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The original current regulators use a linear regulator (LM117). These regulators waste the unused voltage between the power source and the LED voltage in the form of heat. This is not nice.

About a year after the original design, I replaced the regulators with a switching type that reduced the wasted power by a factor of four.

Details of the new regulator are shown below.

 

Electrical Design of Switching Regulator for Deck Lights

 

The regulator for the walkway lights is based on the L5970D from STMicrolectronics. Several design elements were adopted from the application note AN2259 from STMicroelectronics. The fundamental principle of the L5970D is to control an internal switch at a high frequency (250kHz) by adjusting its on-time based on the voltage at the feedback (FB) pin. The L5970D will pass as much current as is needed until the voltage at the feedback pin reaches 1.235V.

Let’s first assume that the resistors R1 and R2 were not there and we want 350mA through the load. This would require a current sense resistor Rsense of 3.53 Ohms. The power lost in the sense resistor would be 0.43 Watts. While that does seem to be a lot of power, there is a trick to reduce it.

The L5973D has an internal reference voltage of 3.3V (at the pin REF). By using a voltage divider R1/R2, the voltage at the feedback pin is biased. This results in requiring a lesser contribution coming from the current sense resistor. Without going into the detailed calculations, selecting values for R1 and R2 as shown below, calls for a current sense resistor value of 1.15 Ohms cutting the power loss there by a factor of three.

The input capacitor C1 is there to allow the regulator to draw high current pulses from the supply side. The inductor L1, the so-called “free-wheeling diode” D1 and the output capacitor C6 smooth out the switch pulses. This is a typical design of a buck regulator where the output voltage is always lower than the input voltage. The capacitor C6 can even be omitted if one can accept a large percentage of ripple in the output current. The rest of the circuit deals with high frequency compensation and was taken directly from the application note.

If you look at the schematic below, you will see some different elements than have been described above. There are two models of the regulator IC and R1 is in series with a trim pot. Rsense is now .392 Ohms. The L5973AD regulator has a 2 Amp switching capacity vs. 1 Amp for the L5970D in the same SOP-8 package. The higher current capacity is achieved by an exposed pad on the chip which can be soldered to the circuit board for better heat dissipation. By making the bias voltage for the feedback pin adjustable by means of the trim pot, the regulated current can be adjusted between about 200 mA to more then 1,000 mA. This makes the design flexible for multiple applications, for example driving the new Luxeon L2 LEDs at 1,000 mA or even higher.
 


All parts of the regulator except the 100uF capacitors are surface-mount devices to reduce the board size. A double-sided epoxy fiberglass board was used. The board can be cut into two different shapes. In its rectangular form, screw terminals are used to accept the input and output wires. However, if the finished board is cut along the circular outline it will fit into a 40 mm diameter space with the wires soldered directly to copper pads. This feature may come in handy if the regulator were to be mounted inside a tubular structure such as a spotlight.

 

RegulatorLayout05
RegulatorCopperTop02
RegulatorCopperBottom02


The images above show the component layout, the top and bottom copper patterns. The larger rectangular outlines serve to align the transfer films. The board will be trimmed to either the smaller rectangle or to the circular outline. Since I have no easy way to make vias (plated-through holes), a few non-component holes were added to the copper patterns where short jumper wires will connect the layers.
 

This is the assembled regulator board. Yes, it is not pretty since all surface-mount components were hand soldered in place. The board measures a mere 2 x 1.5 inches. It can power several LED configurations, using a 24 VDC source.

This regulator has been the work horse for numerous yard lights and is in use for the following:

4 regulators powering the lights in the deck pillars (each driving 2 parallel strings of 5 Luxeon 1 W LEDs

4 regulators powering sets of 5 Luxeon Rebel LEDs at 700 mA for the deck borders (not shown nor discussed here in detail).

WalklightsRegulator02

 

October 2011 Update

When the remnants of the tropical depression Lee passed through Central Pennsylvania, a small thunderstorm passed by to the north of us. However, it decided to leash out several miles to the South and hit our house with a lightning strike. While it did not cause any lasting fires, just everything in the house that contained electronics got destroyed or damaged. The list includes five computers, two TVs, garage door opener, dimmers, thermostats and some 36 outdoor LED light fixtures and more.

I took the opportunity to upgrade the yard lights. In lieu of a larger 24 V DC power supply, a 600 Watt AC transformer is used as the main power source. The LEDs in the outdoor fixtures were replaced with newer more efficient types. The LED drivers were redesigned with newer components and they are now AC/DC compatible. This design should be more resistant to voltage transients.

What about surge protectors? Of course, my sensitive equipment is hooked up to surge protectors or backup  power supplies. The problems are networks, be it the computer network or the power network for the lights. When lightning strikes a network at some point of entry, very large transients travel along the network wires and potentially take everything down along the way.

The associated new electrical and mechanical designs may be discussed in a future update to this web site.

 

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