Power packs come in a very wide variety of quality, ratings, frills, packaging and prices. But price is not a true indication of any of the others. However in general, cheap train-set packs are just that: CHEAP. Often they do not last through the Christmas season. At the other extreme; hype, catchwords and fancy frills cost dearly. Most commercial packs do not handle all desirable functions well, since they are compromised to appeal to the widest market. Any approach to a universal pack must have many adjustments readily available, which may make them too complicated for the general user.
The simplest pack consists of a transformer to lower line voltage to a usable value, a rectifier to convert AC to DC and a device to control output voltage, usually a taper wound rheostat. Some may use a variable transformer, eliminating the rheostat and providing better control. Add ons are commonly: circuit breakers and on-off, and one or more polarity (direction) switches. The latter should be double pole-double throw-center off to permit cutting track power. The circuit breaker is an absolute necessity to avoid burning down your house. These are the bare basics to run trains safely. Beyond these, all kinds of goodies may be added, some of which may not really be useful. Most power packs have accessory AC and DC outputs by tapping off before the recifier or the throttle. It is advisable not to use them, since they rob power from the train. Average grain-of-wheat bulbs draw .1 A, so 10 will totally load down a small pack. If upgrading by replacement; one, that still works, can be used for controlled lighting in a few structures.
Note: Adjust brightness and contrast for optimum viewing.
Schematic of basic pack with location of optional meters.
It should be noted that a power pack is NOT a transformer and should not be called such.
Another add on to basic packs is a PULSE power switch, which eliminates one half cycle from the normally full wave rectification. The remaining spaced pulses tend to jog the motor rotor at slow speeds. However at high speeds, overheating may occur.
Since most users do not examine the internals, quality and workmanship are not often evaluated directly. Only feedback from reliable sources can help in this area.
The most important parameter is the current rating at nominally 12 V, which should be sufficient to handle the heaviest load you plan to run. Older ratings listed current output directly, but newer, more confusing, ratings are in volt-amps or apparent power, often with no reference to the rating voltage. We owe this to Ralph Nadir, who was trying to protect me from me. Now even the oldtime buyer usually asks for help. To find the apparent current, just divide the VA by 12 in your head. Small, short duration, current overloads are not dangerous. But the increased heat usually reduces output voltage, slowing the train, perhaps unrealistically. Added meters are helpful to determine values, if this is critical.
Danger: It is not too wise to
have packs with extra large current ratings; unless the breaker is a fast
acting, reliable, solid state type, such as a "crow bar" and all associated
wiring is of sufficient size. However a 10 amp breaker, not uncommon for
DCC, will not protect stalled locos from very rapid motor burnout or PLASTIC MELT-DOWN, unless internal limiting is
incorprated inside the loco. With pulsed current, it is very difficult to
calculate the effective value that will trip a breaker. This is usually
done empirically and not always correctly.
With the introduction of low priced power semiconductors, power supply design changed rapidly and sometimes drastically. Surplus, newer design, switching regulator types make excellent supplies for home made packs and other electrical or electronc projects. Diodes replaced older selenium rectifiers, which aged over time, increasing forward resistance. Often older packs had secondary voltages as high as 22 V to compensate and still provide 12 V DC output after several years. Far too often, the same transformers were used in newer packs allowing the track voltage to reach as high as 20 V.
Soon after, Darlington power transistor circuits replaced rheostats for throttles, yielding superior control. Even though it was not often reflected in prices, the manufacturing cost was far less than using quality rheostats. Although it rarely was, case size could have been greatly reduced. Even today, the vast majority of solid state devices in packs are not properly heat sinked or ventilated. Too often, the completely enclosed case is used as a sink with mica or plastic mounting insulators interposed. A basic parallel is that a poor electrical conductor is also a poor heat conductor. Although only about a dollar or two more, separate isolated heat sinks should be used along with compound coated, direct contact and adequate ventilation.
"Transistorized" packs paved the way for all sorts of enhancements including momentum, braking, variable PULSES and feedback circuits to overcome binds and up or down hill speed changes. Even CTC circuits can be added for automatic block, speed control.
In my opinion, the most important features are full control of acceleration and deceleration rates, maximum speed, minimum starting voltage, pulse amplitude, width and rate, fast feedback for correcting erratic binds and slow feedback for correcting hill speeds. These were all included in the discrete component, True-Action Throttle (TAT-IV), fostered by Linn Westcott, the best editor MR ever had. Although not very elegant by todays standards, the design was pure and simple, while being extremely functional.
