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Creating a homemade 12VDC LED lighting system

21 Nov 2014  | Joseph Julicher

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The firmware I used in the LMH-2's MiWi wirelessly-controlled ballast performed the following functions:

1) Configure the hardware for SEPIC power supply according to the block diagram.

2) Regulate the output current to the desired level by closing the loop between the average output current and the peak current setpoint.

3) Manage the MiWi network and respond to dimming commands by adjusting the output current setpoint.

We'd hoped to light the entire cabin with a design power budget of 100W but that would have required LEDs with an efficacy of nearly 60lm/W. Unfortunately, the efficacy of today's commercial LED products are typically in the range of 100 lm/W. Planning the cabin's power budget began with this real-world figure which was further adjusted by factoring in the additional losses introduced by power-supply inefficiencies and light-fixture design. Using the LMH-2's specified output Lumens, the measured input power of the SEPIC converter and the LED fixtures, the system's end-to-end efficacy was estimated to be 70 lm/W.


Figure 3: Microchip's PIC16F1788 28-pin 8bit Advanced Analogue Flash MCU serving as a DC-DC converter for one of our off-grid LED lighting fixtures.

System deployment
My father, Mark, and I arrived on site at midnight, after driving from Maricopa, AZ for 17 hours with conduit parts, wire, lights and tools. The conduit had been pre-staged by a relative the previous week, so all was in a state of readiness. We slept for five hours and got ready to work.

Over breakfast, which we prepared by propane light, we discussed the plan. We had 10 LMH-2 lights, each equipped with Microchip's SEPIC controllers, 120' of conduit and plenty of wire. So we decided to partition the lights, as indicated in table 1.

Cabin's lighting system

Table 1: Cabin lighting plan.

This arrangement provides us with excellent light in the two most used locations (kitchen and dining room), and provide somewhere between good and great light everywhere else. The power distribution box I constructed has six available fuses. We decided to pull the wire into the attic for all six fuses, and to save some wire for future expansion. The wiring plan was to run no more than five primary lights on each branch, which we would accomplish by grouping the lights, as shown in table 2.

 Cabin wiring plan

Table 2: Cabin wiring plan.

Day 1
After breakfast, we started work by hanging the power distribution box and the power entry box. The power entry box is a weather-resistant box with brass contacts for attaching a 12V power source. Its brass contacts are large enough for automotive jumper cables. The power box was mounted on the outside of the cabin, close enough to where the vehicles are parked to "jump" the cabin with jumper cables. The power distribution box is located inside the cabin, and contains a fuse block, Watt-meter and master power switch. The fuse block supports six fuses. The wateter was originally designed for RC modelling, which seemed perfect for monitoring this power system.

After mounting the power boxes, we greased six wires and shoved them down the short conduit from a junction box in the attic into the power box. We placed ring terminals on the ends and wired up the box. The attic was now ready for wire.

The next order of business was to drill holes in the ceiling for lights in the bunk room. We drilled the holes and ran the conduit. Some surface-mount Panduit was run down the wall for the light switch, and an always-hot line to the night-light dimmer. The night-light was built from a long strip of 12V white LED tape. These tapes are 24W per 5m and consist of three LEDs and a resistor every 3 inches or so. You can cut them to length in multiples of the three LED circuit.

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