Block of thyristor amplifiers BTU. Control of a powerful AC load The thyristor is turned off

The article describes the use of thyristors and provides simple and visual experiments to study the principles of their operation. Practical instructions for checking and selecting thyristors are also given.

Homemade dimmers

Despite the variety and availability of such devices on sale, you can assemble a dimmer using a fairly simple amateur circuit.

Besides dimmer It is not at all necessary to regulate the light; you can adapt it, for example, to a soldering iron. In general, there are plenty of applications; a ready-made device can always come in handy.

Almost all such devices are made using thyristors, which are worth talking about separately, or at least briefly, so that the principle of operation thyristor regulators was clear and understandable.

Let's repeat something!

Types of thyristors

Name thyristor implies several varieties, or as they say, a family of semiconductor devices. Such devices are a structure of four p and n layers, forming three consecutive p-n ( p-n letters Latin: from positive and negative) transition.

Rice. 1. Thyristors

If conclusions are drawn from the extreme regions p n, the resulting device is called a diode thyristor, in another way dinistor. It is similar in appearance to the D226 or D7Zh series diode, only the diodes have only one p-n junction. The design and circuit of the KN102 type dinistor is shown in Figure 2.

A diagram of its connection is also shown there. If we draw a conclusion from one more pn junction, we get a triode thyristor, called a trinistor. One housing can contain two SCRs at once, connected back-to-back - in parallel. This design is called a triac and is designed to work in circuits AC, since it can pass both positive and negative half-cycles of voltage.

Figure 2. Internal structure and connection circuit of the diode thyristor KN102

The cathode terminal, region n, is connected to the housing, and the anode terminal through a glass insulator is connected to region p, as shown in Figure 1. The inclusion of a dinistor in the power circuit is also shown there. A load must be connected to the power circuit in series with the dinistor, just as if it were a regular diode. Figure 3 shows the volt-ampere characteristic of the dinistor.

Figure 3. Volt-ampere characteristic of the dinistor

From this characteristic it is clear that voltage can be applied to the dinistor both in the reverse direction (in the lower left quarter of the figure) and in the forward direction, as shown in the upper right quarter of the figure. In the opposite direction, the characteristic is similar to that of a conventional diode: a small amount flows through the device. reverse current, we can practically assume that there is no current.

Of greater interest is the direct branch of the characteristic. If voltage is applied to the dinistor in the forward direction and gradually increases it, then the current through the dinistor will be small and will change slightly. But only until it reaches a certain value, called the dinistor turn-on voltage. In the figure this is indicated as Uincl.

At this voltage, an avalanche-like increase in current occurs in the internal four-layer structure, the dinistor opens and goes into a conducting state, as evidenced by the area with negative resistance on the characteristic. The voltage of the cathode-anode section decreases sharply, and the current through the dinistor is limited only by the external load, in this case, the resistance of resistor R1. The main thing is that the current is limited to a level not higher than the maximum permissible, which is specified in the reference data.

The maximum permissible current or voltage is the value at which it is guaranteed normal operation device for a long time. Moreover, you should pay attention to the fact that only one of the parameters reaches the maximum permissible value: if the device operates in the maximum permissible current mode, then the operating voltage should be lower than the maximum permissible. Otherwise, normal operation of the semiconductor device is not guaranteed. Of course, there is no need to specifically strive to achieve the maximum permissible parameters, but if this happens...

This direct current will flow through the dinistor until the dinistor is turned off in some way. To do this, it is necessary to stop the flow of direct current. This can be done in three ways: open the power circuit, short-circuit the dinistor using a jumper (all current will pass through the jumper, and the current through the dinistor will be zero), or change the polarity of the supply voltage to the opposite polarity. This happens if you power the dinistor and the load with alternating current. The same switching methods apply to a triode thyristor - a trinistor.

Dinistor marking

It consists of several letters and numbers; the most common and accessible domestic devices are the KN102 series (A, B... I). the first letter K indicates that this is a silicon semiconductor device, N that it is a dinistor, the numbers 102 are the development number, but the last letter determines the turn-on voltage.

The entire reference book will not fit here, but it should be noted that KN102A has a switching voltage of 20V, KN102B 28V, and KN102I already as much as 150V. When devices are switched on in series, the switch-on voltage is added up, for example, two KN102A will give a total switch-on voltage of 40V. Dinistors produced for the defense industry have the number 2 instead of the first letter K. The same rule is also used in the marking of transistors.

