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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Powerful FET switch, 20 amps. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches Modern powerful key field-effect transistors are distinguished by a very low channel resistance in the open state, often even lower than the resistance of closed contacts of an electromagnetic relay or a mechanical switch, because corrosion, pollution, and burning affect the resistance of mechanical contacts. The key field-effect transistor does not have these shortcomings. In addition, the low open channel resistance, even with a significant current and high load power, makes the power dissipation on the transistor very minimal. Therefore, often, for switching kilowatt loads, a key field-effect transistor does not require even the simplest radiator. Here is a diagram of an electronic switch of two loads with a supply voltage of 5 to 20 V at a current of up to 20 A. The basis of the circuit is two key field-effect transistors APM2556NU, in which the open channel resistance does not exceed 0,006 Ohm. This means that at a voltage of 20 V and a load current of 20 A (that is, at a load power of 400 W), the power on the open channel of the transistor will not exceed 2 ... 4 W. The switch is controlled by two quasi-touch (non-latching) buttons, by short-term pressing of which it is possible to switch loads. The loads cannot be turned on at the same time, even if both buttons are pressed at the same time, both loads are turned off. There is an emergency blocking input, when voltage from the supply voltage up to 50 V is applied to it, both loads are turned off. This input can be used in various protective circuits when you need to urgently turn off any of the switched on loads, and block the possibility of turning them on with the buttons. The loads are connected between the power plus and the corresponding output of the circuit. Switch status is indicated by two LEDs. The circuit diagram is shown in the figure.
The control device is an RS flip-flop on the D1 chip. Pins 2 and 12 are used to switch the stable states of the trigger. These conclusions through the resistors R1 and R3 are pulled up to zero. The resistance of the resistors is taken relatively small (usually resistors of tens or hundreds of kilo-ohms are used in such circuits). In the first version, there were 56 kOhm resistors, but later it turned out that at the moment a powerful load is turned on, an impulse-interference occurs that resets the trigger and puts the circuit into self-oscillating mode. To prevent this from happening, the resistance of the trigger inputs had to be lowered by lowering the resistance of the pull-up resistors, as well as to additionally install capacitors C2 and C3, which increase the stability of the trigger in conditions of impulse noise. Pressing the S2 button leads to the appearance of a logical unit at pin 13. Transistor VT2 opens and turns on load 2. At the same time, pin 1 is zero, so VT1 is turned off and load 1, respectively, is also turned off. When the S1 button is pressed, one appears at pin 1 D1 and the transistor VT1 opens, load 1 turns on, and zero appears at pin 13, so load 2 turns off. Resistors R6 and R7 are needed to reduce the effect of the gate capacitance of the field-effect transistor on the output of the microcircuit. The gate capacitance is quite high, therefore, with a sharp change in voltage across it, a rather large charging current of this capacitance takes place. Resistors limit this current to a safe level for the microcircuit. Diodes VD3 and VD4 help discharge the gate capacitance when the transistor closes. Pins 3 and 11 connected together are used to create a blocking point. These conclusions are pulled up to zero by resistor R2, so as long as there is no voltage at the blocking input (or this voltage is low), they do not affect the operation of the trigger. But when a logic-one level voltage is applied to them, both elements D1.1 and D1.2 are forced to go to a logic zero state at the output. That is, when at a given point a logical unit both loads are turned off regardless of the previous state. The voltage applied to the interlock input may come from some interlock circuit or system. The value of this voltage, preferably, should not be greater than the supply voltage of the circuit. However, the presence of a zener diode VD1 and a resistor R4 allows you to use voltages up to 50 V inclusive for blocking (more is possible, but there is a risk of damage to the zener diode, and after it, the microcircuit). The load supply voltage can be from 5 to 20 V. In this case, the supply voltage of the microcircuit should not exceed 15 V. To reduce the maximum supply voltage D1, the R5-VD2 circuit is installed. This circuit, when powered from a source of more than 15 V, works as a parametric stabilizer and does not allow the voltage on the microcircuit to be exceeded. When powered with voltages below 15 V, the circuit does not work as a stabilizer, since the zener diode is closed, but only together with C1 as a blocking RC circuit along the power circuit. It is impossible to reduce the voltage below 5 V, since in this case the voltage at the gate of an open transistor will be insufficient for its full opening. The transistor channel will not open completely, that is, it will have a higher resistance, and this will lead to the fact that the power dissipated on it will increase sharply, which can damage the transistor. During installation, it is necessary to ensure a sufficient width of the tracks going to the drain and source of the transistors from the load and from the power minus. Mounting conductors must also be thick enough. The conductors of the control circuit on D1 can be thin, that is, any reasonable thickness, since there is little current. APM2556NU transistors can be replaced by others with similar characteristics. If it is not possible to find transistors with such a low open channel resistance, but there are transistors with twice the resistance, you can use two transistors connected in parallel instead of one. Either run at a lower maximum current or use a heatsink to dissipate excess heat. Zener diodes BZV55C15 can be replaced with 1N4744A, KS215, KS515, D814D. In principle, any zener diodes for a voltage of at least 10 V and not more than 15 V can be used here. The K561LE6 chip can be replaced with an analog of the CD4002 or a K561LE10 chip (analogue of the CD4025). The K561LE10 chip is different in that it has three three-input NOR elements. Two are used in this scheme, and one extra remains free. So that it is not damaged by static electricity, the inputs of a free element must be connected to the 7th or 14th output of the microcircuit. All elements of the microcircuit are physically interconnected, therefore, even damage to an unnecessary element can adversely affect other elements of the microcircuit. You can also use the K561LP4 chip, it has two three-input OR-NOT elements and one single-input inverter, it remains free (connect its input to pin 7 or 14). Diodes 1N4148 can be replaced by almost any low-power pulse diodes, such as KD522. The FNR05K220 varistor can be replaced by any varistor with a voltage of about 20 V. LEDs - any indicator. A device assembled without errors does not require adjustment if all parts are in good condition. Author: Lyzhin R.
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