Modern household equipment (washing machines or microwave ovens, for example) is necessarily equipped with built-in time relays. In addition, the devices are installed in single-line power supply circuits and perform the function of controlling the operation of the load, turning it on or off at the right time. To make a time relay at home with your own hands, you need to study the design features and the principle of their operation.
Principle of operation and areas of application
The simplest example that allows you to understand the principle of operation of a relay is a mechanical or electronic alarm clock set for a certain time. To obtain a full-fledged timer, an executive device is added to it that performs the desired function - supplying power to a chandelier or a fan, for example. The order of operation of such a relay:
- As soon as the time interval set on the timer (clock) has expired, the control signal is sent to the relay coil.
- Immediately after this, its working contacts open or close the supply circuit.
- As a result, the device connected to it is turned off or turned on.
In real devices, a similar mode of operation is implemented taking into account a predetermined time delay.
Timers of various types are widely used to control the operation of industrial installations, as well as when turning on and off household appliances. The following are commonly used as switched home loads:
- lighting devices of any class;
- various samples of climatic equipment;
- ventilation systems and similar devices.
The use of time-controlled household appliances allows you to reduce the cost of electricity bills.
Before you make a timer for turning on and off electrical appliances with your own hands, you will need to familiarize yourself with the varieties of these devices.
Relay types
According to the type of output element used in the switching circuit, known samples of time relays are divided into the following types:
- relay systems, the main switching link of which is the working contact;
- transistor switches on key semiconductor elements;
- triac or thyristor switches.
The first of the options is not very suitable for independent manufacture, since its scheme is relatively complex - it contains too many elements.
It is recommended to choose a circuit solution based on thyristors when the connected load is insensitive to the shape of the supply voltage.
In homemade products, it is unreasonable to use modern microcontrollers, which significantly complicate the process of setting up and adjusting the executive part of the circuit. More preferable are transistor relays, which are easy to assemble and pre-debug.
Practical schemes
All the proposed options for homemade relays contain inexpensive items that are freely sold at any radio store. Their scheme works according to the simplest algorithm, according to which the timer built into it is first started, and at the end of the countdown, the executive unit is triggered. As a result, the supply voltage is applied to or removed from the load connected to the relay.
Transistor timing device
The electrical circuit of the time relay on several transistors is the easiest to implement, since it contains only 8 active elements. On its basis, it is possible to assemble devices that control the time the lighting is turned off, for example. To power such a circuit, you will need a 9 Volt battery or a 12 Volt car battery.
To make a homemade relay, you need the following set of parts:
- two fixed resistors and one variable potentiometer - their values are selected for a specific circuit;
- dual transistor KT937A or foreign analog;
- load switching relay;
- constant time-setting capacitor of the required capacity;
- diode under the designation KD105B;
- button to start the relay.
The time delay in a homemade device is organized by charging a constant capacitor to the power level of the transistor key element. During this entire process, until the voltage reaches 9-12 Volts, the output switch remains open, and the lamp connected to it glows at full incandescence. After a period of time set by the current value of the variable resistor, the transistor is completely closed. As a result, the winding in its collector is de-energized, and the load is disconnected from the supply circuit.
Time parameters in seconds or minutes for a relay assembled according to a transistor circuit are selected experimentally - by changing the resistance of an adjustable resistor. For the convenience of the subsequent setting of the moment of switching on or off, it is recommended to apply indicators on the relay housing, which indicate the experimentally obtained time settings.
Chip Relay
The circuit of an electronic timer, assembled by hand on the basis of a microcircuit, allows you to get rid of such disadvantages of a transistor analog as the complexity of calculating the delay time. In addition, in a transistor circuit, each time before the next start, it is necessary to discharge the timing capacitor. The use of microcircuits, on the one hand, neutralizes these shortcomings, and on the other hand, complicates the device somewhat. When choosing a microchip suitable for a time relay, proceed from the following considerations:
- if you need a delay in the range from ten minutes to one hour, the TL431 series chip is best suited;
- if it is necessary to work with a wider range - with a time delay from 1 second to several hours - it is most convenient to make a timer on the classic NE555 series;
- approximately the same time parameters can be obtained using the KR512PS10 microcircuit.
Due to the presence of a reference voltage source, their operation threshold is strictly fixed, which allows you to precisely set the required delay time.
The ability to increase the switching voltage allows the time delay range to be increased upward. Due to the built-in unit for resetting the charging circuits, there is no need to make a forced reset. The advantages of this version of a home-made time relay include a reduction in false alarms, which is explained by more "stringent" current modes.
Relay circuits based on microcontrollers have become very popular. However, for self-copying, they are not entirely convenient. When using them, certain difficulties arise associated with the soldering of microchips and their programming. The proposed variants of timing devices are usually sufficient for home repetition.
Device for switching equipment 220 Volts
To control the operation of power equipment (electric motors, for example), conventional 12 volt circuits are not suitable. In this case, a magnetic starter is required, switching voltages of 220 or 380 Volts (depending on the number of phases). To supply the control voltage to its coil, you will also need a powerful 220 volt driver.
The simplest circuit of such a relay, used only for lighting control, does not need powerful contactors and can be assembled from a limited number of parts. This requires a rectifier bridge of 4 valve diodes and a semiconductor thyristor controlled by an alternating voltage level. When operating as a regulator, the valve passes only the positive part of the 220 volt sinusoid. This mode is permissible only for circuits loaded on incandescent bulbs, as well as on fan motors or heating elements. They are not sensitive to the shape of the applied voltage, unlike other types of switched electrical equipment.
For self-assembly of such a timer, you will need the following components and parts:
- fixed resistances of 4.3 MΩ and 200 Ohms plus a variable resistor of 1.5 kΩ;
- four diodes designed for currents over 1 Ampere and reverse voltage up to 400 Volts (this can be KD202R, for example);
- timing capacitor 0.47 μF;
- thyristor VT151 or similar element.
The principle of operation of the circuit is similar to the cases already considered earlier and is reduced to charging the timing capacitor with the supply voltage.
During the charging cycle, a positive potential is applied to the control electrode of the thyristor, which keeps it open. As a result, half-waves of the mains voltage are supplied to the light bulb connected to the circuit. At the end of the charging process, the current in the chain stops, the thyristor closes, and the light goes out. The time delay is adjusted by setting the voltage value on the charging capacitor using a variable resistor.
Multifunctional devices
Multifunctional time relays produced by the domestic industry have an extended range of options, which means the following capabilities:
- The products are able to work according to the schedule set by the program for a year, month or week.
- They can serve from one to 4 switched channels.
- Equipped with six input control modules.
It makes no sense to copy multifunctional devices on your own. They should be considered as samples for an in-depth study of the capabilities of modern models of time relays.
