Probe for thyristors and triacs. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The probe allows you to control the correct functioning of the triac or thyristor. The performance of the element under test can be assessed by setting the triggering current of the control electrode. So, for a triac, you can show the difference in its behavior depending on the unlocking method: I, II, III and IV. With the help of such a device, the resistance in the control electrode circuit is easily determined, sufficient for a correct start.

The basis of the probe is a switch with resistors R1 - R8, which set the current of the control electrode of the thyristor or triac. Positive or negative control voltage allows you to unlock the element under test in two ways. The choice of the polarity of the control current is carried out using switch SW4. Another switch (SW3) allows you to select the polarity of the power supply between the working electrodes of the element under test or turn it off completely.

Description of the scheme

The circuit diagram of the device is shown in fig. 1. Supply transformer TR1 reduces the mains voltage of 220 V and provides galvanic isolation between the mains and the probe circuits. The secondary winding TR1 is made with a midpoint, from which the voltage is removed with an effective value of approximately 2x9 V. The full-wave rectifier is assembled on a diode bridge D1. Smoothing of positive and negative voltages is provided by capacitors C1 - C4. The voltage at the AP contact relative to the common wire (A1) is approximately +11 V, while the voltage at the AN contact is about -11 V. Switch SW3 changes the polarity of the lamp supply voltage.

A stabilized supply voltage of ±5 V is generated using positive voltage regulators (7805) and negative voltage regulators (7905) and provides a rated control current.

Diodes D2 and D3 do not allow a sharp drop in the input voltage of the stabilizers when the test thyristor or triac is unlocked.

The value of the control current is determined by the resistors R1 - R8, included in the control electrode circuit using switch SW2. The voltage at the control electrode of a thyristor or triac is approximately 1 V, so the control current is calculated using the following formula:

I_у=4/R

The resistance R corresponds to the resistance in the thyristor or triac control electrode circuit. The probe will allow you to determine the maximum value of this resistance for various triggering methods with a control voltage of 5 V.

Various ways of unlocking the triac are realized through four possible combinations of switch positions SW3 and SW4.

For the thyristor, a single + + mode is used, which corresponds to the position AP of switch SW3 and the position GP of switch SW4.

Rice. 1-1. Probe Wiring Diagram

Production

The layout of the printed circuit board of the probe and the placement of radio components are shown in fig. 2 and 3, respectively. The appearance of the assembled probe is shown in Fig. four.

For the first tests, the external switches SW3 and SW4 can be omitted. The necessary connections are made by means of wires with clamps.

First you need to check the voltage between the contacts AP and A1 (approximately +11 V) and between the contacts AN and A1 (-11 V), then measure the two symmetrical supply voltages +5 V between GP and A1 and -5 V between GN and A1.

After a preliminary check, you can connect the switch SW3 to the contacts AP and AC, and the switch SW4 to the contacts GP and GN.

Rice. 2. Probe PCB layout

Rice. 3. Layout of radio components on the probe board

The next step is to connect a triac or thyristor, as shown in the diagram. With a working element, the light should not light up.

Pressing the BP1 button should turn on the thyristor or triac.

If the lamp does not light up, then by increasing the amount of control current with switch SW2 and pressing the start button again, you can turn on the lamp.

When the AP - AC circuit is broken, the light should go out.

If all stages of this test are completed successfully, then the tested radio element is considered serviceable.

With the triac under test, the previous test can be carried out using the other three unlocking methods, i.e. alternating GP with GN and AP with AN by means of switches SW3 and SW4.

Then the light should go out when you change the position of switch SW3.

The list of elements required to assemble the probe is given in Table. one.

Rice. 4. Appearance of the probe

Table 1. Probe Assembly Items List

Author: Kadino E. Color-Musical Installations.-M.: DMK Press, 2000; Publication: cxem.net

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