On Delay Timers
On delay timers can have many uses, but most of them involve making one event follow another after a certain amount of time. These types of timing functions are a staple in most ladder diagrams.
These timer relays are commonly used in a variety of applications, including cooling fans and pumps. Depending on the application, they can be retentive or nonretentive.
Preset Time
The preset time setting on a delay timer is used to set the amount of time that passes between when the timer is activated and when its output becomes active. This time delay is used to ensure that the system is safe before activating an output. The default preset time is one day. You can change this time period as needed.
The Preset Time setting on a delay timer consists of two data table words, the preset value and the accumulated word. These two words are stored as BCD values together with a time base of your choice. The time base may be a real number (like 1.0 or 0.01 for long delays) or it can be an S7-type value based on seconds, minutes or hours.
To change the time delay that you want, you need to open the sequence where the delay is active and double-click on it. If you edit the preset time setting, it will apply to all of the sequence pieces in that sequence.
On delay timers are widely used in industrial automation processes, such as conveyor belt systems and robotic assembly lines. They also play a key role in traffic signal control, as they introduce timed delays to ensure safe transitions between green, yellow and red phases. Their ability to delay the activation of an output makes them valuable tools for adding precision and control to complex systems.
Accumulated Time
The accumulated time setting on a delay timer is what determines when the hot swap controller output will go active. When the timer coil is energized it begins to count down until it hits the preset time. When the preset time is reached the accumulated time is reset and the done bit, word zero, is set. The timer is then said to be nonretentive, as the loss of power flow to the accumulator coil causes it to restart.
This timer type is used for timing things such as conveyor start buttons. You can also use this timer to turn on a warning light when a machine is being serviced.
This is a simple timer. When the rung that contains the TON instruction becomes true it starts to count down, and the output it controls is activated when the accumulated time reaches the preset value. The timer keeps track of the accumulated intervals which have occurred and its done bit (DN) is reset when a rung condition goes true again. It can be replaced with a TOF, or RTO retentive timer to add features such as the ability to keep the timer’s DN bit active after the accumulated time equals the pre-set value. This is a common feature in many machine control applications. This timer also adds a bit to the data table that allows you to set how long it will take for the accumulated time to reach its preset value.
Timed Contacts
A delay timer uses a set of contacts to implement its control and timing functions. These contacts are normally open or closed when the timer coil is energized and change to the opposite state when it loses power. The accumulated count and preset time are reset upon removing power from the coil.
A commonly used type of on delay timer is an interval-on timed relay which has a built-in sequence of on and off cycling times. These are ideal for use with circuits such as burglar alarm systems, electric welding equipment and car seat belt warning lights.
The timers of off-delay relays and solid-state timers are not quite as simple to identify in a ladder diagram. Unlike on-delay timers which have instantaneous contacts, the off-delay contact’s original state is led driver ic not changed when the coil is energized. However, when the coil is de-energized the accumulated time begins to clock down.
To differentiate between an on and off delay timer a circuit designer can refer to the wiring symbol in the ladder diagram. On-delay timers are characterized by a coil that is marked the same way all loads are identified in ladder diagrams with the abbreviation TD to indicate time delay. The timer contacts look like a single pole switch and have an arrow pointing down from the switch. Alternatively, these contacts can also be called normally open timed to close or normally closed timed to open depending on the direction in which the contacts change from their initial state.
Normally Closed Contacts
Once the timer coil is energized the preset time begins to count down. This count is known as the accumulated time. When the accumulated time equals the pre-set time the contact setting changes; contacts that were normally open when the coil was not energized will change to closed and those that were already closed will remain closed for the same amount of time that the coil remains energized. When the accumulated time reaches zero the contacts return to their original state.
This type of timer is often used in circuits that require a delay between the pushbutton control switch and activating industrial devices such as conveyor belt systems or furnace HVAC systems. The delay can range from as low as a few milliseconds up to hours and even days.
Off delay timers are also very popular because they can be set to either an on or off state and offer a wide variety of contact arrangements including normally open, timed to open (NOTO) or normally closed, timed to close (NCTC). They start accumulating time immediately when the coil is energized and change their states instantly when the coil de-energizes.
Ladder diagrams are often used to represent these types of timers and their associated circuits. The timer coil is drawn the same way all loads are represented in ladder diagrams except with an abbreviation of TD to indicate a timing device. The contacts are drawn like a single pole switch and the symbols for the on-delay timed contacts are shown with an arrow pointing down from them.
