DIGITAL COMMAND CONTROL ON THE

TETON SHORT LINE (POWER BOOSTERS)

Wayne Roderick, 3rd Division, PNR, NMRA (life)

03/15/98 rev 01/15/07

TSL BOOSTERS: We have four POWER BOOSTERS using low saturation resistance Hexfets and I think you'll really like the design. The first one was prototyped 01/13/98 and put in service so we could work on and test the software. There were errors in that schematic. I suggest you discard it. Three more boosters were built in March '98 with an improved missing pulse shutdown circuit.


The PROGRAMMING booster was also operational on 01/13/98.


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Schematic, DCC Booster

The TSL uses rack mounting wherever possible to make things neat and tidy. The DCC booster(s) are constructed on plugin circuit card(s) and so it is desirable to keep the heat producing components to a minimum. We chose a "hexfet" Power MOSFET amplifier for it's very low saturation resistance, about 0.06 ohms. The only significant heat is due to the 8kHz switching and a 3/4"x 1" aluminum tab heatsink takes care of that.

A bridge diode adds some protection to the fet's making the circuit rugged comparable to bipolar transistors. The raw power supply is outboard and includes the heat producing items, i.e. the transformer and voltage regulators. Why the two LM317 regulators instead of a single higher current device? Because we just hopped up the existing analog power supplies and there were some LM317's in the junk box.

Our circuit will not put out direct current, or "stretched zero's" so operation of non DCC locomotives is not considered.

The "missing pulse detector" circuit permits output only when TTL level data is coming in and unlike some similiar circuits, it works regardless if the data line fails in a HI or LO mode. Should the output be overloaded, the voltage regulators start current limiting and alarm is indicated locally by a red LED and immediately sent to the computer for fault isolation purposes. The time delay associated with the alternative recycling electronic circuit breakers was not acceptable, if we are to do rapid computerized fault isolation.

02/13/06 update Computerized fault analysis has been abandoned. Experience has shown that fault (short on the track) isolation is of little real concern and is easily coped with.

The TSL's computerized fault isolation scheme depended on the relays for each block, residue that was left over from the days of Automatic Cab Control (ACC) and it was time to let it all go. This meant that I had to add a circuit breaker to each boosters to maintain adequate protection.

It is important to understand that the circuit breaker is to protect the wiring and equipment on the tracks. The booster already has adequate protection for itself. A partial short on the track from a derailment or over-running a switch can develope enough heat to do serious damage to trucks and loco wiring from the three amp DCC power. To visualize this, a worst case partial short would draw the full three amps with the 12 to 14 volts still sustained on the track. that is about 40 Watts of heat! How long could you hold a 40 light watt bulb or soldering iron in your hand? The principle of the self-resetting electronic breaker is to keep the average power down by reacting quickly and then staying off for a relatively long period of time. I chose about 1/10 second on and 3 seconds off.

Each booster is isolated from ground and each other by the supply transformer and optical couplers for greater flexibility in present and future wiring schemes.

(08/19/01 update) As to COMMON rail wiring: There are a number of pitfalls and risks to decoders when DCC is installed in an existing COMMON rail system. This topic has been heavily addressed by myself and others in the DCCSIG forum, but remains controversial to some even to this day. The TSL was wired COMMON RAIL for nearly 35 years using three terminal occupancy detectors. In this situation one terminal of the detector is "GROUND" for the track feeder AND "GROUND" for the logic output so abandonement of COMMON RAIL wiring was not a trivial task. I initially installed my DCC into the COMMON RAIL system mixed with some conventional pulsed analog DC and managed to destroy two decoders and one Kato engine shell.

For COMMON RAIL wiring, one of the output terminals (#5 or #7) on each booster must go to the COMMON/GROUND. For that reason, auto-reversing within the booster is not feasible. For decoder safety, it is essential that all boosters be in phase with each other. If terminals #5 go to COMMON/GROUND then there shall be no significant voltage between the #7 terminals of the booster pairs, if the input signals at #01 and #13 are in phase. External devices that manually or automatically swap feeders to wyes and balloon tracks can create significant risks. See for some risk guidance.

04/14/98 update: Thirty-five years of COMMON rail wiring has been abandoned in favor of what is sometimes called DIRECT HOME wiring to minimize decoder risks. With DCC, the ancient arguments that support COMMON rail wiring mostly fall apart, and there is much to support DIRECT HOME. See my arguments on DO-IT-YOURSELF DCC SUB-SYSTEM.


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Schematic, Program Booster
The program booster is fabricated on a small card and installed near the programming track. During programming, the power is drawn from the common twelve volt buss system. A simple 12 Volt panel lamp, #1816 is used to limit the current so that a loco will not move. The NMRA calls for a minimum current increase of .060a for 20 msec as an acknowledgment that a programming command has been accepted. Detecting this change in the presence of differant loads such as lights, sound systems, smoke generators etc., requires AC coupling between the 15 ohm sense resistor and the acknowledge circuit. An piezo audible device indicates the acceptance with a brief chirp and a signal is sent back to the computer.

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