Time relays are designed to implement a given sequence of turning on and off various devices, circuit elements, and signaling. With the help of temporary control devices, the specified switching and control delays are formed. Most of the designs of time control devices provide for adjusting the duration of the on or off interval. Depending on the design of the time relay, the adjustment can be carried out mechanically, electronically or by software.
Content
The principle of operation of the time relay
The general principle of the time relay operation is to form a time delay for turning on, off or switching the control groups of contacts. The implementation of the delay depends on the design features of the device. Common differences in relays of different types is the switching of the executive part. On this basis, two groups of relay devices are distinguished:
- with turn-off delay;
- with start-up delay.
Many relays allow you to change the type of switching or have both options.
The principle of timing and contact control depends on the design of the relay, but the general algorithm of operation is as follows:
- at startup, a contact group is activated, organized in accordance with the type of switching (for time relays with off-delay, the contacts close);
- at the same time, the time delay mechanism is cocked (starts the clock generator in electronic devices);
- after the specified interval, the contact group changes its state to the opposite.
The three-position relay has a more complex operation algorithm. The sequence of work is as follows:
- The circuit is open.
- Start. The circuit closes, the countdown begins.
- The countdown is over. The circuit is closed.
In cyclic devices, the listed sequence is repeated many times.
The countdown is started manually or automatically by direct closing of the power supply contacts or through an electromagnet acting on the mechanism.
The time relay with on-delay works in a similar way.
Types and classification
The following types of counting time intervals are used, according to which the classification of time-setting devices is made:
- pneumatic;
- motor;
- electromagnetic;
- sentries (anchor);
- electronic.
The next difference lies in the value of the supply voltage of the control electromagnet, which carries out the initial platoon of the actuator or mechanism and the electromagnet that controls the switching of the output terminals. The most widespread are these types of time relays according to voltage:
- 12V DC voltage;
- 24 V DC;
- 220 volts AC.
380V time relays are used in three-phase networks with delta connection.
The operating voltage differs from the switching voltage, which depends on the design and power of the contact groups. The operating voltage is necessary for the operation of the device and must be within strictly defined limits. There is no minimum switching voltage limit. If the permissible values are exceeded, a breakdown of the gap between the contacts is possible.
The same requirements apply to the switching current, the excess of which is more than the permissible value is fraught with burning and sintering of the contact groups, the occurrence of an electric arc at the moment of opening.
The value of the operating voltage is dictated by safety requirements. This takes into account the fact that the greater the power of the control electromagnet, the stronger the current consumed by it. The most widely used time relay is 24 volts, since in this case there is the most advantageous combination of voltage and current consumption of the relay.
In cars, time relays with a supply voltage of 12 V are used, since this is the most common value for the car's on-board network. For example, a time relay for controlling windshield wipers and direction indicators. The contact groups of these devices are highly reliable, have a large current margin to prevent burning, since road safety depends on proper operation.
All listed types allow the release of multi-channel time relays. In this case, circuit switching is carried out by several independent groups of contacts.In simple constructions, groups are triggered simultaneously, in complex ones, depending on the programmed algorithm.
Electronic devices provide a wide variety in the number of groups and the algorithm of operation. Circuits developed using microcontrollers have small dimensions, which are limited only by the type and size of the actuating elements that switch the load.
The reliability of the operation of devices and mechanisms depends on the compliance of the design with the requirements. The choice of a time relay consists in the selection of a type that meets all the requirements, including:
- operating voltage;
- switching voltage and current;
- duration of time intervals;
- shutter speed setting accuracy;
- work on or off;
- on/off adjustment.
Cyclic timers
This type of time relay automatically and continuously generates the specified time intervals. If you ask the question of why cyclic type relays are needed, then we can say that they are most widely used in automatic lighting control systems (outdoors, in livestock farms, in aquariums).
electromagnetic
Electromagnetic devices are also called time relays with electromagnetic delay. They have a simple design and are used in relay automation devices. The electromagnet winding additionally contains a short-circuited coil in the form of a copper cylinder, which prevents the rapid rise and fall of the magnetic flux, as a result of which the armature of the moving system moves with deceleration.The delay time for operation is from 0.07 to 0.11 seconds, and for release from 0.5 to 1.4 seconds. Flaws:
- the impossibility of correcting the delay time;
- work only on direct current.
Pneumatic
The retarding device in this design is a pneumatic damper, the air into which enters through a calibrated hole. Its flow area is regulated by a needle with a special screw.
Pros: Doesn't require power
Flaws:
- low accuracy of time setting (over 10%);
- sensitivity to air pollution.
Motor
It is a synchronous motor that transmits rotation to a shaft with contact groups through a gearbox. It may include an electromagnetic clutch that disengages the motor shaft and gearbox. The holding time ranges from a few seconds to tens of hours.
Flaws:
- low accuracy of time delay;
- performance only in a narrow temperature range;
- the need for regular cleaning and lubrication of the mechanism.
Clockwork or escapement
Arranged on the principle of a mechanical watch. In industry, a current winding is used to charge a spring. Thus, the higher the current in the winding, the stronger the spring is compressed and the faster the movement of the mechanism. Differ in low accuracy of installation of time. Setting a mechanical relay is similar to setting an alarm clock.
Electronic
The most common class of devices. Made on electronic components. As a time-setting element, a clock frequency generator or synchronization from the mains frequency is used.
Differ in the widest limits of frequency tuning.The minimum interval is units of microseconds, and the maximum interval is days, months, and years. The interval adjustment is carried out electronically (with switches) or software (by changing the coefficients of the built-in program or via an interface from external equipment).
An hourly, daily or weekly relay is often an option in electronic watches.
Electronic timing relays provide the widest possibilities for building control circuits, including multi-channel versions or cyclic operation.
As the executive part, semiconductor switches or electromagnets with different groups of contacts are used to switch the relay load.
Advantages of electronic devices:
- the widest range of exposure settings;
- minimum dimensions and weight;
- high reliability;
- the highest accuracy of setting time intervals.
Exposure accuracy depends only on the frequency stability of the master oscillator. The use of generators based on quartz elements with thermal stabilization makes it possible to achieve accuracy in thousandths of a percent.
Flaws: The need for an external power supply for the operation of the electronic components of the circuit.
Time relay circuits have a wide variety. Among them, there are both simple and complex ones based on microcontrollers.
Application area
Time delay relays are used in areas where it is necessary to strictly observe the intervals between turning on and off the equipment, to give signals at specified intervals.
The need to use one or another type of device is dictated by local conditions and requirements for their parameters.
Electronic devices can replace all of the above, provided that external power is available.
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