Thyristor timers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches
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Thyristor switches can be successfully used in timer circuits.
A diagram of one of these timers is shown in Fig. 1. In the initial state, the thyristor VS1 is closed, and the device does not consume current from the power source. When you press the button SB1 ("Start"), the capacitor C1 is discharged through the diode VD1. At the same time, a control voltage is applied to the control electrode of the thyristor through the resistor R1. Thyristor VS1 turns on, and through the load resistance RH current flows. At the same time, the previously discharged time-setting capacitor C1 begins to charge through the resistor R3 and the potentiometer R4. The charge rate of the capacitor depends on the resistance R4.
Rice. 1. Schematic diagram of the thyristor timer 1
When the voltage on the capacitor plates exceeds the breakdown voltage of the Zener diode VD2, the capacitor is discharged through the emitter-base junction of the transistor VT2. It opens, respectively, and the transistor VT1 opens. This transistor shunts a series-connected thyristor VS1 and diode VD3. Since the voltage drop across an open transistor is small, such shunting is tantamount to interrupting the current through the thyristor. Therefore, the thyristor turns off. The charging process stops (R4 is disconnected from the charging circuit), and since the capacitor C1 has already managed to partially discharge, the voltage across it has dropped so much that the zener diode VD2 ceases to conduct current and participate in the charging process. Accordingly, transistors VT2 and VT1 close. The circuit goes into its initial state and is ready for the next inclusion.
The timer exposure interval is determined by the time constant τ=С1(R3+R4) and is 1...20 s for the ratings indicated on the diagram. Due to the short duration of transients, low-power transistors KT1 can be used as transistor VT315. It is desirable to use transistors with high p values in the circuit.
Fig.2. Schematic diagram of the thyristor timer 2
The second timer (Fig. 2) works on the principle of discharging a charged capacitor. In the initial state, the thyristor and transistor are closed. When you press the start button SB1, the control voltage turns on the thyristor. Capacitor C1 is charged at the same time. The voltage taken from C1 is added "in antiphase" with the voltage supplied through an open thyristor, potentiometer R3 and resistor R4, to the base VT1.
After C1 is discharged through the section of the potentiometer R3 connected in parallel to it, VT1 will open and bypass the chain of series-connected VS1-HL1-VD2. The thyristor turns off, opening the load supply circuit and the control circuit of the transistor VT1. The circuit returns to its original state.
The exposure time can be set from 0 (when R3 is fully inserted) to 40 s. The reed relay K1 is used as a load for an operating voltage of 5 V (RMK 11105) with a resistance of 350 Ohm. The HL1 LED indicates the on state of the device. The maximum load current must not exceed 20 mA.
Author: M. Shustov, Tomsk; Publication: radioradar.net
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