Breadboard-1

Wired-N-1A-2-1

About

The Wired-N-1A-2-1 design is a basic 1A N-Scale decoder based off the Breadboard-1 reference design.

• 1A Capacity
• 2 Functions (FOF, FOR)
• ICSP programming exposed via pads.
• Components on top side only enabling cheap fabrication.

3D Render

Actual size 9mm x 18mm:

Intended Use Case

Used for any hard wired N/Z scale application, from locomotives that do not support drop in decoders, to passenger car and caboose lighting operations.

Schematic

View the schematic: Wired-N-1A-2-1.pdf

Supporting CALCULATIONS.md discussions how component values were determined.

Schematic Walkthrough

Power Supply

Power from the two rails is fed into a bridge rectifier made up of four discrete diodes D11-D14. As DCC is a square wave, the output is nearly but not quite continuous DC power with low ripple. That power is split off to drive the motor and functions directly, labeled as Vdrive. A linear regulator U11 is used to generate +5v for the micro-processor, and has two support capacitors.

The project requirements for this design are to operate from 6v to 30v of input.

Micro-controller

A PIC18F06Q40 processor is used in this design due to it's avaialbility in a 3mm x 3mm VQFN-20 package. PIC processors require minimal support components. C21 is a local bypass capacitor that is placed closed to the power pins. C23 and R11 form a hold up circuit for MCLR which must be kept high during normal operation. If MCLR drops, the chip will reset.

Particular notes on this part:

• This design does not use a Vref pin.

H-Bridge

The H-Bridge controls the current to the motor. It is driven by a PWM signal generated by the micro-controller. The software generates 4 PWM signals, P1A-P1D, which are used to control speed and direction.

The H-bridge is simply 4 high power MOSFETs arranged in an H configuration. This design uses two discrete dual MOSFET chips each with 1 PNP and 1NPN MOSFET, which is a common arrangement. Resistors are placed on the gate inputs to prevent inrush from damaging the micro-controller.

External diodes are omitted on this design to save space, the body diode of the MOSFET must do the job.

In order to calculate BackEMF, a 100nf capacitor C31 is added to reduce signal noise. The resistors R35 and R36 then reduce the voltage to an appropriate level for the analog input of the micro-controller. By monitoring this voltage the micro-controller can calculate the load.

Functions

This design supports functions F0F, F0R, F1-F6.

Functions F0F, F0R, and F1-F6 are designed to be connected to external device in a "common anode" configuration.

Vdrive power is provided as a common anode source at both ends of the board. The MOSFET connects to ground when turned on by the micro-processor. A resistor is provided on the gate of each MOSFET to manage inrush current.

External Connectivity

MA & MB are the two motor connection solder pads.

RL & RR are the left and right rail solder pads.

F0F, F0R, & F0-F6 are the function solder pads.

CA are the Common Anode solder pads.

MCLR, +5, GND, PGD, PGD make up the ISCP connection points.