Functional Blocks

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External Power Supply

  • 3-5A Switching PS
  • Integrated or external battery backup
    • 12V sealed lead-acid
    • Connection point for monitoring DC/Battery voltage

Power Input Protection

  • In-line or crowbar diode
    • 6A Silicon rectifier type
    • 30V MOV (10mm)
      • Good for slow transients
      • High energy capability
      • Non-polarized
    • TVS Diode
      • Good for fast-moving transients
      • Quick rise time
  • LC or capacitor filtering
    • Good for fast pulses, general ESD filtering and immunity

Internal Power Supply

  • MC34063 Switching Power Supply
    • Adaptable for 5V or 3.3V
    • Cheap

Output Drivers and Protection

  • Integrated Driver Chip
    • ULN2004 or ULN2803 for 7-8 outputs
    • Includes flyback diodes
  • Suitable for relays or external
    • Current-limit external (LED/buzzer) outputs
  • SMT Resistors

Relays

  • Form C relays for external switching
    • Omron G5SB-1 DC12 (5A, rectangular)
    • Omron G5LE-14 DC12 (10A, square)
  • No ground plane on output traces
  • Trace separation for 250V+ voltage rating

Logic input protection

  • Current-limiting resistor
    • MELF or other type with pulse-handling capability
  • Pulse clamping
    • TVS Diode
    • Zener diode
    • 0.22uF decoupling capacitor
      • Provides ESD protection
      • May cause problems with data signals
  • ESD Suppression chips
  • Op-Amp on inputs
    • Provides impedence matching and logic buffering
    • Good choice for high-impedence and other non-standard devices

Logic pull-up

  • Internal pull-up resistor (software addressable)
    • Provides ~50K impedence
    • Some logic pins may not have pull-ups avialable
    • Weak compared to external resistor/state easily changed
  • External 10K resistor
    • Robust, simple
    • Not as flexible as internal pull-ups
    • Can be configurable via jumper

RS-485 Circuit

  • Integrated driver chips
    • Dallas/Maxim
      • MAX-485
    • Texas Instruments
      • SN65HVD485EDR

Real-time Clock

  • Dedicated chip with battery backup
    • Dallas/Maxim
      • DS-1307 (+/-20ppm, I2C)
      • DS3231 (+/-2ppm, I2C)
      • DS1302 (SPI, supercap backup)
    • NXP
      • PCF8563
  • Crystal

External Power Rails

  • Overvoltage protection
    • MOVs
    • TVS diodes
  • Overcurrent protection
    • High-side switching ICs with built-in protection
    • Polyfuses
  • ESD Protection
    • TVS diodes
    • Decoupling caps

Terminals

  • 3.5mm screw terminals
    • Pluggable, straight
      • Phoenix 8-position header, part #1945151
      • Phoenix 2-position pluggable terminal, part #1984015
      • Phoenix 3-position pluggable terminal, part #1984028
      • Phoenix 4-position pluggable terminal, part #1984057
      • Phoenix 6-position pluggable terminal, part #1984073
    • Pluggable, right-angle
    • Non-Pluggable

Circuit Board

  • PCB Specs
    • FR4 PCB material
    • 0.62" substrate thickness
    • 1oz Copper
    • HASL finish or Gold
    • Double-sided with PTH and vias
    • Solder Mask both sides
    • Silk-screened legend on both sides
  • Grounding
    • Continuous ground plane or mesh on bottom layer
    • Optional double ground or power plane on top layer
    • Exposed ground plane at case screw holes
    • Separate ground for differential signals as needed