As railway systems continue to move towards greater electrification, reliability and reduced maintenance are becoming increasingly important when selecting switching solutions for onboard electrical equipment. To meet these requirements, celduc® relais, a leading manufacturer of Solid State Relays (SSRs), has developed the SMI Series, a new range of compact High-Voltage DC Solid State Relays specifically designed for controlling resistive loads in demanding applications.

Blue celduc SMI high-voltage DC solid state relay with a black built-in heatsink, rated 750 VDC 20 A

A Compact, Self-Protected Solution

The SMI Series is designed to switch loads up to 750 VDC / 15 A, with a typical control input voltage of 24 VDC.

With an installation width of only 45 mm, the SMI combines compact dimensions with advanced protection and diagnostic functions.

One of its key features is its Status output, which provides real-time information about the relay and installation status. This diagnostic capability makes it easier to identify abnormal operating conditions and can contribute to faster troubleshooting and improved system availability.

The SMI Series also integrates several protection functions directly into the relay.

Short-circuit protection reacts when the load current reaches the detection threshold. The power semiconductor is switched off within a few microseconds, cutting the power and extinguishing the current. The Status output is then activated to indicate the fault condition.

An integrated UVLO (Under Voltage Lock Out) function protects the system in the event of insufficient auxiliary or main supply voltage.

The SMI also features active-clamp protection against overvoltages and fast transients. As voltages exceeding the semiconductor’s 1700 V breakdown voltage could otherwise cause damage, the active-clamp function limits the voltage across the relay output terminals. During an overvoltage event, the power element redirects the energy towards the load until it has been absorbed, after which the power element is fully released and the circuit remains open.

Combined with strong input/output insulation, these integrated functions make the SMI particularly well suited to the demanding electrical environment encountered in railway applications.

Designed for High-Voltage Railway Heating Systems

A typical application for the SMI Series is the control of high-voltage heaters supplied directly from the third rail or catenary system.

In such applications, replacing traditional electromechanical contactors with Solid State Relays can provide significant operational benefits. With no moving contacts subject to mechanical wear, SSR technology helps reduce maintenance requirements and increases switching lifetime.

Electronic switching also enables more precise and repetitive switching cycles, helping improve temperature control while increasing the overall reliability of the heating system.

The SMI Series therefore provides railway equipment manufacturers and system integrators with a compact solution combining switching, protection and diagnostics in a single device.

Simple Connection and Reduced Maintenance

The SMI has also been designed to simplify installation and maintenance. Its push-in spring-type power connectors enable fast and secure wiring while eliminating the need for periodic retightening of screw connections.

Key specifications include:

• Compact 45 mm installation width
• Typical input voltage: 24 VDC
• Output voltage: up to 750 VDC
• Switching current: up to 15 A*
• Integrated short-circuit protection
• Integrated UVLO protection
• Active-clamp protection against overvoltages and fast transients
• Real-time Status output
• Push-in spring-type power connections

*A Solid State Relay must be mounted on a suitable heatsink to achieve its rated performance.

For easier integration, celduc® relais also offers a ready-to-use SMI version with a built-in heatsink, providing an optimized thermal solution in a compact assembly.

The new SMI Series complements celduc® relais’ existing portfolio of High-Voltage DC Solid State Relays, which already includes the SDI Series of IGBT-based DC Solid State Relays.

With its combination of compactness, integrated protection, diagnostics and maintenance-free semiconductor switching, the SMI Series provides a new solution for reliable control of high-voltage DC resistive loads in railway applications.

This article was originally published by celduc® relais.

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