What is a Multifunction Time Delay Relay?

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Time delays are often included in industrial control, HVAC, and residential systems. A simple example is a motion detecting light. The light turns on immediately but turns off only after a preset time has passed. A more complex example includes a high-decibel horn that sounds three times before a machine starts.

Today, some of these tasks are performed using a Programmable Logic Controller (PLC). However, for simple operations, discrete time-delay relays such as the one pictured in Figure 1 are used. In fact, these devices are popular as evident by the large number of offerings. At the time of this writing, DigiKey has 5205 line items in the Time Delay Relay category.

In this article we will focus on the Selec 600XU. This small DIN-rail-mounted relay is classified as a multifunction relay. According to the datasheet, this relay may be configured by via the orange selector switches to perform one and only one the listed operations:

  • On delay (A)
  • Interval (B)
  • Cyclic equal OFF first (C)
  • Cyclic equal ON first (Ci)
  • Pulse output, 500ms fixed (D)
  • Delay on break (E)
  • Delay on make / Delay on break (F)
  • Interval after break (H)
  • Single shot (I)
  • Retriggerable single shot (J)
  • Latching relay (K)
  • Delay on make with totalise (Ai)
  • Interval with totalise (Bi)

Tech Tip: The 600XU relay mode and selector switch are read when power is applied across the A1 and A2 “coil” connections. This is a very important non-intuitive consideration as adjustments to a powered-on relay will be ignored. You must turn off relay power for the changes to take effect.

Figure 1: Picture of a Selec 600XU multifunction relay mounted on a DIN rail.

Tech Tip: The term “time delay relay” is an umbrella that covers relays with many unique time related functions. Some relays are purpose built for a single function while other such as the 600XU may be configured for many different operations.

Electrical Connections

The physical connection for the 600XU relay as installed on the trainer is shown in Figure 2. Note that there are two control inputs. The A1 (U input) powers the relay and initiates the time operation for select modes. For example, the on-delay operation is entirely dependent on the U input. In this mode, when power is applied, the relay contacts will activate after the user selected (orange dial) time has passed. On the other hand, the 25 (Y input) is used to control operations that that required the relay to remain powered. For example, the time-off (delay on break) operation requires that the relays logic be active to perform the countdown. Consequently, the time-off operation is based on the Y1 input.

The red and green indicator lamps are connected to the relay’s Single Pole Double Throw (STDT) contacts. The 24 VDC is connected to common, the red lamp to the red to normally closed contact, and green to normally open contact. Note that the 600XU relay is designed for flexibility. It is specified to operate with supply voltage from 20 to 240V AC (50 / 60Hz), and 12 to 240V DC.

Figure 2: Schematic of the test setup for the Selec 600XU multifunction relay.

Tech Tip: The time delay operation may be performed using electrical, mechanical, or even pneumatic methods. My first introduction to time delay relays was a high-power radio transmitter that featured AGASTAT relays as shown in Figure 3. These devices utilized a diaphragm and controlled release of air to perform the time delay. Unlike the 600XU relay featured in this article, the AGASTAT off-delay timer did not require power to keep its timing logic “alive.” Instead, the contacts were held by air pressure. The relay’s contacts were released when the gentle hiss of air was complete. In modern language we would say the AGASTAT featured retentive timing capabilities – it remembered even if power failed whereas the 600XU will reset (forget) upon power failure.

Figure 3: The AGASTAT time delay relay uses air pressure to perform the time delay operation.

Operating Mode Demonstrations

The 600XU relay may be programmed for 1 of 13 operating modes. Before we conclude, we will demonstrate a few of these operating modes. The front of the relay as shown in Figure 1 has three controls:

  • top: course time
  • middle: fine time adjust
  • bottom: mode selector switch


The relay will activate t seconds after power is applied to the A1 (U) input. The mode selector switch is set to A. The desired time is entered into the fine and course controls. Video 1 demonstrates the operations. Recall that the selector switch is connected to the A1 inputs.

The on-delay is often used for sequential operations. An example is starting an industrial heater a few seconds after the blower motor is active.

Video 1: Video showing the Selec 600XU relay configured for a 3 second time-on delay.

Off delay:

The relay will turn off t seconds after the 25 (Y1) input is released. The mode selector switch is set to E. The desired time is entered into the fine and course controls. Video 2 demonstrates the operations. Observe that the selector switch remains in the on position. The relay is responsive to the 25 (Y1) input which is driven by the green pushbutton.

Continuing with the industrial heater application, we may want to keep the blower active as a cooldown for the heater.

Video 2: Video showing the Selec 600XU relay configured for a 3 second time-off delay.

Pulse Output:

In this mode the relay will produce a 500 ms pulse t seconds after it is powered on by the AI (U) input. This pulse may be used to activate a latch. For example, 5 seconds after the blower is active and working to capacity, we may latch the control circuitry that monitors the blower’s performance using a sail or pressure switch.

Video 3: Video showing the Selec 600XU relay configured to produce a 500 ms pulse t seconds after it is powered on.

Interval After Break:

This time we consider the interval after break mode. This is like the delay-on-break (off-delay) however, the relay activates when the 25 (Y1) input is released. This is different than the delay-on-break (off-delay) which activates immediately upon 25 (Y1) activation. The operation is shown in Video 4.

Continuing our example where we monitor a blower motors airflow, the normally open contacts of the 600XU relay mode could release the latch that was previously monitoring the airflow.

Video 4: Video showing the operation of the Selec 600XU in the interval after break mode.


To keep this article short, we will stop here. It is up to you to build the circuit and explore the operation of the remain 9 operating modes.

Parting Thoughts

Traditional time delay relays are still common in industrial, HVAC, and even some residential equipment. There is certainly a move to replace this functionality with embedded microcontrollers and PLCs. However, even if production of these devices stopped today, legacy equipment would continue to the discrete time delay relays.

You are encouraged to explore the operation of relays such as the Selec 600XU as featured in this article. From an educational standpoint, understanding how the relays work will help you improve you PLC programming abilities.

Best Wishes,


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