Understanding a DCC Decoder Circuit Board

[richhotrain]

I am hoping that some of our resident electronics experts will explain the makeup of a DCC circuit board.

When I look at those little green boards, I am familiar with the tabs that are wired for the motor, the rail power pickups, and the function outputs for light and sound.  And I can see and understand the traces throughout the circuit board that are there to carry the current.  But what are those other little components like transitors, etc. that dot the circuit board and what do they do?

Rich

[zstripe]

Rich,

Unfortunately, you are not going to find a DCC, Decoder, schematic,showing components, any where, for the main reason, that they are copyrighted, it would be like giving a secret away. Using one as a example though, think of this one, but in miniature, on a decoder, drivers, amplifiers outputs and so forth:

http://www.build-electronic-circuits.com/electronic-schemat

Cheers, 

Frank

[gregc]

richhotrain

When I look at those little green boards, I am familiar with the tabs that are wired for the motor, the rail power pickups, and the function outputs for light and sound.  And I can see and understand the traces throughout the circuit board that are there to carry the current.  But what are those other little components like transitors, etc. that dot the circuit board and what do they do?

the primary componet in the pico-processor that decodes the received signal and drives the transistors to control the motor and optional output (e.g. LEDs).

Since the packet is transmitted by reversing the polarity of the power, a bridge rectifier is used to provide a constant DC for the processor, motor, ...   Capacitors filter this output.   The processor also looks at the non-rectified track signal to decode the DCC packets.

The motor is controlled using pulse waveform modulation, PWM.   Since the motor needs to be reversable, there are two power transistors (e.g. mosFets)  on each lead of the motor, creating a bridge.   One gating positive and the other ground voltage.  The PWM signal is applied by contolling the positive transistor on one lead while the ground transitor on the other lead is enabled.   Using the other pair of transistors reverses the motor.  (see microchip app note, AN893 for more info).

resistors are added btween the processor outputs and transistor to limit the current the processor needs to provide.  Reverse biased diodes are added across inductive paths (e.g. motor) providing a path for the current until is dissipates when the transistors are turned off.

while the pico-processor can turn an output lead on (plus voltage) or off (ground), the processor outputs can't source/sink much current.   Therefore eternal transistors that have higher current capacity are used to driver the LEDs.

 

 

[richhotrain]

gregc, u da man.  That's what I was looking for.  Thank you much.

Rich

[richg1998]

There are integrated Circuits, IC's, sometimes called chips, resistors, capacitors, transistors, sometimes two transistors in a common case, diodes. UK people usually call the complete decoder a chip. Chipping the loco you might see sometimes.

Decoder use surface mount devices. SMD.

 The AC, DCC signal feeds four diodes, full wave bridge first. That pulsating DC is filtered by one or two capacitors to produce nearly pure DC. A portion of the DCC before the bridge supplies the digital info to the microprocessor, the black square device with the most leads.

 Sound decoders have a few more components. Layout differs between different manufactures.

Below is a very simple layout of a non sound decoder. The added components are for stay alive. Another discussion.

 

Rich

 

[rrinker]

 There are a few DIY DCC decoders (yes, people do make their own) and the sites for those will have full schematics. Not that these will exactly match any commercial decoders, but the concept is close.

 The truck pickup wires generally first go to a set of diodes to rectify the DCC power into DC. This is also tapped off to the microcontroller to provide a source of the DCC signal. The diodes will be 4 relatively large rectangles, usually black, with two connections to each one.

 Small black rectangles with numbers like 103 on them are resistors. 103 means 1000 ohms, or 1k. Other small rectangles of different color are usually capacitors. Small black elements with 3 connections are transistors, tracing these often finds them connected to function outputs. The biggest block with the most pins is the main microcontroller, near the motor lead connections where will be either 4 transistors or a smaller chip which internally contains 4 transistors in an H bridge configuration. If it's a sound decoder, there will be resistors and capcitors between the main chip and speaker leads, possibly transistors or another chip of the amplifier is external to the main chip. If the functions are current limited (those decoders where you cna connect LEDs directly, without external resistors), you should be able to trace back the function connection and find a resistor.

                     --Randy