Time relays, also called timers, depending on their construction, principle of operation and installation, are divided into analog and electronic devices, which are used to switch electrical circuits and consumers, according to a pre-selected timing diagram and a time interval. According to their maximum time range, there are 24 – hour timers, weekly timers, time relays with switching delay (ON-delay or OFF-delay), cyclic time relays, multifunction time relays and various combinations of all of the above.
Analog timers also include electromechanical and mechanical relays which do not require electrical power. They are also called clock relays. They consist of a mechanical winder, connected to a contact system and clockwork, which serves to adjust the switch-off time.
Turning the winder to the desired time activates countdown and after the manually set time has expired, the contact system changes its state from ON to OFF or vice versa. It is possible that after the pre-set time is over, an audio signal is activated (kitchen clock). A major disadvantage of this type of timers is that they are not very accurate.
This type of action can be realized by electronic timers as well and it is called normal relay time mode. The mode consists of a preset time, after which the relay is activated and the normally open contacts close and the normally closed ones open. A version of this mode is when the relay has a preset switch-off time, but it begins to run when the relay is power supplied – manually or through some automated process.
When we want to achieve the reverse effect, we use a switch-on delay timer. With this function, the countdown time is set in advance and it starts when there is power supply to the relay. After the time is expired, the output switches (i.e. the normally open contact of the output closes and the normally closed contact opens) and remains in that position until the next pulse, interruption or power restart.
A combination of the two modes described above is a timer relay performing a cyclic mode (trigger mode). The name trigger comes from the shape of the voltage curve, which represents sequence of rectangular pulses with length, corresponding to the time of on and off state of the relay (Figure 1). Timers in this mode perform a sequence of intervals: work and pause. In other words, operation time and pause time are pre-set. After supplying power to the relay, it activates the contact system at its output and switches-on until the expiry of the operation time, then resets (switches-off) until the expiry of the pause time. After this time is over, the relay reacts again and this cycle is repeated until the next pulse, interruption or power restart.
When a relay performs the above described functions (ONN-delay, OFF-delay, trigger mode), as well as additional ones, it is called a multifunctional relay.
The principle of operation of weekly and 24-hour timers, also called time programmers, is slightly different. They are mechanical or electronic timers, which are set in real time. Depending on their memory, we can preset a number of switch times of the electrical circuits. According to their installation, there are panel or contact relays.
Contact timers, except the relay mechanism, have in-built contact and are plugged-in directly into a selected mains socket. The managed consumer is plugged-in after the timer.
Panel timers need wiring according to a defined electrical circuit.
With weekly timers we initially set the exact switch-on and switch-off times of consumers for each day of the week and we can choose different modes for each of them. Then, power is supplied to activate the programming of the relay. The weekly operation mode is repeated every week according to the settings of the previous week.
In 24 hour timers, also called daily timers, depending on our willingness, we set only once the times of switching of the consumers for a day, and the cycle is repeated every day.
There are relays which are used mostly for managing staircase lighting in residential buildings, commercial buildings, etc. Their principle of operation is based on the delay switch-off mode or the already familiar normal mode. When connecting a relay in a specific pattern to consumers and after pulse is supplied (start signal) by a staircase button, the loads connected to the relay are activated and work as long as it is set.