Timer relays are a variation of the standard instantaneous control relay in which a fixed or adjustable time delay occurs after a change in the control signal before the switching action occurs.
Timers allow a multitude of operations in a control circuit to be automatically started and stopped at different time intervals. The use of timers can eliminate the labor-intensive process of trying to manually control each step of a process. Timer functions include timing a cycle of operation, delaying the starting or stopping of an operation, and controlling time intervals within an operation.
Timer Relay Working Principal
There are different types of timer relays, out of them the solid state timing relays are very popular. Solid-state timing relays use electronic circuitry to provide their timing functions. The two broad categories of solidstate timers are analog and digital.
Different methods are used to control the time-delay period. Some use a resistor/ capacitor ( RC ) charge and discharge circuit to obtain the time base, while others use quartz clocks as the time base. These electronics-based timers are very accurate and can control timing functions ranging from a fraction of a second to hundreds of hours.
In order to maintain their timing operations, solid-state timers are normally constantly powered. Some are equipped with batteries or internal memory to retain their settings during power failures. The timing functions are initiated when the electronic circuit of the timer is energized or a triggering signal is received or removed.
Electronic timers are available in a variety of rated input operating voltages. Figure 1 shows a typical solid-state timing relay. The operation of the device can be summarized as follows.
- Connections provided include timed contacts (C1, C2), voltage input (L1, L2), and external trigger switch (S1, S2).
- A timing-delay period of from 0.1 to 2 seconds is set by the adjustment of an internal potentiometer located on the front panel of the timer.
- The timer is energized continuously, and timing is initiated when the external trigger circuit is closed.
There are four basic timing functions: on-delay, off-delay, one-shot, and recycle.
On-Delay Timer: The on-delay timer is sometimes referred to as DOE, which stands for delay on energize. The time delay of the contacts begins once the timer is switched on; hence the term on-delay timing. Figure 2 shows the NEMA symbols for the on-delay timer normally open (NO) and normally closed (NC) contacts. The operation of the timed contacts can be summarized as follows.
- Once initiated, DOE timed contacts change state after a set time period has passed.
- After that time has passed, all normally open timed contacts close and all normally closed contacts open.
- Once the timed contacts change state, they will remain in this position until the power is removed from the coil or electronic circuit.
The circuit shown in Figure 3 illustrates the timing function of an on-delay timing relay. In this example a timer with a time delay setting of 10 seconds can be assumed. The operation of the circuit can be summarized as follows.
- When the switch is closed, power is applied to the coil but the contacts are delayed from changing position.
- With the switch still closed, after the 10-second timing period the normally open contacts (TR1-1) close to energize load 1 and the normally closed contacts (TR1-2) open to deenergize load 2.
- If the switch is then opened, the coil deenergizes immediately, returning both timed contacts to their normal state, switching load 1 on and load 2 off.
Off-Delay Timer: The off-delay timer is sometimes referred to as DODE, which stands for delay on deenergize. The operation of the off-delay timer is the exact opposite of that of the on-delay timer.
When power is applied to the coil or electronic circuit, the timed contacts will change state immediately. When power is removed, however, there is a time delay before the timed contacts change to their normal deenergized positions.
Figure 4 shows the standard NEMA symbols and illustrates the timing function of an off-delay timing relay.
One-Shot Timer: With a one-shot timer, momentary or continuous closure of the initiate circuit results in a single timed pulse being delivered to the output. The one-shot causes this action to happen only once, and then must be reinitiated if it is to continue to operate.
Recycle Timer: The contacts of a recycle timer alternate between the on and off states when the timer is initiated. Solid-state circuits within the device drive an internal electromagnetic relay. The operation of the recycle timers shown in Figure 5 can be summarized as follows.
- Upon application of input voltage, the first delay period (TD1) begins and the output remains deenergized, or off.
- At the end of the first delay, or off period, the relay coil will energize and the second delay (TD2), or on period, begins.
- When the second delay period ends, the relay deenergizes.
- This recycling sequence will continue until input voltage is removed. In some recycling timers the on time may be configured for the first delay. Removing input voltage resets the output and time delays, and returns the sequence to the first delay.
Recycle timers are available in two configurations:
- symmetrical and
- asymmetrical timers.
- In symmetrical timing, the on and off periods are equal. The length of the timing period is adjustable but the time between the on and off operations remains constant. Flashers are an example of symmetrical timing.
- Asymmetrical timers allow independent adjustments for the on and off periods. They come equipped with individual on and off time adjustment knobs and use standard NO and NC contact symbols referenced to the timer that controls them.
Multifunction and PLC Timers
Multifunction Timer: The term multifunction timer refers to timers that perform more than one timing function. Multifunction timers are more versatile in that they can perform many different timing functions and therefore are more common. Figure 6 shows a multifunction digital timer that is capable of performing all of the basic timing functions.
PLC Timers: Programmable logic controllers (PLCs) can be programmed to operate like conventional timing relays.
The PLC timer instruction can be used to activate or deactivate a device after a preset interval of time. One advantage of the PLC timer is that its timer accuracy and repeatability are extremely high. The most common types of PLC timer instructions are the on-delay timer (TON), off-delay timer (TOF), and retentive timer on (RTO).
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