Multivibrator on field-effect transistors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Multivibrator on field-effect transistors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Novice radio amateurs, of course, know that multivibrators (balanced and unbalanced) perform on bipolar transistors. Unfortunately, such multivibrators have a drawback - when working with a sufficiently powerful load, for example, incandescent lamps, large base currents are required to fully open the transistors.

If the arms of the multivibrator switch with a frequency of 3 ... 0,2 Hz, it is necessary to install high-capacity oxide capacitors in the frequency-setting circuits, and hence large dimensions. We should not forget about the relatively high saturation voltage of open transistors.

The proposed multivibrator (see figure) uses domestic field-effect n-channel transistors with an insulated gate and an induced channel. Inside the case, between the gate and source terminals, there is a protective zener diode, which significantly reduces the likelihood of a transistor failure if it is handled ineptly.

FET multivibrator

The switching frequency of the multivibrator transistors is about 2 Hz, it is set by capacitors and resistors. Multivibrator transistor load - incandescent lamps EL1, EL2.

Resistors connected between the drain and the gate of the transistors provide a soft start of the multivibrator. Unfortunately, they "tighten" the switching off of the transistors a little.

Instead of incandescent lamps, it is permissible to include LEDs with 360 Ohm limiting resistors or a telephone capsule, for example, TK-47, in the transistor drain circuit (for this option, the multivibrator must operate in the audio frequency region). In the case of using only one capsule, it is necessary to include a resistor with a resistance of 100 ... 200 Ohm as a load in the drain circuit of another transistor.

Resistors R1, R2 of the ratings indicated in the diagram can be made up of several smaller resistances connected in series. If this option is not available, install smaller resistors and larger capacitors.

Capacitors can be non-polar ceramic or film, for example, series KM-5, KM-6, K73-17. Incandescent lamps are used from a Chinese-made "flashing" Christmas tree garland for a voltage of 6 V and a current of 100 mA. Small-sized lamps for a voltage of 6 V and a current of 60 or 20 mA are also suitable.

Instead of transistors of the specified series, withstanding direct current up to 180 mA, it is permissible to use keys of the KR1064KT1, KR1014KT1 series designed for higher current. If you use a multivibrator with a more powerful load, say, car incandescent lamps, you will need other transistors, for example, KP744G, which allow a drain current of up to 9 A. But with this option, you need to install protective zener diodes for a voltage of 8 ... 10 V between the gate and the source ( cathode to the gate) - KS191Zh or similar. At high load currents, transistors will have to be installed on heat sinks.

The multivibrator is adjusted by selecting capacitors until the desired switching frequency of the transistors is obtained. To operate the device at audio frequencies, the capacitors must be 300 ... 600 pF. If you leave the capacitors of the capacitance indicated on the diagram, you will have to select resistors of lower resistance - up to 47 kOhm.

The multivibrator is operational at a supply voltage of 3 ... 10 V, of course, with an appropriate load. If it is supposed to be used as some node in the design being developed, a blocking capacitor with a capacity of 0,1 ... 100 μF is installed between the power wires of the multivibrator.

Author: A.Butov, village of Kurba, Yaroslavl region

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