This logic of the dinistor operation allows you to collect enough simple pulse generators. A diagram of one of the options is shown in Figure 4.

Figure 4. Generator on a dinistor

The principle of operation of such a generator is quite simple: the mains voltage rectified by diode VD1 through resistor R1 charges capacitor C1, and as soon as the voltage on it reaches the switching voltage of dinistor VS1, the latter opens and the capacitor is discharged through light bulb EL1, which gives a short flash, after which the process is repeated at first. In real circuits, instead of a light bulb, a transformer can be installed, from the output winding of which pulses can be removed, used for some purpose, for example, as opening pulses.

The article describes how a thyristor power regulator works, the diagram of which will be presented below

In everyday life, very often there is a need to regulate the power of household appliances, such as electric stoves, soldering irons, boilers and heating elements, in transport - engine speed, etc. The simplest amateur radio design comes to the rescue - a power regulator on a thyristor. Assembling such a device will not be difficult; it could become the first homemade device, which will perform the function of adjusting the temperature of the soldering iron tip of a novice radio amateur. It is worth noting that ready-made soldering stations with temperature control and other pleasant functions are an order of magnitude more expensive than a simple soldering iron. A minimal set of parts allows you to assemble a simple thyristor power regulator for wall mounting.

For information, surface mounting is a method of assembling radio-electronic components without using printed circuit board, and with good skill it allows you to quickly assemble electronic devices medium difficulty.

You can also order thyristor regulator, and for those who want to figure it out on their own, a diagram will be presented below and the principle of operation will be explained.

By the way, this is a single-phase thyristor power regulator. Such a device can be used to control power or speed. However, first we need to understand this because this will allow us to understand for what load it is better to use such a regulator.

How does a thyristor work?

A thyristor is a controlled semiconductor device capable of conducting current in one direction. The word “controlled” was used for a reason, because with its help, unlike a diode, which also conducts current only to one pole, you can select the moment when the thyristor begins to conduct current. The thyristor has three outputs:

• Anode.
• Cathode.
• Control electrode.

In order for current to begin flowing through the thyristor, it is necessary to perform following conditions: the part must be in a live circuit, and a short-term pulse must be applied to the control electrode. Unlike a transistor, controlling a thyristor does not require holding the control signal. The nuances do not end there: the thyristor can be closed only by interrupting the current in the circuit, or by forming a reverse anode-cathode voltage. This means that the use of a thyristor in DC circuits is very specific and often unwise, but in AC circuits, for example in a device such as a thyristor power regulator, the circuit is constructed in such a way that a condition for closing is ensured. Each of the half-waves will close the corresponding thyristor.

Most likely, you don’t understand everything? Do not despair - below we will describe in detail the process of operation of the finished device.

Scope of application of thyristor regulators

In what circuits is it effective to use a thyristor power regulator? The circuit allows you to perfectly regulate the power of heating devices, that is, influence the active load. When working with a highly inductive load, the thyristors may simply not close, which can lead to failure of the regulator.

Is it possible to have an engine?

I think many of the readers have seen or used drills, angle grinders, which are popularly called “grinders,” and other power tools. You may have noticed that the number of revolutions depends on the depth of pressing the trigger button of the device. It is in this element that a thyristor power regulator is built in (the diagram of which is shown below), with the help of which the number of revolutions is changed.

Pay attention! Thyristor regulator cannot change speed asynchronous motors. Thus, the voltage is adjusted to commutator engines, equipped with a brush unit.

Scheme of one and two thyristors

A typical circuit for assembling a thyristor power regulator with your own hands is shown in the figure below.

The output voltage of this circuit is from 15 to 215 volts; in the case of using the indicated thyristors installed on heat sinks, the power is about 1 kW. By the way, the switch with the light brightness control is made according to a similar scheme.

If you don't need to fully regulate the voltage and just want an output of 110 to 220 volts, use this diagram, which shows a half-wave thyristor power regulator.

How does this work?

The information described below is valid for most schemes. Letter designations will be taken in accordance with the first thyristor regulator circuit

A thyristor power regulator, the operating principle of which is based on phase control of the voltage value, also changes the power. This principle is that under normal conditions an alternating voltage acts on the load. household network, varying according to a sinusoidal law. Above, when describing the operating principle of a thyristor, it was said that each thyristor operates in one direction, that is, it controls its own half-wave from a sine wave. What does it mean?

