Bypass Diodes vs Blocking Diodes: What are the differences?

The solar industry often sees the terms “Bypass Diode” and “Blocking Diode” used interchangeably. The truth is that they are not interchangeable at all, but have very different functions in a photovoltaic system.

Bypass diodes are diodes found on solar panels that shunt current around underperforming or faulty sections of a solar module that affect the module's overall output.

However, blocking diodes are installed on a combiner box to prevent reverse current flow through a solar module. They are installed on different components of a solar power system and serve entirely different purposes.

What is a Bypass Diode?

A bypass diode is a special type of diode that is designed to bypass the current around underperforming or faulty sections of a solar module. It allows all modules to operate at an optimal level without affecting the output of other modules.

When a part of a solar cell is shaded or defective, that section is unable to produce current as it should. This may result in a drop in power output from the entire module. By using bypass diodes, a string of connected cells or panels can continue to supply electricity at a lower voltage rather than no power at all.

Every solar panel on the market today has at least one bypass diode or more. It is a standard component included in the manufacturing process. You can see bypass diodes by opening up the junction box on the back of your solar panel.

The number of diodes indicates the number of strings of cells on a solar panel. This is not the same as how many cells are on a panel. The bypass diodes will be placed across every string of cells in the solar module, so if there are four sets of cells, there will be four bypass diodes.

Bypass diodes are wired parallel to each string to ensure that if one of the cells in the string doesn't produce any power, the remaining cells' power can still be used.

The purpose is to prevent current from having to flow through the series of cells when one cell in a string fails or produces no power for some reason, such as shading and bad weather conditions (rain and snow).

How does a Bypass Diode work?

Cells in solar panels are connected in series to form strings. Within each string, the current flows from cell to cell in a single direction (from the positive terminal of one cell, through the negative terminal of that same cell, and into the negative terminal of the next successive cell).

A bypass diode is installed across every string so as to allow current flow even if any one or more cells become shaded.

A bypass diode functions by allowing current to flow in the direction that will allow the maximum voltage possible. This is achieved by providing a low resistance path around the part of the cell that is shaded or defective.

When sunlight hits an individual cell it generates a small amount of power. If the cell is ‘shaded' or defective in any way it cannot generate power. However, it is not the main problem that affects the performance of a solar array.

The problem is the series of cells that are ‘shaded' or defective could cause voltage drops along the entire line and potentially shut down your system completely which is why it's important to have bypass diodes in solar panels.

What is a Blocking Diode?

A blocking diode prevents current from flowing in the reverse direction. It is an essential component in solar panels that stops your solar array from discharging at night or when the batteries are full.

Blocking diodes are also known as ‘rectifiers' because they allow current to flow in one direction and prevent the reverse flow. The simplest way to think of a blocking diode is like a check valve on your water sprinkler that only allows the water to go up rather than down.

In the past, blocking diodes were often used in solar power systems because they could do a much better job of protecting batteries from over-charging.

In modern designs using MPPT charge controllers and lithium-based battery technologies, this is less important as your system can withstand hours or even days without being under load which means that there isn't often a concern about over-charging.

Blocking diodes are relatively cheap and easy to install, so it is still common for people to use them in their solar power systems despite the fact that they can limit system performance without adding any real benefit.

How does a Blocking Diode work?

Blocking diodes are connected in parallel with the solar panels (or another charging source). When they are working correctly, this will prevent current from flowing backward through the panel when it is dark or low on voltage.

However, blocking diodes has a secondary purpose that keeps them useful. If you have arrays facing various directions, you can use a blocking diode to ensure that when those strings are connected in parallel, the voltage of one does not drag down the voltage of another.

Assume you have a roof with two solar arrays built on it, one on the east and one on the west side of your roof. One of your arrays will generate more power than the other in the morning.

When you have two parallel arrays, you want their voltages to match; otherwise, if one of them is underperforming, you can actually lower the voltage of both arrays.

A blocking diode prevents this from happening by allowing current to flow in only one direction from each string.

When you use blocking diodes correctly, you'll produce more power over the course of the day. However, if all of your solar panels are pointed in the same direction and tilted at the same angle, a blocking diode is pointless.

It is good to have blocking diodes on the combiner box. However, any diode has resistance and will lower the energy production of the array by a couple of watts.

However, keep in mind that the inclusion of blocking diodes in a combiner box implies that people are connecting their strings in parallel.

If you only have a single series string and are connecting it to a single MPPT solar charge controller, no blocking diode is required.

We only employ a blocking diode when the strings are joined in parallel. If you have a single series string, you should never use a blocking diode.

How do bypass and blocking diodes compare?

Conclusion

Bypass diodes and blocking diodes are different in their operation and in what they are intended to accomplish. Bypass diodes are installed in the parallel configuration on the solar panel while blocking diodes are installed in a series configuration on the combiner box.

Bypass diodes serve as a shunt path to allow current to bypass the faulty or underperforming module. Blocking diodes prevent current from flowing backward through the string, discharging the battery at night or other times when the solar panels aren't working.