Searchlight_Power

• Searchlight Power
  • Features
  • Block Diagram
  • Discussion / Rationale

Searchlight Power

This project is a small power converter/regulator intended for powering LED lighting from a DCC Track Bus or other moderately high voltage AC or DC power source on a Model Railroad layout. It is based on a Texas Instruments TMS40210 switching power supply controller.

There are, admittedly, much simpler and cheaper ways to solve this particular problem, but this project has some odd purposes. First, it is a design challenge in itself. Second, it provides better power efficiency than other common options. Third, it provides a "reuse block" for a power source for other projects.

Features

• Wide input range (AC or DC, 9-24V, including DCC Track Bus)
• Selectable output voltage (2.5, 3.3, or 5.0V DC)
• 800mA output current (~ 40 LEDs per board)
• Four independent outputs with remote on/of switches
• High efficiency (target 90-95%)
• Small size

Block Diagram

This is an initial concept block diagram. Subject to frequent and radical change.

Discussion / Rationale

LEDs are an excellent choice for model railroad lighting. They are tiny, inexpensive, bright, highly efficient, and give off negligible heat (very important for lighting the interiors of plastic buildings!). However, they require a current limit resistor to protect them from burning out by controlling the current flow through them. The resistor uses power in direct relation to the supply voltage used for the lighting. This power is essentially wasted, gone, put to no useful purpose.

The power lost in the resistor is easily recovered by reducing the supply voltage to 5V or even 3.3V DC. While it is easy enough to supply a 5V power source to a new layout, owners of existing layouts may be reluctant to retrofit yet another power distribution system just for lighting. Modular layouts like NTrak already have standards for distributed accessory power, and may not wish to add yet another connection.

Common power sources already available on typical model railroad layouts include:

• DCC Track Bus (12-18V AC, depending on scale)
• 16V AC accessory power (common from older DC controllers)
• 12V DC

There are existing circuits, usually based on the common LM317 or 7805 linear power regulators that can provide lower voltage locally for very low cost and extremely simple parts count. However, linear regulators are generally not very efficient in terms of power.

This circuit, while admittedly significantly more complex (and expensive) than linear regulator circuits, combines high power efficiency (target > 90%) with usability features (input flexibility, selectable output voltage, remote control) that will make it an interesting and useful circuit for those who are interested.