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| DCC System Diagram • DCC Basics • How Does the Decoder Get Power? • Speed Control |
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Digital
Command Control consists of several parts: throttle, command station,
booster, decoder, and a power supply to run it all. An optional throttle
network can be added to have multiple walk-around throttles. |
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Briefly (referring to the illustration at right):
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Some systems combine
some of these components into one unit. For example, Chief combines
the command station and booster. Regardless, all of these components
must be present, in one form or another, to make up the system. |
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NOTE: The information here is not meant to be complete and
totally technically accurate. Since most readers will be non-technical
individuals, efforts have been made to describe everything in general
terms that non-technical persons can understand. If you need exacting
technical information for research and development, contact the NMRA's
DCC working group for current technical information. |
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| Before we go any further, there are three things that need to be made clear, so you don't have a wrong pre-conception going in.
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First, the DCC power and signal
on the rails is square wave AC. Think of a normal DC computer signal
square wave, as illustrated in the diagram at right, and double it
- as explained below. |
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Computer signals
like this are usually measured in milliamps, with a voltage swing
from zero to 5V DC. This provides a good signal circuit, but could
hardly power a train. So, the booster takes this signal, and amplifies
it - voltage and amperage. What you wind up with is a digital signal
with enough voltage and power to run trains. |
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Now for the AC part of the system.
With DC power on the rail, a non-decodered loco would simply take
off at full speed. So, the same exact signal that rises above zero
is mirror-imaged to the negative side, making a square wave signal
that swings from negative to positive - square wave AC - as illustrated
in the diagram. |
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The actual voltage
is dependent upon the booster you're using, and at which scale you
have it set to operate. Regardless, a short duration pulse represents
a one (1), and a long pulse represents a zero (0). A group of eight
pulses represents one byte of an information packet. |
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As illustrated in the diagram
here, a basic packet contains a preamble of "1" bits, a "0" start
bit, an address/data byte, a "0" start bit, an instruction byte, a
"0" start bit, an error detection byte, and a "1" end bit - which
leads into the preamble of "1" bits for the next packet. |
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| As long as the duration of the negative (-) pulse matches the duration of each positive (+) pulse, a non-decodered loco will not move. It gets an instant of positive power trying to make the loco go forward, then gets an instant of negative power trying to make it go backwards. The result is a loco that sits there buzzing. (Note: a drop of Conducta Lube on the commutator can reduce this noise). "Zero stretching" is implemented to make the non-decodered loco go. That is, the zero (0) pulses are stretched longer on one side than the other. To make the loco go forward, the zeros on the positive side are stretched. This provides more positive power than negative. The loco still gets short pulses of negative power, but the longer positive pulses overpower the negative to make the loco go forward. Obviously, to make the loco go backwards, the negative zeros are stretched. As you've probably experienced, the faster the analog loco goes, the less it buzzes - because the longer periods of positive power reduces the number of AC pulses per second. While this analog loco is running, all DCC locos continue to run as if nothing else is happening to the signal. That's because the decoders are designed to recognize short pulses as ones (1), and long pulses, no matter how long, as zeros (0). Stretched zeros are still read as zeros by decoders. If you want to install meters to monitor track power, you have to use AC volt and amp meters. However, even though AC meters work, they won't be exactly accurate - because the typical AC meter is not designed for square wave. Even so, it will be accurate enough for general layout operation purposes. As for how decoders use square wave AC to control and power DCC locos, there are two parts to a decoder that use this power/signal: the control part, and the power part. Turning track power on also turns the decoder on. Even if the loco has never been addressed, the controlling part of the decoder starts reading the digital packets - all of them. Each packet is "decoded" to get the address. If that address matches, it acts upon the commands in the packet. If the address doesn't match, it continues doing what the last packet for that loco said to do. That is, if the decoder has never been addressed, it continues doing nothing. But if the loco is running, or has a light or other function turned on, it continues passing power to the appropriate wires. |
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As for the power
portion of the decoder, the first thing that has to be done is convert
the AC power to DC. With AC, the current flows in one direction (phase
A, below) for one pulse, then the other direction (phase B. below
that) for one pulse, and so on, alternating back and forth - hence,
alternating current. But, the decoder needs DC (direct current) to
work. |
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Converting AC to
DC is done with a device called a bridge rectifier, or with four diodes
configured to make a bridge rectifier (a diode is a device that lets
electricity flow in only one direction). So, when four diodes are
configured as illustrated, the rail's AC power is converted to DC. |
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| When AC flows into
the AC1 connection (Phase A, at right), diode D blocks it in that
direction, but Diode A allows it to go in that direction. Since Diode
C blocks it in that direction, it has to go out DC+. It continues
through the device being powered, back into to DC - through diode
B and out the AC2 connection. |
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| On the next cycle,
Phase B, power flows into the AC2 connection through diode C, out
DC+ through the device, then back in DC-, through diode D to go out
the AC1 connection. |
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Since this alternating
transition happens so fast, it effectively provides a constant flow
of DC (direct current) power. This explains how AC is converted to
DC, but doesn t explain how it can operate a motor in both directions. |
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| The motor is driven
by power transistors. Transistors also allow power to go only in one
direction. Consider this illustration: to make the loco go forward,
transistors T1 and T4 are activated by the controlling circuitry of
the decoder. T1 allows positive current to enter the positive terminal
of the motor, while T4 connects the negative terminal of the motor
to the negative terminal of the bridge rectifier. To make the motor
go in reverse, transistors T2 and T3 are activated. |
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| Since there is full voltage on the rails at all times, there is also full voltage available at the power transistors at all times. If the transistors are turned on to pass available voltage, the loco will take off at full speed. Unlike a water faucet, transistors are designed to pass full voltage, or no voltage. With a water faucet, if you want to pass 10 gallons per hour, you can simply turn the volume down and it will dribble 10 gallons per hour. But what if you have a valve that is either on or off. How would you get an even flow of water that would produce 10 gallons per hour? Simple, turn it on for one second, then off for a few. So, these power transistors are turned on (triggered) for a brief period of time, then turned off for a time depending on how fast the loco is to go. The difference is that the transistors are turned on and off hundreds of times per second - so fast that the motor can't respond instantaneously to each pulse. The motor has so much mass to it (including flywheels) that a single pulse can't budge it. But, a whole lot of pulses can make it go. This is part of why locomotives run so much better on DCC than with DC analog. A motor takes more voltage to get it started than it does to keep it going. For example, you've probably experienced increasing analog voltage to the tracks to a point where the loco lurches to a start, but can then reduce voltage to keep it going at a slower rate than it started. Well, DCC hits the motor with pulses of full voltage, so the full-voltage pulse can bump the motor to start, but doesn't provide the power long enough to make it lurch. Consider this: to make a loco run at 30% throttle with DC analog, you will be providing 30% of the available voltage all the time. But with DCC, you provide full voltage 30% of the time. |
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