheme of Fig.1. It uses a regulated constant voltage source in the range of 0...500 V with a current of up to 200 mA. The resistor, which limits the current of the thyristor, depends on the type of thyristor and is selected within 1 ... 6 kOhm. The supplied DC voltage is measured using a voltmeter (it is gradually increased), and the moment of opening the thyristor is determined using a milliammeter (the current increases sharply from fractions to several tens of milliamps). The highest voltmeter reading before opening the thyristor is the maximum allowable forward voltage for this thyristor.

Thyristor selection device

A regulated 500V DC supply is hard to come by. The author proposes a portable voltage converter, powered by a 6-9 V battery or from a mains power supply, designed for portable radios and tape recorders. The converter works as a multivibrator assembled on two P217 transistors and a transformer with the following data: iron 16×16 mm, winding W1 - 10 turns of wire D0,3 mm, W2 - two windings of 18 turns of wire D0,6 mm, winding W3 - 1160 turns of wire D0,18 mm, wound until the frame is completely filled.

The rectifier is assembled according to the voltage doubling scheme (about 450 V is obtained). With switch B1, the circuit can be switched to half-wave rectification (we get 220 V to check low-voltage transistors). Double voltage is applied to the storage capacitor C3 and terminal "A", to which the anode of the thyristor is connected. A high-resistance voltmeter is connected in parallel with the capacitor.

The minus of the rectifier through the terminal "K" is connected to the thyristor cathode. All connections to the thyristor are made with colored wires and alligator clips.

The device is switched on for a short time only (at the time of the thyristor test) by pressing the KN1 button. At the same time, the multivibrator starts working and voltage appears on the storage capacitor, it will gradually increase and be recorded by a voltmeter.

At a certain moment, the readings of the voltmeter will begin to decrease sharply, which indicates the transition of the thyristor to the conducting mode. The highest readings of the voltmeter before they decrease are the switching voltage of this thyristor. In some cases, even the maximum voltage does not open the thyristor under test.

In this case, the thyristor is checked for serviceability by pressing the KN2 button, while a positive voltage is applied to the control electrode of the thyristor, sufficient to open it. If at the same time the thyristor does not open, then it is faulty. You can check the health of any thyristor by applying a voltage equal to 80% of the maximum allowable voltage to its anode and pressing the KN2 button.

Thyristor selection device

As can be seen from the diagram, when the thyristor is open, the high voltage "+" is shorted to "-". But for this circuit, this is not scary, since the power coming from the device is very small, so the device is not afraid of short circuits. Powerful transistors such as P217 can be replaced by transistors of the same medium power structure.

Replacement with germanium transistors is quite acceptable, the circuit may not work with silicon ones.

Author: V.I. Zorenko

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