Low voltage load current interrupter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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The current interrupter proposed below differs from similar devices of a similar purpose by a small voltage drop across an open switching element and a small intrinsic current consumption during that part of the period of operation when this element is closed.

The breaker is capable of operating in a wide range of load current - from a few milliamps to tens of amperes at a frequency from fractions of a hertz to tens of kilohertz. An incandescent lamp, an LED with a current-limiting resistor, a dynamic head, a car siren, a winding of a relay or a transformer, and other current consumers can be used as a load.

Fig. 1

Schematic diagram of the interrupter is shown in fig. 1. On the field effect transistor VT1 and logic elements DD1.1, DD1.2, a rectangular pulse generator is built. Their repetition rate is set by resistor R3 and capacitor C2, and resistor R4 and diode VD1 provide a pulse duty cycle equal to two. Resistor R5 forms a positive OS circuit, which increases the switching speed of the elements of the DD1 chip.

This construction of the generator significantly reduces the current consumed by the DD1 chip, since only the VT1 transistor operates in a linear mode. In addition, it greatly simplifies the frequency-setting circuit.

The node on transistors VT2, VT3 blocks the operation of the breaker when the supply voltage drops below 8 V. This is necessary to prevent overheating of the powerful field-effect transistor VT4 due to its incomplete opening and low supply voltage.

When the supply voltage decreases, the emitter junction of the VT3 transistor, which works as a zener diode, closes, followed by the VT2 transistor. The high level at the lower input of the element DD1.2 according to the circuit is replaced by a low level, as a result of which the generator stops in a position where the output of the elements DD1.3, DD1.4 is low. Transistor VT4 is closed, the current does not flow through the load.

Resistor R9 improves the start of the generator with a smooth increase in supply voltage. Capacitors C4, C5 protect the generator control unit from interference.

The low-current breaker nodes are powered by a parametric stabilizer assembled on the elements R11, VD2, VD3. Diode VD4 protects the device from emergency polarity reversal of the supply voltage. When the load RH is de-energized, the capacitor C4 accumulates the energy necessary to maintain the gate-source voltage of the transistor VT10 at a level of at least XNUMX V during those time intervals when it is open and the breaker supply voltage is close to zero.

At the rated supply voltage, the zener diodes VD2, VD3 are closed and do not increase the current consumed by the interrupter, which in pauses, when power is not supplied to the load, does not exceed 300 μA. The varistor RU1 protects the field-effect transistor VT4 from voltage surges created by the inductive component of the load (for example, the winding of the step-up transformer of the voltage converter for a fluorescent lamp or a Chizhevsky chandelier). The fusible insert FU1 protects the transistor VT4 from overload with excessively high current.

The device is hinged assembled on a circuit board measuring 64x45 mm. All resistors - MLT, C1 -4 or imported. Capacitor SZ - imported, C2 - any small-sized film, for example K73-17, the rest - any.

Instead of 1N4148 diodes, any of the KD521, KD522, KD103 series will do. Zener diodes KS168A can be replaced by KS407D, 1N4736A. We will replace the KP501A field effect transistor with any of the KP501, KP502, KP504 series or imported ZVN2120, BSS88. Instead of 2SA933, any p-n-p transistor from the B89012, KT3107 series will do. The KT645B transistor can be replaced by any of the KT645, KT315 series. They usually have an avalanche breakdown voltage of the emitter junction of about 7 V, while transistors of the KT3102 series are about 8 V, and, for example, imported 2SC3330, 2SC1740 are about 9 ... 10 V. The lower this voltage, the lower the supply voltage will stop working generator. When replacing transistors, one should not forget about the differences in their pinout.

The IRFZ30 field effect transistor has an open channel resistance of no more than 0,05 ohms, is designed for a drain-source voltage of 50 V, a maximum drain current of 30 A, and can dissipate power up to 90 W. It can be replaced by IRFZ34, IRFZ42. The pinout of these transistors is the same.

Instead of CNR07D470K, varistors FNR-05K470, FNR-07K470 are suitable.

With the ratings of the elements C2, R3, R4 indicated in the diagram, the switching frequency of the breaker is about 1 Hz, which is suitable, for example, for its operation in the car alarm system, direction indicators, in New Year's illumination. With a capacitor C2 with a capacity of 470 pF, the generator frequency will increase to 2 kHz, and then, at a supply voltage of 12 V, the interrupter can be loaded with the secondary winding of the TV-ZSh sound transformer (its magnetic circuit should be completely disassembled and reassembled, placing the plates "overlapped") from a tube-semiconductor TV , and an incandescent lamp with a power of 15 ... 25 W for a voltage of 20 V connected to the primary winding of this transformer will shine with full heat. In this capacity, the interrupter can be used as a voltage converter, for example, to power a low-voltage low-power electric soldering iron.
If it is necessary for one reason or another to change the duty cycle of the generator pulses, this can be implemented by selecting the resistor R4.

The described device can also operate at a supply voltage greater than 12 V, you only need to select a resistor R11 of such resistance that the current through the zener diodes VD2, VD3 is close to the nominal value. So, for example, at a supply voltage of 24 V, the resistance of this resistor should be equal to 3,6 kOhm, and the dissipation power should be at least 1 W.

The greater the operating frequency of the chopper, the greater the power loss for switching transistor VT4. This will require its installation on a heat sink with a larger heat sink surface.

A photo of one of the options for the design of the current interrupter is shown in fig. 2.

Literature

1. Kozhurov A. Load switch. - Radio, 1991, No. 7, p. 37-39.
2. Chudnov V., Dialectov V. Operation of a switch with a low-current load. - Radio, 1997, No. 11, p. 53.

Author: A.Butov, village of Kurba, Yaroslavl region; Publication: radioradar.net

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