TSS Protector Applications: Telecom, RS-485 and Security Interfaces

A TSS protector, or thyristor surge suppressor, is designed for communication lines that must survive lightning-induced surges and other transient overvoltages. After its breakover voltage is reached, the device switches into a low-voltage conducting state and diverts surge current. It resets when line current falls below its holding current.

This snapback behavior can produce a lower residual voltage than a conventional TVS diode during a strong surge, making TSS devices useful as secondary protection for sensitive telecom and industrial ports.

1. Telephone and xDSL Line Protection

Customer-premises equipment, modems, gateways and line cards connect to long outdoor copper pairs. A nearby lightning event can couple high surge energy into the cable.

A coordinated protection path commonly uses:

Line entry → GDT or primary protector → current-limiting impedance → TSS protector → transformer or line-interface IC

The primary stage handles high energy, while the TSS limits the voltage closer to the electronics. Select a device whose repetitive off-state voltage remains above ringing, battery-feed and normal signal peaks. Check surge waveforms used by the applicable telecom requirement rather than comparing peak current alone.

2. RS-485 and RS-422 Industrial Networks

RS-485/422 cables may run between buildings, machines or control cabinets. Ground-potential differences, inductive coupling and lightning surges can damage transceivers.

A TSS can be used in a coordinated secondary stage when its breakover, holding current and capacitance are compatible with the interface. Place protection at the connector, ahead of the transceiver. Long field wiring may also require a GDT, series impedance, common-mode protection and a defined chassis-earth discharge path.

TSS is not automatically interchangeable with a low-capacitance TVS array. Verify that normal common-mode voltage cannot trigger the device and that available current will fall below the holding current after the surge.

3. Alarm, Access-Control and Security Lines

Outdoor cameras, alarm loops, access-control readers and intercom systems often combine long communication cables with exposed installation points. TSS protectors can provide secondary surge protection for low-speed signal pairs, control lines and legacy telephone interfaces.

Install the protection where the cable enters the enclosure. Keep the surge return separate from logic ground until the intended bonding point. For powered security ports, protect the power pair and data pair according to their different normal voltages and fault currents.

4. Industrial Controllers and Remote I/O

PLCs, remote I/O modules, building controls and metering equipment may expose serial or sensor wiring to switching noise and outdoor transients. A staged network can use a GDT for high-energy diversion, a resistor or resettable fuse for coordination, and a TSS close to the interface IC.

The series element must limit follow current without causing excessive signal loss. Confirm thermal behavior during repeated surges and verify communication after every required immunity test.

5. Medical and Measurement Communication Ports

Medical equipment and laboratory instruments can contain telephone, serial or service interfaces connected to cables outside the protected PCB area. A TSS may be considered where low residual voltage and low leakage are required, but patient isolation, creepage, clearance and product-specific safety standards still govern the complete design.

How to Select a TSS for an Application

• VDRM: must exceed the maximum normal line voltage.
• Breakover voltage: must be low enough to protect the downstream circuit.
• On-state voltage: determines the residual voltage after triggering.
• Holding current: the circuit must fall below this value so the TSS can reset.
• Surge current and waveform: match the actual telecom or IEC test pulse.
• Capacitance and leakage: verify compatibility with data rate and line bias.

YINT's thyristor surge suppressor portfolio covers communication-line protection applications including xDSL, RS-485/422 and security systems.

Common TSS Application Mistakes

• Using a TSS on a DC rail that can continuously sustain its holding current
• Selecting VDRM below the highest valid signal or ringing voltage
• Omitting primary protection on a high-energy outdoor line
• Sending surge current through the digital ground plane
• Assuming a peak-current rating applies to every waveform

Contact YINT Electronics

YINT Electronics supplies TSS, TVS, GDT and coordinated surge-protection components. For help with telecom, industrial or security interface protection, contact global@yint.com.cn.