Low Volt Wiring – Unit 4

Low Volt Wiring – Unit 4


Sometimes our keypads and card readers control Panic or Exit devices with Electric Latch Retraction (ELR). Usually these high current devices have their own power supplies. We don’t try to energize the lock with our system. We just send a Normally Open, two conductor (input) wire to the power supply input terminals *(REX). When the correct card is presented, our C and N/O terminals close. The power supply sees the close across its own input terminals. Power is then sent to the panic bar and the door is unlocked.

*Request To Exit (REX) is an input that energizes the main relay to add or remove power to a device like an electric strike or mag lock.

An Output provides both Positive and Negative power when the control energizes the input terminals. 12 or 24 VDC is then sent out to the connected devices. Example: The siren terminals of a small alarm system are usually wired to an output. Power is sent during an alarm. Some access control systems use outputs to control the lock hardware. Some power supplies use N/O inputs from the access control panel, to control a power output to the lock devices.


Wiring Diagrams


In this scenario, a technician needs to install and wire an electric strike door release. This is a common request. There are many facilities that need to control customer access. A keypad or card reader could be added for employee access.

A wire is run from the electric strike and from the power supply to the button. At the button, the black wires are terminated to each other. The positive (Red) wire from the power supply is terminated to the common. The positive (Red) wire from the electric strike is terminated to N/O button.

As stated earlier, the Common and Normally Open terminals don’t connect naturally. It requires the button to be pushed. Power is now connected as long as the button is depressed. In a nursing home, the button may need to be held for quite a bit while the resident slowly moves toward the door. You might want to consider adding a timer module. See next…


Wiring Diagrams


Once again, Grampa wants to get through the door but it’s going to take him a minute to get there. Adding a timer will allow the door to stay energized for an extended period of time with a single push of a button.

Power is run from the power supply, electric strike and button to the timer relay. At the timer, positive and negative power is terminated. The negative of the electric strike is connected to the same – terminal. The Positive from the strike is terminated to the N/O (Normally Open) position on the relay. At this point, there is no Positive wire terminated to the Common on the relay. We must provide a short jumper from the Positive terminal to the Common. The relay is now ready to energize the electric strike for up to an hour.

Almost Finished! The button is your control. Run a pair of wires to the timer. Terminate your normally open wired button to the Positive and the Trigger. A button push provides the closure the timer needs to begin its cycle. The door remains unlocked for the programmed time and the nurse can get back to the real work.


Wiring Diagrams


This 1 door scenario has 5 different devices needed to install a controlled opening.

1 – Power Supply with Fire Alarm over-ride, 1 – Access Control Panel Relay, 1 – REX Motion Sensor, 1 – Securitron Timed Egress Button and 1 – Magnetic Lock.

The access control system, motion sensor and button are all powered with continuous power. The Negative (Black) of the mag lock is powered by the same power supply.

From our previous discussion, you understand how a normally closed relay works, right? The positive power is supplied to the Common Terminal. The Normally Closed terminal allows power to flow continuously. After leaving the control panel, the mag lock positive feed is terminated to the common on 1 relay in the motion sensor. Power is continuous in and out of the motion. The button is Normally Closed. It also transfers continuous power to the mag lock positive wire.

Now that the magnetic lock is energized, any valid card read, motion sensor movement or button depress will break Positive power and unlock the door.

In this example, the motion breaks the mag lock power directly. Some technicians use the 2nd motion sensor relay to send a signal back to the control panel and have the panel unlock the mag lock. Using both methods is called “Double Break Wiring”.