Switching two 12V lamps on transistors circuit. Logic circuits on current switches. Schema parameters and elements

Characteristics of the KT969 transistor:

Structure - NPN,
Max. eg k-b, V - 300,
Maximum permissible collector current, A - 0.1,
Current transfer coefficient h21e - 50,
Cutoff frequency h21e fgr, MHz - 60,
Maximum power dissipation, W - 1.

As you can see, the maximum current is 100 mA, which is why I decided to use them here, since I have about 50 of them lying around, and I haven’t figured out where else to put them yet. I use 3 LEDs in the switch arm, the arm current is approximately 75-80 mA. Almost with a small margin. Electrolytic capacitors with a capacity of 100 microfarads are just suitable for clearly switching transistors with the desired frequency.

I took the LEDs already soldered on boards from a (once) Chinese tape recorder. A positive current limiting resistor of 120 ohms is soldered in series to each LED. In one arm of the multivibrator, 2 yellow LEDs light up, in the other - 4, 2 on each side. The effect is very beautiful.

I placed the diagram in the bottom of a cocoa can. It is transparent and just the right diameter. I glued the boards to the sides and they are drying out. It can be powered from 9-12 volts DC at a load of 150-170 mA.

The design is simple (hinged installation) and requires virtually no adjustment; it begins to work immediately after assembly and connection of the supply voltage. And if you don’t want to look for a suitable power supply, you can power the flasher directly from the network, through a transformerless power supply. The modification scheme is shown below:

It provides an open circuit (open circuit) when it is in the off state and provides a closed circuit when it is in the on state. This is a very important function, without which the operation of many devices would be simply unthinkable.

In other words, a switch can be said to provide infinite resistance or total resistance during its off state, and it provides zero resistance or total resistance during its on state.

From this it turns out that the switch can be called a kind of controlled on/off resistor, which provides both zero and infinite resistance for the circuit without any average value. Yes, perhaps such a name may not seem the most accurate to some, but it more or less conveys the essence of the switch’s activity in a concise form.

On the other hand, a transistor can be considered as a controlled resistor, because the resistance between the emitter and collector is controlled by the current in the base-emitter junction. Due to the fact that the current at the base of the emitter is controlled, the resistance at the emitter-collector can be set to infinite, but in this way it will not be possible to make the resistance equal to zero (the result will not be ideal). However, despite the fact that the ideal value does not work out, this does not prevent the transistor from being very popular as a switch.

The transistor provides quite a lot of resistance to the circuit, but it is not perfectly infinite. The transistor also provides very little resistance, but it is also not perfectly zero.

There are 3 areas in the transistor characteristics:

— shutdown area;

— linear region;

— saturation region.

In the linear region, in order for the collector-emitter (VCE) voltage to have a wide range, the collector current (IC) is kept constant. Due to the fact that the voltage has a wide range and the collector current is almost constant, there will be a very large loss of energy if the transistor operates in this region.

But in practice, in a switch, when it is turned off, the voltage that passes through it will be equal to the voltage on the open circuit, but the current is zero, which means that there is no loss of energy. Similarly, when a switch is turned on, the current passing through the switch is as strong as the current in the closed circuit, but the voltage passing through the switch is zero, which means that there is also no loss of energy.

If you want to make a transistor act like a switch, then you need to make it work in such a way that the energy losses during the on and off states are close to zero, or very low. The only case where this is possible is when the transistor operates only in the limiting range of characteristics. There are two extreme regions in the characteristics of a transistor. These are the turn-off region and the saturation region.

In a figure where the base-emitter current or simply the base current is zero, the collector (IC) current will have a very small constant value over a large range of collector-emitter (VCE) voltage. So if the transistor operates with zero or less than zero base current, then the current passing through the collector to the emitter (IC) is very small.

Hence the transistor is in the off state, but at the same time, energy loss through the transistor (switch) i.e. IC x VCE is not significant due to the fact that IC is very small. This implies that the transistor works as an open circuit switch or as an off switch.

Now, let's assume that the transistor is connected in series with a load of resistance RL. In normal condition, the voltage passing through the load is VL. Hence the current passing through the load is:

If the transistor operates with a base current of I1 for which the collector current of C1 is greater than IL, then the transistor operates in the saturation region. Here, for any current (C1) passing through the collector of the transistor to its emitter (IC), there will be a very small voltage at the collector-emitter (VCE).

It follows that in this situation the current passing through the transistor is as strong as the current across the load, but the voltage passing through the transistor (VCE) is quite low, which means that the loss of energy in the transistor is again negligible.

The transistor behaves much like a closed circuit switch or on switch. So to use a transistor as a switch, you need to make sure that the current applied at the base-emitter is strong enough to keep the transistor in the saturation region to provide current to the load.