If you periodically connect a load using a thyristor at a strictly defined moment, the value of the effective voltage will be lower, since part of the voltage (the effective value that “falls” on the load) will be less than the mains voltage. This phenomenon is illustrated in the graph.

The shaded area is the area of ​​stress that is under load. The letter “a” on the horizontal axis indicates the opening moment of the thyristor. When the positive half-wave ends and the period with the negative half-wave begins, one of the thyristors closes, and at the same moment the second thyristor opens.

Let's figure out how our specific thyristor power regulator works

Scheme one

Let us stipulate in advance that instead of the words “positive” and “negative”, “first” and “second” (half-wave) will be used.

So, when the first half-wave begins to act on our circuit, capacitors C1 and C2 begin to charge. Their charging speed is limited by potentiometer R5. this element is variable, and with its help the output voltage is set. When the voltage necessary to open dinistor VS3 appears on capacitor C1, the dinistor opens and current flows through it, with the help of which thyristor VS1 will be opened. The moment of breakdown of the dinistor is point “a” on the graph presented in the previous section of the article. When the voltage value passes through zero and the circuit is under the second half-wave, the thyristor VS1 closes, and the process is repeated again, only for the second dinistor, thyristor and capacitor. Resistors R3 and R3 are used for control, and R1 and R2 are used for thermal stabilization of the circuit.

The principle of operation of the second circuit is similar, but it controls only one of the half-waves AC voltage. Now, knowing the principle of operation and the circuit, you can assemble or repair a thyristor power regulator with your own hands.

Using the regulator in everyday life and safety precautions

It is impossible not to say that this scheme does not provide galvanic isolation from the network, so there is a danger of electric shock. This means that you should not touch the regulator elements with your hands. An insulated housing must be used. You should design the design of your device so that, if possible, you can hide it in an adjustable device and find free space in the case. If the adjustable device is located permanently, then in general it makes sense to connect it through a switch with a dimmer. This solution will partially protect against electric shock, eliminate the need to find a suitable housing, and has an attractive appearance and manufactured industrially.

♦ It is known that electric current in household and industrial networks varies according to a sinusoidal law. Form of alternating electric current frequency 50 hertz, presented on Fig 1 a).

Over one period, cycle, the voltage changes its value: 0 → (+Umax) → 0 → (-Umax) → 0 .
If you imagine simple generator AC (Figure 1 b) with one pair of poles, where the receipt of a sinusoidal alternating current determines the rotation of the rotor frame per revolution, then each rotor position in certain time period corresponds to a certain value of the output voltage.

Or, each value of the sinusoidal voltage per period corresponds to a certain angle α frame rotation. Phase angle α , this is the angle that determines the value of a periodically changing quantity in at the moment time.

At the moment of the phase angle:

• α = 0° voltage U = 0;
• α = 90° voltage U = +Umax;
• α=180° voltage U = 0;
• α = 270° voltage U = — Umax;
• α = 360° voltage U = 0.

♦ Voltage regulation using a thyristor in alternating current circuits uses these features of sinusoidal alternating current.
As mentioned earlier in the article “”: a thyristor is a semiconductor device that operates according to the law of a controlled electric valve. It has two stable states. Under certain conditions may have a conducting state (open) and non-conducting state (closed).
♦ A thyristor has a cathode, an anode and a control electrode. Using the control electrode, you can change the electrical state of the thyristor, that is, change the electrical parameters of the valve.
A thyristor can only pass electric current in one direction - from anode to cathode (the triac passes current in both directions).
Therefore, for the thyristor to operate, the alternating current must be converted (rectified using a diode bridge) into a pulsating voltage of positive polarity with the voltage crossing zero, as in Fig 2.

♦ The method of controlling a thyristor is to ensure that at the moment of time t(during the half-cycle Uс) through the transition Ue – K, switching current has passed Ion thyristor.

From this moment, the main cathode-anode current flows through the thyristor, until the next half-cycle transition through zero, when the thyristor closes.
Turn-on current Ion A thyristor can be obtained in different ways.
1. Due to the current flowing through: +U – R1 – R2 – Ue – K – -U (on the diagram, Fig. 3) .
2. From a separate unit for generating control pulses and feeding them between the control electrode and the cathode.

