Taking on the challenge of finding robust alternatives to failing electronic components often requires some creative thinking and a little customisation.
It is a scenario which is all too common in the rail industry – a suitable component is chosen, it is equipped with additional protection to help it to cope with the harsh operating conditions, and then the environment throws up another challenge which causes it to fail.
The simple fact is that rail environment is tough on electronics. So many of the standard devices used in other industries can fail when confronted by the daily operational realities of vibration, dirt, contamination or high-voltage transients.
The reliability challenge
Take the example of a power supply that was used to drive an on-board Train Protection and Warning System (TPWS). Although the original unit provided a good fit for the system’s electrical and mechanical specifications, concerns over its operational reliability soon began to surface and a more reliable alternative was needed urgently.
The original approach was to use three different variants of an open-frame power supply to cover input voltages of 24V, 72V and 100V. To provide protection against vibration, water, dust and other potential contaminants, the open-frame supply was enclosed in a metal case with wires connecting the printed circuit board to the outside world.
With reliability at the core of the challenge, the starting point for the replacement unit was a 150W PCMDS150WK-IP65 DC-DC converter manufactured by MTM Power. The standard version of this supply features patented thermoselective vacuum encapsulation and an IP65 rating for maximum protection against external contaminants.
The thermoselective vacuum process completely and permanently encapsulates the power supply, creating a cemented joint that provides an inseparable link between the potting material and the components. This ensures that ageing, heat, cold, rapid temperature changes and other environmental influences do not result in delamination, cracking or air pockets, which can compromise reliability and potentially lead to failure.
The combination of the patented encapsulation process and the IP65 rating eliminated the hand assembly that was required to protect the original open-frame unit. This resulted in a 20 per cent reduction in the overall unit cost of each supply.
In addition to successfully overcoming the main reliability challenge, the replacement was also able to deliver additional benefits. The standard unit’s nominal input ranges of 24V (16.8V to 33.6V) and 110V (50.4 to 154V) were sufficient to cover the three input voltages required by the system. This allowed the OEM to replace the three original versions of the open-frame supply with just two PCMDS150 variants. A secondary circuit, with fully independent isolation and regulation, was also added to the supply to provide a 40V rail for use as a reset, operating independently of the 12V board net.
To provide assured reliability for operation in the rail environment the new supply was fully tested for compliance to EN50155 and EN50121-3-2 by the manufacturer. A built-in heatsink further improved reliability by eliminating the need for external cooling fans – often a cause of failure in power supplies.
A quick fix for transients
The high voltages present in rail systems can cause serious damage to sensitive electronics, as another OEM discovered when the door controls on rolling stock started to fail. The source of the problem was identified as transients on the 110V DC input supply and a quick fix was essential. Ideally, the fix also had to avoid the high cost and time-consuming process of replacing the system on every single door within the fleet.
The proposed solution was for Charcroft to supply an active transient filter, from MTM Power, that could be retro-fitted to the input supply in front of the door-control modules. When a transient voltage is detected, the filter automatically clamps the input supply and buffers the output supply to continue to provide power to the doors, enabling them to operate normally.
Terminal problems
Fixings and terminations can be another potential point of failure for power supplies in rail applications. Conventional pin-style connections on the input and output of power supplies can easily become corroded, leading to failures in the field and unplanned downtime.
For one OEM, the solution was to switch to a drop-in replacement with modified screw terminals. These terminals not only provide greater protection against corrosion but also ensure a more secure and reliable connection.
Creative customisation
Whilst drop-in alternatives are the best option for many projects, it is not always possible to find a direct replacement. For one upgrade project, the only option for replacing a legacy circuit breaker with non-standard fixings appeared to be significant and costly alterations to the metalwork of the distribution panels.
Cooperation between Charcroft’s e-mech specialist Jeff Gurr, circuit-breaker manufacturer Sensata, and a third-party fixings company, provided adaptor plates for the new breakers. These allowed the replacement breakers to be fitted directly into the existing footprint of the legacy component and eliminated the time and cost which would have been incurred to re-configure the metalwork.
Although vital in helping to provide solutions to failing electronic systems, creative customisation can also deliver commercial benefits to rail operators. Take component labelling for example. The use of custom vinyl labels with QR codes, applied to a power supply unit before despatch, can help to minimise maintenance time. Each QR code label contains information such as manufacturing date, customer part number, input voltage range, output voltage and current as well as a unique serial number for each converter. This provides maintenance crews at the depots with all the information they may need at the click of an app in addition to providing complete component traceability.
Avoiding customisation
Of course, customisation can be avoided if the products themselves offer sufficient flexibility. A high degree of configurability certainly helped with the refurbishment of an HVAC system on a legacy carriage. The purchasing department had already spent considerable time searching for spares inventory only to discover that the manufacturer had withdrawn the thermostats some years earlier.
The buyer called in his engineering colleague who liaised with Charcroft to configure a standard Klixon 1NT thermostat from Sensata to match the electrical, mechanical and thermal parameters of the legacy unit.
Selecting from a broad range of terminal options, the decision was made to use a quarter-inch quick-connect mounting which offered either horizontal or vertical terminals. By specifying the vertical terminals, the thermostat was able to meet the very tight space-constraints within the HVAC duct. This configuration also allowed the existing electrical connections to reach the terminals of the new thermostat.
Because this was a high-current application, silver contacts were specified rather than gold-plated contacts, which would typically be specified for switching lower voltages and currents. As the thermostat was being used as a safety device, the operation of the contacts was specified to open on rise and close on fall.
Finally, the thermostat was fixed inside the duct using a standard Sensata airflow mounting bracket. Unlike a flat mounting bracket, the airflow bracket allowed the thermostat to measure the temperature of the air within the duct rather than the temperature of the duct material. This solution also provided an added benefit for the buyer and the maintenance teams as it ensured that the thermostats would be available on a short lead-time.
Whilst there are rail applications which can successfully use conventional off-the-shelf components, the potential impact of failures in both cost and downtime is considerable. The use of robust, rail-approved components, with the addition of flexible configuration options or creative customisation, can help electronics engineers to find a more reliable and often more cost-effective approach to rail electronics.
This article was written by Chris Lee, a power product specialist at Charcroft.
