Acoustic relay on a field effect transistor. Scheme, description

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Acoustic relay on a field effect transistor. Encyclopedia of radio electronics and electrical engineering

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

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The acoustic relay is connected in series with the load, on command it smoothly turns on, turning off after a certain period of time. It does not interfere and practically does not reduce the brightness of the incandescent lamp. In addition, the device has a voltage indication when the load is off and an emergency shutdown for overcurrent. The device is assembled on a board with dimensions of 28x50 mm and is easily placed in a light switch for internal wiring.

Acoustic relay on a field effect transistor. Acoustic relay circuit

The device is powered by 220 V through the load (filament lamp EL1). The supply voltage is supplied to the VD4-VD7 diode bridge through the terminal block. The key element of the device is a field effect transistor VT5, included in the diagonal of the bridge through the resistor R17, which is a current sensor. In the initial state, all capacitors are discharged, the voltage between the gate and the source of the field-effect transistor VT5 is zero, as a result of which this transistor is in the closed state. There is practically no current through the load. The source-drain voltage of the transistor VT5 is partially protected from possible surges in the network by capacitor C10. Under the influence of this voltage, the capacitor C4 is charged through the resistor R12 and the LED HL1 to the reference voltage of the zener diode VD1 (15 V).

The signal amplifier from the electret microphone BM1 is assembled on a transistor VT1 and operates in the low collector current mode - about 0,15 mA. The microphone is powered through resistor R1 with a current of less than 0,3 mA. Coupling capacitor C1 of small capacity suppresses low-frequency signals. Sensitivity adjustment is carried out by a trimmer resistor RP1 included in the negative current feedback circuit. The signal, amplified to an amplitude of 1 ... 2 V, is fed through the decoupling capacitor C2 to the input of the transistor switch assembled on the transistor VT2. The negative half-wave of the signal, exceeding 0,6 V in amplitude, opens the transistor VT2 and charges the capacitor C2 through the VD7 diode and the current-limiting resistor R5. The same result can be obtained by pressing the SB1 button (momentary button). Through the divider R10R11, this voltage is applied to the gate of a low-power field-effect transistor VT3, opens it, as a result, the bipolar transistor VT4 closes. The voltage on the capacitor C5 for about 0,5 ms reaches a level slightly lower than the voltage on the capacitor C4. Through the high-resistance resistor R9, capacitor C9 begins to charge, which is connected directly to the gate circuit of the field-effect transistor VT5. Together with the C8R15 negative feedback circuit, a smooth opening of the VT5 field effect transistor is provided. The soft start time is a little more than 1 s and can be changed by selecting the values of the resistor R9 or capacitor C8. After opening the transistor VT5, the diagonal of the VD4-VD7 bridge becomes closed, the incandescent lamp EL1 lights up at full brightness.

The drain-source voltage of the open transistor VT5 is a fraction of a volt, the current in the circuit R12, HL1 stops, the LED goes out and the voltage across the capacitor C4 drops to zero. The signal amplifier from the microphone on the transistor VT1 stops working. Capacitors C5, C9 are smoothly discharged through resistors R10, R11.

Transistor VT4 serves to quickly discharge capacitors C5, C9 and close the key transistor VT5 in two cases:

- upon expiration of the exposure time, when the voltage at the gate of the field-effect transistor VT5 approaches the threshold value and voltage appears on its drain;
- in the event of an overcurrent, when the negative voltage on the resistor R4 relative to the negative terminal C17 exceeds the voltage in absolute value
opening the emitter-base transition of the transistor VT4 (more than 0,6 V).

The field-effect transistor VT3 is necessary to close the transistor VT4 during soft start, since in the initial state (transistor VT5 is closed) the transistor VT4 is in the open state, which is created due to the voltage drop across the divider resistors R13R14. When discharging capacitors C5, C9 through resistors R10, R11 and a smooth decrease in voltage across them, transistor VT3 must close before the voltage across capacitor C9 reaches the closing threshold of transistor VT5. This is ensured by a selection of resistance divider R10R11. When the closing threshold of VT5 is reached, a voltage appears on its drain, which, acting through the resistor R13 on the base of the transistor VT4, ensures the rapid discharge of capacitors C5, C9 through the current-limiting resistor R8 and the collector-emitter junction of this transistor.

Transistor VT5 closes and turns off the load. The HL1 LED starts to glow and voltage appears on the capacitor C4. The device is ready to be switched on again. The time from turning on the load to turning it off at the ratings indicated in the diagram is about 3 minutes and can be changed by selecting the capacitor C5 or the resistances of the resistors R10, R11 up or down. Capacitor C7 improves noise immunity. The device uses small-sized resistors of size 1206 (all resistors in the diagram without power designation) and capacitors of size 1206 (C1-C3, C6, C7). Resistors RIO, R11 - high-resistance C3-13, C3-14, R17 - wire. The remaining resistors are MLT, C2-23, C1-4 in accordance with the indicated power. Capacitor C4 - imported oxide for a voltage of at least 25 V, the remaining film capacitors, for example, K73-17, capacitors C8, C10 for a voltage of at least 400 V. Diode bridge VD4-VD7 for a voltage of at least 600 V and a current exceeding the rated current load at least twice.

Zener diode VD1 - BZX55C15 for a voltage of 15 V, it can be replaced with KS515A1, diodes VD2, VD3 - 1N4148, instead of them you can use KD521, KD522 with any letter index. As a diode VD2 (prevents the discharge of capacitors C5, C9 through the collector-base junction of the transistor VT2, when the voltage across the capacitor C4 drops to zero), it is better to turn on the collector-base junction of the transistor BC547, which has a lower reverse current and allows you to slightly increase the exposure time. The key transistor VT5 - IRF840 for a voltage of 500 V and a current of 8 A can be replaced with a domestic KP707B1, KP707V1. When switching a lamp with a power of up to 100 W and with an exposure time of 3 min, the transistor can operate without an additional heat sink, since its significant heating occurs only during switching on. Red LED HL1 with low operating current L-53LSRD. The field effect transistor VT3 ZVN2120 can be replaced with its analogue KP501A.

Author: A. Begiev, Volzhsky, Volgograd region; Publication: cxem.net

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