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DCC Auto Reversers

Bob compares Digitrax PM4-2, Lenz LK100, MRC AD520 and Tony’s PSRev (replaced since by PSX-AR by DCC Specialties).

TONY’S TRAIN EXCHANGE PSReverser (replaced by DCC Specialties PSX-AR)

There are two distinctly different module types offered by Tony’s. One is an autoreverser with built-in circuit breaker, called the PSReverse, and the other is a circuit breaker only, called the PowerShield. These modules provide totally silent, reliable operation because they are completely solid state and use no mechanical relays. They are very easy to install. They can be used with most dcc systems right out of the box. Track current passes through completely silent and long-lasting semiconductor junctions rather than mechanical relays which are noisy and will fail eventually as the metallic contacts wear out with age and use.

The PSReverse and PowerShield modules are reasonably priced. Extremely fast, silent, solid-state switching reduces arcs at the insulated track gaps and helps preserve wheel plating. These modules are very flexible and are easily configurable for a variety of circuit breaker current settings and autoreversing switching times, if so desired. The PSReverse module provides both autoreversing and circuit breaker protection within the module itself, the only product to do both. No separate power supply or ground wires are required. Only four wires must be connected for easy installation. They provide on board LED status indication and remote LED mounting, also.

The TTE PSReverse is the best autoreversing module for most applications. When coupled with a PowerShield your entire layout is totally protected against overvoltage, overcurrent and short circuits. The PSReverse and PowerShield modules run away with top honors for simple, effective, flexible, economical autoreversing and short circuit protection for higher current dcc systems. Compatible with ALL brands and models of DCC.


The PM42 is a 4 section, mechanical relay, user configurable, autoreverse and/or circuit breaker module. It requires programming with a Digitrax throttle, considerable pre-planning and moderate soldering skills of many wires. The PM42 is the same as its predecessor, the problematical PM4, except it has a firmware (chip) update to allow the user more necessary control over autoreverse and circuit breaker switching times. The PM42 is a difficult product to integrate into the average dcc layout.

The PM42 circuit board plugs into a 44 pin dual edge connector. The connector’s holes are smaller than the wire sizes Digitrax recommends for installation. This requires the user to fabricate ‘necked down’ connections, going from the bigger gauge wire to short lengths of a smaller gauge wire so they can fit into the connector’s solder holes. The results can be ugly, time consuming and difficult for most users. This becomes extremely difficult when more than one wire must be soldered or ‘daisy-chained’ into each tiny connector hole, as is the case for configuring the PM42’s four sections. Patience and moderate soldering skills are required.

The PM42 requires a separate, unshared AC or DC 125mA minimum output power supply – the only product of its kind to require one. The only device that Digitrax recommends can share a PM42 power supply is another PM42 and only if the power supply puts out sufficient current for two or more PM42’s. Some users share the PM42 power supply with Digitrax block occupancy detector modules but, Digitrax tech support frowns on this practice. The PM42 also requires a heavy gauge ground wire between itself and the Digitrax booster for proper functioning – the only product of its kind to require one.

Each of the four sections of the PM42 can be independently configured either as an autoreverser or circuit breaker but, not both at the same time in one section. Unfortunately, configuration requires that the 44 pin edge connector be permanently soldered in such a way as to create an autoreverser or circuit breaker section. These ‘hardwired’ settings are permanent in nature and changing them can be difficult or impossible for any user. Additionally, a Digitrax throttle must be used to program specific option switches to complete the autoreverse configuration of any given section. All four sections are equally affected by the circuit breaker current setting, which can be set between 1.5 and 12 amps in 1.5 amp steps so, they are not independent in that regard.

As an example of its configurability, one section can be used as a circuit breaker to protect up to three other sections set up as autoreversers. Or, any combination of four autoreversers and/or circuit breakers can be configured using one or more boosters as input sources. Each section, whether configured as an autoreverser or as a circuit breaker has independent switching time selection. Four user-selectable switching times can be set by programming the module with a Digitrax throttle. This process is not adequately explained in the Digitrax instructions. Typographical errors and conflicting recommendations for timing settings plague the poorly written instruction manual. Unfortunately, following the instructions and setting the PM42 to its lowest 1.5 and 3 amp current levels causes problems for the average user because the PM42 has a distinct tendency to oscillate and switch unpredictably at these lower current settings. Even at the higher current levels of 4.5 amps and up, the average dcc user will have to ‘tweak’ his layout wiring and option switch timing settings to get the PM42 to work satisfactorily. If the switching times are incorrectly selected with the option switches, and that’s easy to do under the circumstances, autoreversing action and short circuit protection can be rendered completely inoperative. The PM42’s only claim to fame is that it ‘reports’ the status of each of its sections to other devices on the Loconet network or to a pc running layout automation software.

The PM42 offers you a four-section product which works fairly well after tedious installation and tweaking of the layout. I wouldn’t recommend it to anyone except a ‘hardcore’ Digitrax user, based upon the difficulties I’ve had with mine.


LENZ LK100 – This 5 amp, mechanical relay autoreverse module can be used with most low and medium current dcc systems up to 5 amps. The LK100 is very easy to install, requiring only two wires to the booster or track and two wires to the reversing section. Convenient screw terminals are provided for a solderless connection to the module itself. The user can optimize autoreverse switching time simply by setting one trimwheel located on the module case. The LK100 has no internal short circuit protection so, the booster’s circuit breaker must be relied upon in case of a short circuit in the reversing section. This is a slick, well-designed autoreverse device which does its job unobtrusively and reliably.


This elegantly simple, mechanical relay, autoreverse module can be used with any low current dcc system, limited to 2 amps. Very easy to hook up – four wires and no adjustment required. 2 amps maximum current is less than most popular dcc systems provide to the track. There’s no short circuit protection for the autoreverse module if the booster’s circuit breaker doesn’t trip immediately, as can happen with typical wiring installations.

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