Guide to Auto Reversers for DCC

by Don Fiehmann

DCC in Reversing Loops

Operation and wiring of a reversing section has always been a big mystery for most modellers. The problem is that somewhere in the loop the polarity of the rails reverses and when the metal wheels come to the rail reversal, there is a short circuit. Fortunately with DCC, because the decoder determines the direction of a locomotive, and not the polarity of the rails, the polarity of the loop can be “flipped” while the train is in the loop. This has lead to the development of DCC “Automatic Reversers” that allow the loop to be automated. These Automatic Reversers work by sensing the short at the gap and reversing the polarity quickly.

Types of Auto-Reversers

The polarity can be switched using either Relays or Transistors. The advantage of transistor type auto-reversers is high speed switching and no mechanical contacts to arc, pit and fail. Also, there are no sparks at the reverser gaps to cause wheel pitting. A reversing loop is not immune from derailments or someone neglecting to throw a turnout (shorts happen). This means the reversing loop needs to be protected from a short circuit with a circuit breaker. Some reversers come with built-in circuit breakers. If you are using a low-power system 2 Amps or less, be sure the circuit breaker trip current is less than the low-power rating of the DCC system. Also, check that the reverser will reverse the polarity at the lower current without shutting down the DCC system.

Product	Digitrax		Lenz	MRC	Tony’s	DCC Specialties
Features	AR-1	PM42	LK100	AD250	PSRev	OG-AR	PSX-AR (11)
Unit Cost (List Price)	$29.99	$80 / 23 (1)	$57.00	$34.95	$56.95	$39.95	$59.95
Trip Amps (2)	0.25 to 8	1.5 to 12	1 to 10	2	3.8 to 8 4	4	1.27 to 19.1 (5)
Built-in Circuit Breaker	No	Yes (3)	No	No	Yes	Yes	Yes
Response Timing (6)	Slow	Slow	Slow	Slow	Fast	Fast	Fast
Manual Reset Optional	No	No	No	No	Yes	No	Yes
Programmable Settings	Adjustable	w/DCC System	Knob	No	Jumper	1	Jumper or System
Status Lights	No	Yes	No	No	Yes	Yes	Yes
Method of Switching Power	Relay	Relay	Relay	Relay	Solid-State	Solid-State	Solid-State
Extra Power Supply Needed	No	Yes	No	No	No	No	No
Network Feedback	No	Yes	No	No	No	No	Yes
Spike Current Inrush Control (7)	No	No	No	No	Partial	No	Yes
Integrated Stationery Decoder (8)	No	No	No	No	No	No	Yes
Integrated Block Detection (9)	No	No	No	No	No	No	Yes
Annunciator Output (10)	No	No	No	No	No	No	Yes

(1) The PM42 has 4 outputs, so the cost ($80.00) plus power supply ($12.00) equals $92.00 divided by 4; (2) Some units have adjustable trip current settings; (3) PM42 also requires outputs for short circuit protection when using Auto Reverse (4) Special units for low-power DCC systems are available; (5) Currents over 10 Amps require added heat sinks; (6) Slow is in milliseconds (1/1,000) and fast is in microseconds (1/1,000,000); (7) Start-up of Sound Locos creates 4-8 Amp spike currents; (8) Automates reverse block turnouts with integrated stationary decoder; (9) Block Occupancy output for LEDs and Networks; (10) Off the shelf, Status Annunciator option; (11) Patent pending.

Rules for Reverse Loops and Reversing Sections

Auto Reversers can be used for sections of track like wyes, turntables, and loops. Some general rules that apply: 1. The reverse block track must have double gaps at both ends and be totally isolated. 2. The length of the reverse section should be longer than the longest train to use the section. If the track is shorter, wheels can contact on both ends gaps shorting the reverser. There are special gaping procedures that permit having reverse sections shorter than your longest train.