♦ In the first case, the control electrode current flows through the junction Ue – K, gradually increases (increasing along with the voltage Uс), until it reaches the value Ion. The thyristor will open.

phase method.

♦ In the second case, a short pulse formed in a special device right moment time given for transition Ue – K, from which the thyristor opens.

This method of controlling a thyristor is called pulse-phase method .
In both cases, the current that controls the switching on of the thyristor must be synchronized with the beginning of the transition of the mains voltage Uс through zero.
The action of the control electrode is reduced to controlling the moment when the thyristor is turned on.

Phase method of thyristor control.

♦ Let's try it on simple example thyristor lighting controller (diagram on Fig.3) analyze the features of the operation of a thyristor in an alternating current circuit.

After the rectifier bridge, the voltage is a pulsating voltage, changing in the form:
0→ (+Umax) → 0 → (+Umax) → 0, as in Fig. 2

♦ The beginning of thyristor control comes down to the following.
When the network voltage increases Uс, from the moment the voltage crosses zero, a control current appears in the control electrode circuit Iup along the chain:
+U – R1 – R2 – Ue – K – -U.
With increasing tension Uс The control current also increases Iup(control electrode - cathode).

When the control electrode current reaches the value Ion, the thyristor turns on (opens) and closes the points +U and –U on the diagram.

The voltage drop across an open thyristor (anode - cathode) is 1,5 – 2,0 Volt. The control electrode current will drop almost to zero, and the thyristor will remain in a conducting state until the voltage Uс the network will not drop to zero.
With the action of a new half-cycle of the network voltage, everything will repeat from the beginning.

♦ Only the load current flows in the circuit, that is, the current through the lamp L1 along the circuit:
Uс – fuse – diode bridge– anode – thyristor cathode – diode bridge – light bulb L1 – Uс.
There will be a light bulb light up with each half-cycle of the mains voltage and go out when the voltage passes through zero.

Let's do some calculations as an example. Fig.3. We use the element data as in the diagram.
According to the thyristor reference book KU202N switching current Ion = 100 mA. In reality, it is much less and amounts to 10 – 20 mA, depending on the instance.
Let's take for example Ion = 10 mA .
Controlling the moment of switching on (brightness adjustment) occurs by changing the value of the variable resistance of the resistor R1. For different resistor values R1, there will be different breakdown voltages of the thyristor. In this case, the moment of switching on the thyristor will vary within the limits:

1. R1 = 0, R2 = 2.0 Com. Uon = Ion x (R1 + R2) = 10 x (0 + 2 = 20 volts.
2. R1 = 14.0 Kom, R2 = 2.0 Kom. Uon = Ion x (R1 + R2) = 10 x (13 + 2) = 150 volts.
3. R1 = 19.0 Kom, R2 = 2.0 Kom. Uon = Ion x (R1 + R2) = 10 x (18 + 2) = 200 volts.
4. R1 = 29.0 Kom, R2 = 2.0 Kom. Uon = Ion x (R1 + R2) = 10 x (28 + 2) = 300 volts.
5. R1 = 30.0 Kom, R2 = 2.0 Kom. Uon = Ion x (R1 + R2) = 10 x (308 + 2) = 310 volts.

Phase angle α varies from a = 10, up to a = 90 degrees.
An approximate result of these calculations is given in rice. 4.

♦ The shaded part of the sine wave corresponds to the power released at the load.
Power control by phase method, possible only in a narrow range of control angle from a = 10°, to a = 90°.
That is, within from 90% to 50% power allocated to the load.

Start of regulation from phase angle a = 10 degrees is explained by the fact that at the moment of time t=0 – t=1, the current in the control electrode circuit has not yet reached the value Ion(Uc has not reached 20 volts).

All these conditions are fulfilled if there is no capacitor in the circuit WITH.
If you install a capacitor WITH(in the diagram in Fig. 2), the voltage regulation range (phase angle) will shift to the right as in Fig.5.

This is explained by the fact that at first (t=0 – t=1), all the current goes to charge the capacitor WITH, the voltage between Ue and K of the thyristor is zero and it cannot turn on.

As soon as the capacitor is charged, the current flows through the control electrode - the cathode, and the thyristor turns on.

