Seal In Circuit in Ladder Logic: What It Is and How It Works

A seal in circuit in ladder logic is a method to keep an output coil energized even after the input that started it turns off. It uses a contact from the output coil itself wired in parallel with the start input, creating a 'seal' that holds the circuit closed until a stop input breaks it.

How It Works

Imagine you want a machine to keep running after you press a start button, even if you release that button. A seal in circuit does exactly that in ladder logic. It uses the output coil's own contact wired in parallel with the start button. When you press start, the coil energizes and closes its own contact, which keeps the coil energized even after the start button is released.

This is like turning on a light switch that stays on by itself until you press a stop button. The stop button is wired in series to break the circuit and turn off the coil. This method 'seals' the circuit closed, maintaining the output without continuous pressing of the start input.

Example

This example shows a simple seal in circuit with a start button, stop button, and a motor output coil.

ladder_logic

(* Ladder Logic Pseudocode *)
|---[ Stop ]---+---[ Start ]---+---( Motor )---|
               |               |
               +---[ Motor ]---+

(* Explanation: Stop is normally closed, Start is normally open, Motor coil contact is normally open. Motor coil contact seals the circuit after start is pressed. *)

Output

When the Start button is pressed, the Motor coil energizes and closes its own contact, keeping the Motor running after releasing Start. Pressing Stop breaks the circuit and de-energizes the Motor.

When to Use

Seal in circuits are used when you want to keep a device running without holding down a button. Common examples include starting motors, pumps, or conveyors that should run continuously until stopped. It simplifies control by allowing a momentary start input to maintain a continuous output.

Use seal in circuits when you need a simple latching mechanism in your PLC program without complex memory or timers. It is ideal for manual start/stop control in industrial automation.

Key Points

• A seal in circuit keeps an output energized after the start input is released.
• It uses the output coil's own contact wired in parallel with the start input.
• The stop input breaks the circuit to turn off the output.
• Commonly used for motor and device latching in PLC control.

Key Takeaways

A seal in circuit latches an output coil using its own contact in parallel with the start input.

It allows a momentary start signal to maintain continuous output until a stop signal is given.

Seal in circuits are simple and effective for manual start/stop control of motors and devices.

The stop input is essential to break the seal and safely turn off the output.

This method avoids holding down the start button and simplifies control logic.