A common question when building your own solar panel is quite simply, how does a solar panel work? This article will answer one very important part of that question; what is the junction box and how does it work?
When a solar panel is manufactured, the final component is almost always the junction box. It is a plastic enclosure with its own mechanical assembly inside. The internal mechanism includes connectors and diodes. This box is usually bonded with silicone adhesive after it has connected to the electrical contacts located at the back-sheet. If for whatever reason you have a solar panel without this box or if the box was damaged and you need to replace it, connecting is as easy as plug and play. You just align the box to the back contacts and then insert it. But you must have already applied the silicone adhesive on the side of the box that touches the back-sheet prior to insertion.
The junction box usually comes with two PV wires and a pair of M.C.4. or M.C.3. connectors. One M.C.4./M.C.3. pair consist of a female connector and a male connector for identification of the negative and positive wire. Usually the male M.C.4./M.C.3. is for positive and the female for negative. There are a number of bypass diodes found inside the junction box and they protect the solar cells from receiving unwanted power that may damage them. There are also a series of metal contacts inside but only two areas are used for connection with the positive and negative PV wires. Connecting the wires to those contacts usually require soldering to ensure that the connection stays for the next 25 years or so. Once the wires are connected, they each pass through two small openings that leads outside the box. The wires are then stripped and terminated with an M.C.4. or M.C.3. MC stands for the name of the original manufacturer of this connector which was Multi-Contact USA and the numbers 3 and 4 identify the type.
The junction box is made of high-temperature thermoplastic of which the main component is either Poly(p-phenylene ether) (PPE) or Poly(p-phenylene oxide) (PPO). These two types of plastics are very similar and differs only in applications and they are usually mixed with another compound called polystyrene because in their pure form, PPE or PPO, they are very difficult to process. This composition together with how the box is designed are the basis on how this component is rated in terms of protection which is the IP Code. The IP rating of a junction box is usually I.P.65 or I.P.67. IP stands for “international Protection” or “Ingress Protection”. The first number pertains to solid objects and 6 means that the box has complete protection from foreign objects even dust to get inside it. The second number is for liquids, and 5 (in the case of I.P.65), means it is also protected from water (water-resistant), and 7 (in the case of I.P.67), means it is also protected even when immersed in water up to 1 meter deep (water proof).
The junction box also is rated in terms of voltage capability and there are usually two ratings, a 600 V or 1000 V rating which is dependent on the electrical mechanism of the box and the wire used.
Hopefully, you have enough information dealing with the junction box and we have helped solve one piece of the puzzle to, how does a solar panel work?