Momentum
The delay in starting and stopping the motor is called momen-
tum or acceleration and deceleration delay. It is generated by
the microprocessor. You turn the throttle from 0 speed to 50%
speed and the microprocessor tells the motor to go 0%, then
1%, then 2%, etc., until it reaches 50%. The quicker these com-
mands to the motor are sent, the less the momentum.
Stopping works in reverse: 50%, 49%, etc.
Emergency stop removes the delay: 50% goes directly to 0%.
Function Driver and Lighting Functions
The function driver switches full voltage to the function pads
or wires. It doesn’t have to work with reversing the polarity, as
the motor driver needs to do.
10
10: Function Driver equivalent circuit with an external
bulb or 12 volt LED – rear (yellow) function shown.
DCC Impulses Column - 7
The blue wire provides the positive voltage for the LED or what-
ever a function wire is controlling. The colored wire (yellow in
figure 10) connects the bulb, in this case, to the negative voltage.
Once again, the driver is just a switch. If you have a light on at
full brightness, the switch closes when you select a function
and opens when you turn it off.
The microprocessor can be programmed to generate lighting
effects, like Mars lights or firebox flicker. The decoder adjusts
light level the same way that it adjusts speed, Pulse Width
Modulation (PWM). Figures 8 and 9 show how PWM can pro-
vide 50% or 75% power to whatever is connected to it.
Some decoders have built in voltage regulators or current regu-
lators or dropping resistors for direct connection of LEDs or
low-voltage bulbs. Using these “features” may be addressed in
a future column.
So, the designer of the decoder can tell the microproces-
sor that a sequence of pulses will look like a Mars light. Here
comes a rub. Incandescent bulbs and LEDs generate different
amounts of light based on the applied power. So, what looks
good for a bulb won’t look correct for an LED – the LED will
come on sooner. Many manufacturers allow the user (through
CVs) to tell the decoder whether a particular function is con-
nected to a bulb or an LED. The decoder will change the
algorithm (program) accordingly. Telling the microprocessor
to generate a LED-style function won’t (at least in currently-
available decoders) adjust the voltage or current available on
a function output for LED operation. The installer still needs to
provide a resistor or other control component(s).
MRH-Feb 2013