The adjustment angle depends on the capacitance of the capacitor and moves approximately from a = 30 to a = 120 degrees (with capacitor capacity 50 µF).
The load power will vary approximately from 80% to 30%.

Of course, all the calculations given are very approximate, but the general reasoning is correct.

All of the above voltage diagrams, at different time values, were clearly visible on the oscilloscope screen.

If you have an oscilloscope, you can see for yourself

Thyristors are often used to turn on and off loads (incandescent lamps, relay windings, electric motors, etc.). The peculiarity of this type of semiconductor devices and their main difference from transistors is that they have two stable states, without any intermediate ones.

It is an "on" state when the resistance of the semiconductor device is minimal, and an "off" state when the resistance of the thyristor is maximum. Ideally, these resistances approach zero or infinity.

To turn on the thyristor, it is enough to apply the control voltage at least briefly to its control electrode. You can turn off the thyristor (lock it) by briefly turning off the power to the thyristor, changing the polarity of the supply voltage, or reducing the current in the load below the holding current of the thyristor.

Typically, thyristor switches are turned on and off using two buttons. Single-button thyristor control circuits have become significantly less widespread.

Methods for single-button control of thyristor switches are discussed here in detail. The operating principle of thyristor single-button control devices is based on dynamic charge-discharge processes in the thyristor control circuit.

Single-button thyristor control circuit

Figure 1 shows one of the simplest single-button control circuits for a thyristor switch. In the diagram (hereinafter) buttons are used without fixing the position. In the initial state, the normally closed contacts of the button bypass the thyristor control circuit.

The thyristor resistance is maximum, no current flows through the load. Diagrams of the main processes occurring in the circuit in Fig. 1, are discussed in Fig. 2.

To turn on the thyristor (ON), press the SB1 button. In this case, the load is connected to the power source through the contacts of the SB1 button, and capacitor C1 is charged through resistor R1 from the power source.

The charging rate of the capacitor is determined by the time constant of the circuit R1C1 (see diagram). After the button is released, capacitor C1 is discharged to the control electrode of the thyristor. If the voltage across it is equal to or exceeds the turn-on voltage of the thyristor, the thyristor is unlocked.

Rice. 1. Schematic diagram control the thyristor using one button.

Rice. 2. Diagrams of the main processes occurring in a circuit with a thyristor.

You can turn off the load (OFF) by briefly pressing the SB1 button. In this case, capacitor C1 does not have time to charge. Since the button contacts bypass the thyristor electrodes (anode - cathode), this is equivalent to turning off the thyristor's power source. As a result, the load will be disconnected.

Therefore, to turn on the load, you need to press the control button for a longer time, and to turn it off, briefly press the same button again.

Simple power switches based on thyristors

In Fig. 3 and 4 show variants of the circuit idea presented in Fig. 1. In Fig. 3, a chain of series-connected diodes VD1 and VD2 is used to limit the maximum charging voltage of the capacitor.

Rice. 3. A variant of the thyristor control circuit with one button.

This made it possible to significantly reduce the operating voltage (to 1.5...3 V) and the capacitance of capacitor C1. In the following circuit (Fig. 4), resistor R1 is connected in series with the load, which allows you to create a two-pole load switch. The load resistance should be much lower than R1.

Rice. 4. Circuit of an electronic key based on a thyristor with a serial load connection.

Thyristor switch with two buttons

A thyristor load control device (Fig. 5) can be used to turn the load on and off using any of several buttons connected in series that operate to open the circuit. The operating principle of a thyristor switch is as follows.

When the device is turned on, the voltage supplied to the control electrode of the thyristor is not enough to turn it on. The thyristor, and, accordingly, the load is turned off. When you press any of the buttons SB1 - SBn (and hold it pressed), capacitor C1 is charged through resistor R1 from the power source. The thyristor control circuit and the thyristor itself are disabled.

Rice. 5. Diagram of a simple thyristor load switch with two buttons.

After releasing the button and restoring the thyristor power circuit, the energy accumulated by capacitor C1 is applied to the control electrode of the thyristor. As a result of the discharge of the capacitor through the control electrode, the thyristor turns on, thereby connecting the load to the power circuit.

To turn off the thyristor (and load), briefly press any of the buttons SB1 - SBn. In this case, capacitor C1 does not have time to charge. At the same time, the thyristor's power supply circuit opens and the thyristor turns off.

