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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING The device for maintaining the operating temperature of heat-inertia loads on the Schmitt trigger. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers This electronic device can be used to maintain the operating temperature of loads with large thermal inertia, such as electric irons, electric stoves, electric boilers, etc. The device can also successfully replace the bimetallic contact in case of its breakdown in the above devices. Thanks to this device, you can save electricity and extend the life of heat and power consumers. The Schmitt trigger (TSh) with a field-effect transistor at the input can be used in electronic automation devices, where the AC signal must be converted into pulses. These are diagnostic circuits, phase shift meters and other devices. The TS itself is assembled on transistors VT1, VT2 and operates in the frequency range from zero hertz to units of kilohertz, has a large input impedance and an adjustable response threshold. The device allows you to connect heating devices with a power of up to 1,3 kW and stepwise control the power in the connected load according to the following range: 0, 17, 34, 50, 65 and 100%. The electrical circuit (see figure) consists of a bridge rectifier on VD2, a stabilizer on VD3, VD4, the TS itself on VT1, VT2, a current amplifier - an emitter follower on VT3, a loaded relay K1 and a relay K2 with a powerful contact group for connecting powerful thermal loads .
As you know, a trigger on conventional pnp type transistors is an electronic device with emitter connections, in which the emitters of transistors are connected together and work for a common current load (R11 in the figure), and the own loads of transistors (R1 and R2) due to the feedback created by the above circuit and a common current resistor R11, can only be in relay mode, i.e. one transistor is open, the other is closed and vice versa. This TS differs from the one described above in that instead of the usual pnp transistor, a field (channel) transistor is turned on at the input of the circuit. TS is widely used in electronics, for example, in the first domestic color televisions "Electron 701" and "Rubin 401-1" (for tracking color synchronization). The TS circuit in this case is a lamp-semiconductor one. The initial state of the trigger: the transistor VT2 is open, VT1 is closed. If no negative voltage is applied to the trigger input (connection point of R4 and R5), the trigger is always in its initial state. If a negative voltage is applied to the trigger input above the threshold for triggering it, then at a certain voltage (threshold) it will go into another stable state. In this case, VT2 will close and VT1 will open. When the response threshold at increased input voltage, as well as the voltage at which the trigger returns to its original state when the voltage at its input decreases, is not equal, then there is a so-called hysteresis equal to dU. Principle of operation. When the supply voltage is applied to the TS (Upit = 15 V), the capacitor C2 through the resistor R4 and the normally closed contact of the relay K1.1 starts charging. The negative voltage at the trigger input (on capacitor C2) rises. When a certain voltage is reached (about 4,5 V), the transistor VT2 jumps into the closed state. Relay K1 turns on (HL1 lights up), and contact K1.1 breaks the charge circuit C2. Capacitor C2 is discharged through the circuit C2-R5-R8. At a certain voltage (about 3 V), the TS returns to its original state. Transistor VT2 opens and relay K1 turns off. Capacitor C2 is charged again via contact K1.1 and the cycle repeats. With the ratings indicated in the diagram, relay K1 is on for 7 s, off for 14 s. Thus, a scale of power consumption is obtained with (with the position of the toggle switch SB1 indicated on the diagram) values of 0, 35, 65, 100%. If the SB1 toggle switch is turned on, then a powerful diode VD5 is connected to the load circuit, which allows you to get a discrete overall heating scale of 0, 17, 34, 50, 65, 100%. This scale can be changed if necessary. For example, when the authors use resistors R4 = 100 kOhm, R8 = 75 kOhm (Upit = 15 V), the time the relay was in the on state was 8 s, in the off state 24 s. As a result, the heating scale looked like: main 0, 25, 75, 100; additional 0, 12, 37, 50. The advantage of this heating power control circuit, in contrast to the thyristor circuits published earlier [1-4], is that without any