Multivibrators on field-effect transistors KR504NT. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Multivibrators on field-effect transistors KR504NT. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment

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Symmetrical and asymmetric multivibrators for various purposes can be built not only on bipolar transistors, but also on field ones. One example of this can be found in [1]. Given that field-effect transistors have a number of advantages over bipolar ones, the main of which is the extremely low current in the control circuit when operating at low frequency or in static mode, it can be assumed that a conventional two-transistor multivibrator, but only on field-effect transistors, will be in a winning position in front of similar nodes assembled on their bipolar counterparts.

You can see the scheme of the first mulvibrator in fig. 1. Its operation is in many ways similar to the operation of a multivibrator on pn-p bipolar transistors - the LEDs will also wink. The difference is that to close each of the transistors VT1.1, VT1.2, it is necessary to apply a positive gate-source voltage, which must exceed the cut-off voltage of these transistors (about 4 V). This happens every time the multivibrator arms are switched, due to the presence of time-setting capacitors C1, C2. That is why there is no need for a bipolar power supply.

Multivibrators on field-effect transistors KR504NT. Schematic diagram of the multivibrator on KR504NT
Fig. 1

The switching frequency of transistors in this generator is once every 6 s. When installing high-quality electrolytic capacitors (with a low leakage current), with a capacity of 100 ... 4700 μF, it is possible to achieve transistor switching with a period of several tens of minutes, which is unattainable for simple devices based on bipolar transistors.

The resistances of resistors R2 and R3 can differ by several thousand times, for example, R2 can be taken as 30 MΩ, and R3 as 10 kΩ. The multivibrator will then become asymmetrical. The capacitances of the capacitors change in the same way. Having properly selected these elements, it is possible to obtain very short pulses at the drain output of one of the transistors, following with a large duty cycle (100 ... 10000). If in a device made according to the scheme of Fig. 1, instead of ordinary LEDs, turn on blinking ones as a load of transistors, for example, L-36BSRD, then any of them, blinking several times, will rest while its neighbor blinks. If you need to operate the multivibrator at audio frequencies, then the resistance of the resistors R2 and R3 must be reduced by 10 ... 20 times, and the capacitors should be taken with a capacity of several hundred picofarads.

Instead of conventional resistors R2, R3, you can install photoresistors (FSK, SF2-x, SFZ-x, FR117, etc.). In this case, the switching frequency of transistors will change several thousand times depending on the level of illumination. It should only be noted that if the resistance of the resistors R2, R3 is less than 3 kΩ, the generation may fail.

A multivibrator made according to the scheme shown in fig. 1, requires the use of field-effect transistors with a large initial drain current (10 ... 30 mA). In the absence of such assemblies from the KR504 series, it is possible to assemble a similar multivibrator according to the scheme shown in Fig. 2. Here, field-effect transistors operate with a lower drain current, and in order to obtain sufficient brightness of the LEDs, current amplifiers are installed on bipolar transistors VT1, VT4. The switching frequency of this multivibrator is about 1 Hz. If you install powerful composite transistors from the KT1 series in place of transistors VT4, VT829, then incandescent lamps can be used as their load. In this case, R2, R6 are not installed, since transistors of the KT829 type contain their own built-in resistors.

If this multivibrator "refuses" to work, then resistors R3, R7 should be more accurately selected. In a node assembled according to the scheme shown in Fig. 1, it is possible to use microassemblies of matched pairs of field-effect transistors of the KR504, (K504, 504) series with an initial drain current of more than 10 mA. KR504NT4V, KR504NTZV are most suitable, but you can try with indexes A, B. When changing the polarity of the supply voltage and connecting LEDs, instead of a transistor assembly, you can use two separate field-effect n-channel transistors from the KP302, KP307 series. If they have a large cutoff voltage, then the supply voltage can be increased to 15 V.

Multivibrators on field-effect transistors KR504NT. Schematic diagram of the multivibrator on KR504NT
Fig. 2

For the node, the scheme of which is shown in Fig. 2, KR504NT1, KR504NT2 microcircuits with any letter index are suitable, and when selecting resistors R3, R7 - KR504NTZ, KR504NT4. In addition, many field-effect transistors of the KP103, KP101 series will also work without tuning. It is better to use non-polar capacitors, for example, small-sized K73-17 for 63 V. "Regular" LEDs can be any of the AL307, KIPD21, KIPD35, KIPD40 series, as well as 1-1513, L-934, etc. Flashing - L-816BRSC-B, L-769BGR, L-56DGD, Т1ВК5410 and others.

Since the field-effect transistors of the KR504NT (1 ... 4) assemblies allow a maximum source-drain voltage of not more than 10 V, the supply voltage of the multivibrators should not exceed 10 ... 12 V.

Literature

A. Butov. Multivibrator on field-effect transistors. - Radio, 2002, N4, p.53.
Microcircuits and their application. - M.: Radio and communication, 1984, p.73.

Publication: cxem.net

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