Security & Access System Circuit Protection

Using transient voltage suppressing (TVS) Diodes or metal-oxide varistors (MOVs) in protecting alarm and access control circuits

Circuit protection is largely ignored and has greatly contributed to failing field components.   When diagnosing failing hardware you will often find missing TVS protection, a lack of surge protection, or faulty or insufficient grounding.

 

“Transients are temporary spikes or surges in voltage or current that can potentially impact circuits in ways ranging from minor glitches to catastrophic failure. A voltage transient can be anywhere from a few millivolts to thousands of volts, and they can last from nanoseconds to hundreds of milliseconds. Some transients are repetitive, such as those caused by inductive ringing in a motor, while other transients are more sporadic, such as ESD events.”

[Reference: All About Circuits – An Introduction to Transient Voltage Suppressors (TVS) ]

 

In the realm of security and access control systems the biggest cause of transients are electric door strikes which utilize either an electric motor or a solenoid coil for operation or Maglocks which contain large coils.  Transients are often generated by inrush currents which are needed to active motors and solenoids.

 

Most access control panels come with either TVS diodes or MOVs but many techs ignore and often discard them because they lack the understanding of why they are important and or how to properly install them.

 

Installation Standards

 

1. Always use the recommended TVS devices provided by the manufacturer and install exactly as described in the manufacturer’s instructions

  •  If a TVS device is not provided or specified by the manufacturer, use one of the recommended TVS Diodes or MOVs listed below
  • Follow the installation standards and best practices here in the event instructions are not provided

 

2. Install the device nearest to the strike as possible.  Often this is at the harness junction in the door jamb or mullion.

  • Diodes only work with DC devices.  If the diode is directional, install the diode with the stripe side toward the positive terminal.  If the diode is bi-directional, polarity does not matter.
  • Direction does not matter with MOVs.  MOVs should be used exclusively when powering A/C devices

 

3. For best practices, solder the connections between the wires and Diode/MOV.

 

4. Use separate power supplies for locking devices and panels.  Inrush surges and wide voltage variations can cause problems for the access/security panel.  Use a dedicated power supply when possible.