In addition to the throttle, the switchable , primary functions included 2 acceleration rates and 4 deceleration rates; of drift with throttle closed, plus slow, fast and emergency brakes. Braking was interlocked to disconnect throttle. With the secondary settings, the starting voltage could be set to avoid the usual momentum wait, when power is applied. Even the maximum speed could be established to prevent visiting engineers from derailing the train with excessive speed. For various load simulations, both accelerations and decelerations could be adjusted independently.
Other features were added through the use of differential amplifier type circuits, the basis of operational-amplifiers (op-amps). Both pulse amplitude and rate could be adjusted to suit the loco. A cleverly designed squelch circuit caused pulses to reduce as throttle voltage increased and their usefulness decreased. This reduced potential motor overheating. An adjustable, short time constant feedback rapidly increased voltage, when gear or side rod binds drew more current, smoothing operation of some very poor locos. A slower constant circuit compensated for uphill voltage drop and slowing plus downhill voltage rise and speedup. A slight paradox existed in that excessive feedback settings could cause trains to accelerate uphill or to stop going downhill.
Probably because all the possible control settings overwhelmed the average railroader, it was only produce commercially by a few kit makers and in the UK. MRC offered a walk around version which used an external power supply. Including one by Heath-Kit, several integrated circuit versions were offered, but none captured the full flavor. For some unearthly reason, one overdone version used a bilateral switch IC, which at the time cost over $40. At least one enterprising modeller built a version into a simulated cab complete with brake stand.
One good enhancement idea was offered in which the the adjustment potentiometers were replaced externally by fixed resisters mounted on a plug-in header. Once the optimum values for each loco were measured, mounted and the header cover was marked, the matching loco could be run with no adjustments simply by plugging in the header. For owners of large numbers of locos, major drawbacks of the scheme, based on quantity, are the cost of headers, measuring values, mounting resistors and storage.
Unfortunately DCC does not lend itself to this advanced type of throttle operation easily or inexpensively. IMHO, a more practical approach would be a system based on 2 hexidceimal counters, which selects any one of up to 256 locos to be powered and leaves the driving to me on a good throttle. This would only be needed possibly in terminal areas or for coupling for double heading. As a computer system engineer, I might also add: Leave the the desktop computers to their primary functions and let me run my trains. Simple hard wired, dedicated computers have their places in controlling reverse loops, interlocking crossings, indexing transfer and turntables, even automated mining or logging operations. All of which remain virtually untapped.
Since only you know what you want or need, selection is a matter of taste. Commercial packs vary extensively in characteristics and parameters and none is universal for all applications. Characteristics of control can vary drastically even from the same manufacturer. Quality and warranty are prime considerations, along with service. Over the years, MRC has set the standard by which to judge others. Current capability is probably the first operation factor. In HO, for switching , terminal areas, traction and short trains, a 1 1/4 amp rating should be sufficient. For double or triple heading with 60 cars, about 2 1/2 amps will handle it. For safety the upper limit should be about 3 amps. A small pack can stall heavy trains on steep grades, due to internal heat losses.
The other characteristics are highly dependent on application. Diesels, switchers, traction and steamers all respond differently. Gearing, motors, flywheels and load have their influences. To cover all optimally, with commercial packs, might require 10 different units. Proper power trains that achieve prototypical maximum speeds at 12 V, can help narrow the range, These are two of the best arguments for adjustable parameters and proper loco speeds. Except for testing at lower speeds, where the resistance will drop voltage under excessive currents, rheostats should be avoided. If possible try the pack at the shop or on a friend's layout with your loco, before buying.
Ideally road locos with train should have smooth starting and acceleration, controlled running at speed plus smooth deceleration and stopping. This will require good pulses at the low ends, good momentum circuit and clean, no pulse, regulated power at speed. With all the combinations of packs and locos, it would be difficult to list them. But in general older momentum packs by Troller and MRC seem to handle heavier current motors better than newer ones.
Slow speed; for switcher, traction and industrial operation; presents entirely different requirements. Momentum is of little use and may be undesirable. With speed limits of less than 25 SMPH, the aspect changes drastically. At 12 V most motive power on the market have speeds double or triple the prototype. Operation is normally below 5 V., which limits the standard throttle to about 1/3 turn at the low end. Even with excellent pulses, this yields very poor control. Lowering the pack voltage does not help, since intermittent pickup at low voltages causes very erratic operation. The best solution is to repower, so the maximum speed occurs closer to 12 V. Then very fine control and smoother operation can be achieved and pulse requirements are less stringent. Not often available commercially, a walkaround, SCR throttle provides excellent operation, with its own disappearing pulses.
The basic advice is: Try it, you might like it.
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