Delaying an Output On and Off

Introduction to Delaying an Output On and Off

Today, we’ll discuss delaying an output on and off. In this case, I have a heater in the office, and a blower (fan). When the heater energizes, the blower (fan) will start after 2 minutes. This allows the heater time to warm up. If the fan starts too early, we will simply be circulating cold air. Furthermore, when the heater shuts off, the blower will run for an additional 10 minutes.

Whether you are using the SLC, PLC, or ControlLogix, the logic is very similar. In this case, I’ll use the SLC processor. Remember, this is for example only. Always take all safety considerations into account. Your process may work differently.

“ON” Transition when Delaying an Output On and Off

At this point, let’s take a look at the following logic. When the thermostat calls for heat, the heater’s relay will energize. At the same time, T4:1 (TON) begins to time.

At this time, we’ll look at the run which controls the blower. Notice that T4:1 is still timing, and will time for 120 seconds. As long as the timer is timing, our rung is false, and the blower relay does not energize.

Looking at the same rung, after 120 seconds, the thermostat is still calling for heat. However, the TON instruction has timed out, making the bottom branch true. The blower relay energizes.

“OFF” Transition when Delaying an Output On and Off

In the meantime, I’ll turn down the thermostat. As you see, we are no longer calling for heat. The TOF begins to time.

The DN bit of the TOF is still high, and obviously, the TON is not timing, so the fan’s output is still on.

After the TOF has timed out, it’s DN bit goes to 0. This shuts off the blower relay.

The next time the thermostat calls for heat, T4:1 will begin timing, and the blower’s relay will stay off for 120 seconds… The cycle repeats continuously.

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— Ricky Bryce

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