changes in the circuit by introducing additional elements (power relay K2', SA1' SB1', VD5', as well as sockets for connecting the load) you can independently regulate another thermal load, similar to the main one. In the case of refinement of the device for regulating two or three loads, it is necessary to select the capacitance of the capacitor C3. Details. C3 - for an operating voltage of 400..500 V. The circuit uses a K73-11 capacitor 2,2 μF x 250 V. Capacitors C1, C2, C6 of the K50-6 type. The power of the resistor R12 is 0.5 or 1 watt. Resistor R13 - 2 W with a resistance of 47..68 ohms. The power of the remaining resistors is 0,125 or 0,25 watts. Diode VD1 - germanium type D9 with any letter index. Bridge VD2 - high-voltage, for example, KTS403A ... V, KTS404A..V. Zener diodes VD3, VD4 are installed on radiators with an area of 1x1 cm2. They can be replaced with one D815E zener diode. VD5 is mounted on a radiator. As VD5, you can use any high-voltage type D245, D245A, D246, D246A, D247. Toggle switch SB1 type TV1-2. Switch SA1 type PM2 (passport 5P2N or 11P1N). Transistor VT1 with low cutoff voltage type KP103E, KP201E or 2P103A. Particular attention should be paid to the connection of the gate VT1. Transistor VT2 type pnp with a gain of at least 50. Relay K1 type RES22 (passport RF4.500.129 or 0230502), relay K2 type REN18 (passport РХ4.564.509). K2.1 - two parallel contacts of the REN18 relay. To suppress the interference created by the device, elements C4 and C5 are introduced into the circuit, the capacitance of which is selected empirically. By turning on any radio receiver tuned to the MW or LW range, by turning the K2 relay on and off, they minimize the interference introduced by the circuit into the operation of other devices. In parallel with the winding of relay K2, it is also recommended to install a diode connected by the cathode to a common wire. Due to the fact that the electrical circuit has a galvanic connection with the 220 V network, it is necessary to observe all safety measures during installation and commissioning of the device. Installation is recommended to be carried out in two stages, dividing the circuit into two nodes. The first node is all elements to the right of the zener diodes VD3, VD4 (TSh, relay K1), the second node is the left (according to the diagram) part, including VD3 and VD4. This approach during installation is due to the fact that the main node (TSh and relay K1) is configured with a constant 15 V power supply that is not connected to the network, which prevents electric shock when setting up the device. Adjustment. Assemble a node with elements K1, R6, R7, HL1. By connecting an ohmmeter (or any other probe), on the free contact of relay K1, check the on and off voltage of K1. Choosing R6, they achieve that the relay K1 turns on at 7..9 V, and turns off at 3,5 ... 4,5 V. Then the debugged node is connected to the circuit. Connect an ohmmeter between the common wire ("+" C1 and C2) and the VT3 emitter. A constant voltage of 15 V is applied to the TS. If the circuit is assembled without errors, then the TS immediately starts to function correctly. At the same time, two voltage values are recorded on the voltmeter (voltage is duplicated on HL1): low level (approximately 3 V, relay K1 is off) and high level (about 11 V, relay K1 is on). When fixing U = 3 V on the voltmeter, the trigger is in its initial state, and when fixing 11 V, the TSh is in an "inverted" state. In this case, the contact K1.1 opens, the capacitor C2 begins to discharge, the relay K1 will be in this state until the voltage on C2 drops to the lower threshold of this trigger, which abruptly switches to another stable state. Contact K1.1 closes, C2 charges again and the cycle repeats. After checking the normal functioning of the TS with the help of R4 and R8, the necessary heating scale is selected. The right side of the circuit is disconnected from the source. Then carefully check the correct installation of the left side of the circuit, after which the entire circuit is assembled. Having connected the assembled device to the network, the voltage is checked with a voltmeter (probe on "+" C1 and C2). The voltage on the collector VT3 should be 15 ± 0,5 V, and the voltage on the "-" VD2 20 ± 2 V. When using a TS with an adjustable response threshold in the circuit, it is necessary to install a variable with a limiting additive instead of a constant resistor R1. References:
Authors: V.G.Nikitenko, O.V.Nikitenko
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