The value of resistor R2 depends on the supply voltage of the device: at a voltage of 15 V its resistance is 10 kOhm at 9 V - 3.3 kOhm at 5 6-1.2 kOhm.

Circuit with the equivalent of a thyristor on transistors

When using its transistor analogue instead of a thyristor (Fig. 6), the value of this resistor changes, respectively, from 240 kOhm (15 V) to 16 kOhm (9 V) and to 4.7 kOhm (5 V).

Rice. 6. Scheme of an electronic load switch with a transistor equivalent of a thyristor.

Analogue of a multi-button switch using thyristors

A thyristor device that makes it possible to create an analogue of a multi-button switch with dependent position fixation and uses push-button elements operating without fixation for control is shown in Fig. 7. Several thyristors can be used in the circuit, however, to simplify the circuit, only two channels are shown in the figure. Other switching channels can be connected similarly to the previous ones.

Rice. 7. Schematic diagram of an analog multi-button switch using thyristors.

In the initial state, the thyristors are locked. When you press a control button, for example, button SB1, capacitor C1 of a relatively large capacity is connected to the power source through diodes VD1 - VDm and load resistances of all channels.

As a result of charging the capacitor, a current pulse occurs, leading to a short-circuit of the anodes of all thyristors through the corresponding diodes VD1 - VDm to the common bus.

Any of the thyristors, if it was turned on, turns off. At the same time, the capacitor stores energy. After releasing the button, the capacitor is discharged onto the control electrode of the thyristor, unlocking it.

To turn on any other channel, press the corresponding button. The previously involved load is disconnected (reset) and a new load is turned on. The circuit provides a button SB0 for general shutdown of all loads.

Multi-button switch with transistor analogue of thyristors

A version of the circuit, made on transistor analogues of thyristors and diode-capacitive charging circuits using small capacitors, is shown in Fig. 8, 9.

Rice. 8. Diagram of equivalent replacement of a thyristor with transistors.

The circuit provides LED indication of the activated channel. In this regard maximum current The load of each channel is limited to 20 mA.

Rice. 9. Diagram of a multi-button switch with a transistor analogue of thyristors.

Devices similar to those shown in Fig. 7 - 9, as well as in Fig. 10 - 12, can be used for program selection systems for radio and television receivers.

The disadvantage of circuit solutions (Fig. 7 - 9) is that at the moment you press any of the buttons, all loads are at least momentarily connected to the power source.

Multi-position switch circuits

In Fig. 10 and 11 show a thyristor switch of a discontinuous type with an unlimited number of series-connected elements.

When you press one of the control buttons, the power supply circuit of the thyristor analogues opens according to DC. Capacitor C1 is connected in series with the thyristor analogue.

Rice. 10. Diagram of the basic element for a homemade multi-position load switch.

Rice. 11. Schematic diagram of a homemade multi-position load switch.

At the same time, the control voltage (zero level) through the activated button and resistor R2 (Fig. 10) is supplied to the control electrode of the thyristor analogue.

Since in the first moments when the button is pressed, a completely discharged capacitor turns on in series with the thyristor analogue, such inclusion is equivalent to a short circuit in the power circuit of the corresponding thyristor. Consequently, the thyristor is turned on, thereby turning on the corresponding load.

When you press any other button, the previously activated channel is turned off and another channel is turned on. When you press any button for a long time (about 2 seconds), capacitor C1 is charged, which is equivalent to opening the circuit and leads to the locking of all thyristors.

Advanced Electronic Switch Circuit

Rice. 12. Schematic diagram of a thyristor switch for multiple loads.

Among the thyristor switches, the most advanced is the circuit shown in Fig. 12. When the control button is pressed, an inrush current occurs, equivalent to a short circuit.

The previously activated thyristors are turned off and the thyristor corresponding to the pressed button is turned on. The circuit provides an LED indication of the involved channel, as well as a general reset button.

Instead of high-capacity capacitors, diode-capacitor chains can be used (Fig. 12). The operating principle of the circuit remains the same. As a load, you can use low-voltage relays, for example, RMK 11105 with a resistance of 350 Ohms for an operating voltage of 5 V.

Resistor R1 limits the current short circuit and maximum consumption current of 10... 12 mA. The number of switching channels is not limited.

Literature: Shustov M.A. Practical circuit design (Book 1), 2003.