# KB521: What is "Smooth" DC?

This article was last updated on Aug. 20, 2011, 8:23 p.m. | Print Article | Leave Feedback

The term smooth DC is used to describe the power provided to the layout by conventional model railroad power packs (analog transformers). Traditional model railroad power packs convert household line current into 12-18 volt current to power your model railroad.

Voltage is supplied to the track, and as the voltage increases, the speed of the locomotive's motor increases, which makes it run down the tracks at faster speeds.

Some power packs put out only AC (alternating current), while others put out DC (direct current) and AC (for accessories such as track switches and building lights). AC powered trains tend to be in the "toy" category, while DC motors are most often used in "scale" trains because DC motors offer more realistic slow speed control.

DC motors use a magentic field around the motor armature, and the direct current from the tracks is applied to coils in the motor armature. The motor turns as the magentic field created in the armature by the flow of electricity repels the magentic field created by the motor's magnet.

As the armature turns, current is applied to another set of coils, again causing magnetic repulsion. As the voltage increases, this process happens more quickly.

Changing the polarity of the power on the rails causes the motor to turn in the opposite direction.

At very slow motor speeds, the magentic field of the motor tends to "grab" the armature, holding it until the force of the voltage exceeds the magentic force of the motor magnet.  At slow speeds, the motor turns abruptly as the magnet grabs the motor armature until the voltage again breaks the field.  This choppy motion is called "cogging" and results in uneven locomotive performance at slow speeds.

Power pack manufacturers solve this problem by using pulse power, where 1/4 wave AC is applied on top of the DC current.

The DC power makes the motor armature turn, while the AC power causes the armature to vibrate slightly, helping to break up the effects of the motor magnet. As the locomotive speed increases, the pulse power is turned off and the motor continues to operate with pure DC.

A later enhancement of this technique was to use automatic pulse injection that slowly adjusts the wave pattern of the AC as the motor progressively increases its speed.

Typically, power packs manufactured after the 1970's include this feature.

When using a power pack to provide power for a DCC booster, pulse power is not acceptable.  DCC devices requrie "pure" DC power because these devices have timing circuits as part of their electronics. Partial wave form DC and automatic pulse injection contain elements of AC that interrupt the timing ciruitry of the DCC devices.

Likewise, the Jump Ports of the Zephyr also require pure DC inputs; the Zephyr determines the level of DC voltage and adjusts the command control signal accordingly. Pulsed DC interrupts this process as well.

The DCC signal contains both power voltage and a variety of command signals; higher level mobile locomotive decoders can be controlled with these signals to offer a variety of DC motor speed controls for slow speed performance, including BEMF (Back EMF), Speed Stabilization and other terms.

DCC decoders sense the action of the DC motor in each locomotive in relationship to the actual speed command and adjust the power levels of the motor. This produces better slow speed control than that available with pulse power schemes in power packs.