As already mentioned, the energy loss in a transistor, which is a switch, is very low, but not zero. It follows that this is not a perfect switch, but it is acceptable for specific applications. Now, to regulate the DC energy at the input to the load, it is necessary to use a switch transistor in such a way that it periodically turns the circuit on and off, thereby providing the desired energy at the output.

To do this, you will need a specific current waveform at the base, due to which the transistor goes into its turn-off region and saturation region, periodically, to provide current to the load. The typical periodic base current waveform is generally achieved by a microprocessor-based pulse generator.

When selecting a transistor for use as a switch, care must be taken regarding the rating of the transistor. The fact is that during the on state, all the current in the load will flow through the transistor. If this current is greater than the safe value of the transistor's collector-emitter current carrying capacity, then the transistor may permanently fail due to overheating.

Once again in the off state, all the voltage across the open circuit, the load, will appear in the transistor. The transistor must be able to withstand this voltage, otherwise the collector-emitter junction will be broken and the transistor will become on instead of off.

One more detail must be taken into account when using a transistor as a switch. A properly sized heat sink and design that is always needed for a transistor. Each transistor needs some time to transition from the off state to the on state and vice versa.

Despite the fact that this time is very short and it can be less than a few microseconds, it is still not zero. During the period during which the switch is in the on state, the current (IC) will increase while the voltage across the collector-emitter (VCE) will drop to zero.

As the current increases from zero (ideally) to its maximum, and the voltage drops from its maximum value to zero (ideally), there will be a point when both of them are at their maximum values. Peak energy loss occurs at this point.

In the same way, the maximum energy loss occurs in the transistor when it switches to the off state from the on state. It follows that the maximum energy loss occurs in the transistor during the transition period of the state change, but the energy waste is still quite average, since the transition period is quite short.

For low frequency operation, the heat generated can be medium. But if the operating frequency is very high, then there will be a significant loss of energy and corresponding generation of heat. It is worth noting that heat generation does not occur only during the transition state. It also occurs during the on or off state of the transistor. However, the amount of heat during a constant state is quite small and insignificant.

Using a transistor as a switch may seem difficult to some after the above, but it is not. You just need to pay attention to some necessary points and remember certain things. The theoretical part covering this topic, although not small, is relatively simple.

Write comments, additions to the article, maybe I missed something. Take a look at, I will be glad if you find something else useful on mine.

The electronic switch circuit was designed to control loads remotely from a distance. We'll look at the complete structure of the device another time, but in this article we'll discuss a simple electronic switch circuit based on everyone's favorite 555 timer.

The circuit consists of the timer itself, a button without fixing a transistor as an amplifier and an electromagnetic relay. In my case, a 220 Volt relay with a current of 10 Amperes was used, these can be found in uninterruptible power supplies.

Literally any medium and high power transistors can be used as a power transistor. The circuit uses a reverse bipolar transistor (NPN), but I used a direct transistor (PNP), so you will need to change the polarity of the transistor connection, that is, if you are going to use a forward transistor, then the plus power is supplied to the emitter of the transistor, when using reverse transistors conductivity, minus power is supplied to the emitter.

For direct transistors, you can use transistors of the KT818, KT837, KT816, KT814 or similar series, for reverse transistors - KT819, KT805, KT817, KT815 and so on.

The electronic switch operates in a wide range of supply voltages, personally it supplied from 6 to 16 Volts, everything works clearly.

The circuit is activated by briefly pressing the button, at this moment the transistor instantly opens, turning on the relay, and the latter, when closed, connects the load. The load is turned off only when pressed again. Thus, the circuit plays the role of a latching switch, but unlike the latter, it works exclusively on an electronic basis.

In my case, an optocoupler is used instead of a button, and the circuit closes when commanded from the control panel. The fact is that the signal to the optocoupler comes from a radio module, which was taken from a Chinese radio-controlled car. This system allows you to control multiple loads from a distance without much difficulty.

This electronic switch circuit always shows good operating parameters and works flawlessly - try it and see for yourself.

The touch switch is a very simple circuit that consists of only two transistors and several radio elements.

Sensor – sensor – with English language- a sensitive or receptive element. This circuit allows you to apply voltage to the load by touching the sensor with your finger. In this case, our sensor will be a wire coming from the base. So, let's look at the diagram:

The operating voltage of the circuit is 4-5 Volts. Maybe a little more.

The scheme is very simple. On a mm breadboard it will look something like this:

The yellow wire from the base of the KT315 transistor, which is in the air, will be our sensor.

For those who don’t remember where the emitter, collector and base are, the photo below shows the pinout (location of pins) of the KT361 transistor (left) and the KT315 transistor (right). KT361 and KT315 differ in the location of the letter. For KT361 this letter is in the middle, and for KT315 it is on the left. It doesn’t matter what letter it is. In this case, the letter “G” means transistors KT361G and KT315G are used

In my case, I used KT315B transistors (well, whatever came to hand).

