DIY Solar Panel Build – Part 4 of 4

Part 4 – Solar Panel Electrical Connections, Junction Box, Schottky Diode and Testing

Step 13 – Assembling the junction box

For my junction box I purchased a Radioshack project enclosure P/N 270-1801 which is 3″ Long x 2″ Wide x 1″ Deep.

Junction Box Position

Junction Box

I placed the junction box on the back of the frame, 3/8 inch from the left hand edge (right hand looking from the back), and oriented lengthwise along the centerline directly below the frame divider. I drilled 3 holes through the bottom of the box and secured the box to the frame with 3 screws.

I needed to route four wires into the box. Two from the cell array and two to be routed outside to connect to the load. To connect to the cell array I drilled two 3/16 inch holes in the bottom of the box and through the frame, one each side of the divider. To connect to the external load I drilled a further two 3/16 inch holes in the side of the box. I would later route 12 gage connecting wires into and back out of the box.


Step 14 – Installing the terminal block

Terminal BlockI used a Radioshack terminal block P/N 274-658 to make the connections in the junction box. I wanted to install the terminal block lengthwise in the junction box but I realized that this would not allow me enough room to properly fasten the connecting wires. However, the block was two long to fit sideways so I had to cut it to fit. I trimmed a little off each end with a small hacksaw and placed it in the box.

Junction Box With Terminal BlockHere is the terminal block placed in position in the junction box.


Step 15 – Installing 12 gage bus wires

Junction Box With Solar Panel WiresI cut 2 12 gage wires to 30 inches in length, one red and one black. On one end of each wire I installed an 8 gage ring terminal. On the other end I stripped off about 1 inch of insulation and tinned the exposed wire. I passed the wires through the junction box and into the frame where the cells were installed making sure to install the red wire to the positive end of the array and the black wire to the negative end. I left about 4 inches of wire in the box to make the connection to the terminal block.

Solar Panel Wire RoutingTo route and secure the wires within the frame I used 3/16 plastic wire clamps with a small modification. I removed the nails from the clamps and drilled larger holes to accept a small screw. I then secured each wire in four places.

Solar Panel Positive Bus Wire Connection
Solar Panel Negative Bus Wire Connection

To make the connection to the cell array I soldered the wires to the preinstalled bus wires. I checked continuity again, this time at the junction box. All was good.


Step 16 – Installing the lead out wires and Schottky diode

Junction Box With WiresI cut 2 more 12 gage wires, this time 36 in length and again installed an 8 gage ring terminal on each wire. These wires were passed trough the holes in the side of the box once again leaving sufficient slack inside the box to make the connection to the terminal block.

Final Wiring Of Junction BoxIt was now time to make all of the connections in the junction box and install the Schottky diode. I purchased a 40 volt, 5 amp diode P/N SR504 (can’t recall the manufacturer). The diode is used to prevent the batteries from “backflowing” current through the panel when it is producing less than battery voltage (nightime and other low light conditions). Notice that the diode is installed in the positive lead with the white band pointing towards the load. Sometimes this end is also marked with an arrow.


Step 17 – Final Testing

Completed Solar Panel Voltage OutputOn to the final test. I temporarily installed the acrylic front and closed the junction box (no final potting until I was sure my panel worked) and gave the panel a test. Here is the open circuit voltage of the panel in bright sunlight. 19.49 volts. Not bad.

Completed Solar Panel Current OutputHere is the short circuit current, also in bright sunlight. It’s a healthy 3.18 Amps. Not bad also.

I ran the panel for several days to make sure it would produce good output without any problems. Once I was satisfied all was good I sealed the panel:

(1) I removed the acrylic front cover, applied a bead of silicon caulk on the rim strips, then reinstalled the cover. To ensure that the silicon spread evenly and to prevent the cover from cracking I tweaked the cover screws so that at all times I was applying even pressure to the cover.

(2) I filled the junction box with caulk making sure that all the connections were completely covered. I allowed the caulk to fully cure then attached the cover plate.


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