Here is a video of this circuit in action:

What if you use such a touch switch to control a powerful load? For example, a 220 Volt incandescent lamp? We can just use an SSR instead of an LED.

In this circuit I used a Solid State Relay (SSR), although an electromechanical relay can also be used. When using an electromechanical relay, do not forget to place a protective diode in parallel with the relay coil

My modified TTP circuit looks like this:

And this is how it works:

On the Internet, this circuit uses three transistors. I simplified it a little. The operating principle of the circuit is very simple. When you touch the base output of transistor VT2 with your finger, a sinusoidal signal from our body is sent to the base. Where does it come from? Pick-ups from a 220 Volt network. So, these interferences are quite enough for transistor VT2 to open, then the signal from VT2 goes to the base of VT1 and is amplified even more there. The power of this signal is enough to light an LED or send a control signal to a relay. Everything is brilliant and simple!

Let's consider the diagram shown in Fig. 2.3. This circuit, which by means of a small control current can produce a much larger current in another circuit, is called a transistor switch. The rules given in the previous section help you understand how it works. When the switch contact is open, there is no base current. This means, as follows from rule 4, there is no collector current. The lamp does not light up.

Rice. 2.3. Example of a transistor switch.

When the switch is closed, the voltage at the base is 0.6 V (the base-emitter diode is open). The voltage drop across the base resistor is 9.4 V, therefore the base current is . If you use rule 4 without thinking, you can get the wrong result: (for a typical value . What is the error? The fact is that rule 4 only applies if rule 1 is met; if the collector current reaches , then the The voltage across the lamp is 10 V. In order for the current to be even greater, the collector potential must be less than the ground potential. But the transistor cannot go into this state. When the collector potential approaches the ground potential, the transistor goes into saturation mode (typical values). saturation voltages are in the range, see Appendix G) and the change in collector potential stops. In our case, the lamp lights up when the voltage drop across it is 10 V.

If an excess signal is supplied to the base (we used current, although it would be enough to have , then the circuit does not waste this excess; in our case this is very beneficial, since a large current flows through the lamp when it is in a cold state (the resistance of the lamp in cold state is 5-10 times less than when the operating current flows). In addition, at low voltages between the collector and the base, the coefficient decreases (3, which means that in order to put the transistor into saturation mode, additional base current is needed (see Appendix). G). Sometimes a resistor (with a resistance of, for example, 10 kOhm) is connected to the base so that when the switch is open, the base potential is certainly equal to the ground potential.

This resistor does not affect the operation of the circuit when the switch is closed, since only a small fraction of the current flows through it.

When designing transistor switches, you will find the following guidelines useful:

1. It is better to take a lower resistance of the resistor in the base circuit, then the excess base current will be greater. This recommendation is especially useful for circuits that control the switching of lamps; since at a low value the coefficient also decreases.

Rice. 2.4. When connecting an inductive load, you should always use a suppression diode.

It should also be remembered when developing high-speed switches, since at very high frequencies (on the order of megahertz) capacitive effects appear and the value of the coefficient decreases (3. To increase speed, a capacitor is connected in parallel to the base resistor.

2. If the load potential is for any reason less than ground potential (for example, if the load is AC or inductive), then a diode should be connected in parallel with the collector junction (you can also use a diode connected in the opposite direction with respect to the positive potential supply), then the collector-base circuit will not conduct current when the load voltage is negative.

3. When using inductive loads, the transistor should be protected by a diode connected to the load, as shown in Fig. 2.4. If the switch is open, then in the absence of a diode, a large positive voltage will act on the collector, most likely exceeding the breakdown voltage for the collector-emitter circuit. This is due to the fact that the inductance tends to maintain the on-state current flowing from the source to the collector (remember the properties of inductances in Section 1.31).

Transistor switches allow switching to occur very quickly, with switching times typically measured in fractions of microseconds. With their help, you can switch several circuits with one control signal. Another advantage of transistor switches is that they allow remote "cold" switching, in which only DC control signals are supplied to the switches. (If you “drive” the switched powerful signals themselves, then when transmitting them through cables, capacitive surges may occur, and the signals may be greatly attenuated).

Transistor in the form of a person.

Rice. 2.5. The “transistor man” monitors the base current and adjusts the output rheostat so that the output current is greater than the base current.

It should be remembered that at any given moment in time the transistor can:

a) be in cut-off mode, i.e. turn off (no collector current);

b) be in active mode (low collector current, voltage at the collector is higher than at the emitter);

c) go into saturation mode (the voltage at the collector is approximately equal to the voltage at the emitter). The transistor saturation mode is described in more detail in Appendix G.