What is a rectifier bridge?

The topic of this article is the bridge rectifier, but what is it? For many people, there is a huge complexity involved in this subject, but we are here to clear up any doubts and explain how the rectifier bridge works.

Voltage rectifier bridge – What is it?

The voltage rectifier bridge transforms an alternating voltage into a continuous voltage. In this quoted phrase we have 5 words that are very important to understand this concept, they are Bridge, rectifier, voltage, alternating, and continuous.

To explain the importance of each one, let’s start with tension. It is popularly known as voltage, which comes from the English “voltage”, but the most correct is to use the term voltage. When you say that the outlet of your house is 127V, you are saying that the voltage in the outlet is 127V.

Therefore, the alternating voltage means that the voltage value alternates between positive and negative all the time. At one moment it has a value greater than zero and at another less than zero. You may not know it, but the voltage in the socket changes between negative and positive 60 times per second!

We have a small problem here, most electronic devices need DC voltage to work and not AC! Continuous tension is one that remains positive all the time, that is, always greater than zero, but that does not mean that it must necessarily be constant!

Let us now explain the term rectifier! If you don’t know, rectifying means “straightening” or aligning. The rectifier bridge acts maintaining the “straight” alternating voltage, that is, continuous voltage! Now we just need to explain the term bridge, right? It is believed that the name bridge is related to the Wheatstone bridge, which is made with resistors and not diodes, but has a very similar design as you can see in the image below!

See the reason for the name rectifier bridge!

Ready! Now that all the main terms have been clarified, we can continue with the explanation, delving further into the way the bridge rectifier performs its function! For this, let’s take as an example some alternating voltage source to explain the operation of the rectifier bridge. This alternating voltage can come from, for example, a socket or a transformer output.

An alternating voltage source at one moment has a positive voltage and at another the voltage is negative. Each part is a semi cycle, that is, the voltage alternates between positive and negative semicycles, as shown in the image below.

What is the main characteristic of AC voltage?

There are rectifier bridges ready to be used, but it can also be mounted using 4 diodes, which is the case of the example we will show. If you don’t know, diodes are electronic components that conduct electricity in one direction only. We have already discussed the diodes in detail in another article here on the website, we recommend this complimentary reading.

The interesting thing about the rectifier bridge is how the diodes are connected together, each in a strategic position! You need to understand that the electric current passes through the diode in the direction of the drawing arrow on the diode, that is, the current goes from the anode to the cathode, as shown in the following image:

Understand the importance of the polarity of the diodes for the correct operation of the rectifier bridge!

An interesting detail is that there will only be electrical current if the voltage at the anode was greater than at the cathode. Never forget that detail! So, when we switch on the AC voltage on the rectifier bridge, see what happens! When the voltage is positive at the top, the electric current generated reaches the point p1 and needs to decide between D1 and D2.

How does the current flow through the diodes on the rectifier bridge?

Remember the allowed current direction in the diode? So in this case, the permitted path is through D1, following the current direction in the diode.

How does the current flow through the diodes on the rectifier bridge?

Now at p2 the current cannot flow through D3, as the direction is prohibited! So the current flows through the resistor until it reaches p3.

How does the current flow through the diodes on the rectifier bridge?

Now in p3 both D2 and D4 are in the right direction to conduct current. The energy could return through D2, but this is not because the other side of D2 is also positive. Soon the current flows through D4!

How does the current flow through the diodes on the rectifier bridge

Upon reaching p4, the current could return through D3, but this is not because the other side of D3 is positive too! Thus the current flows back to the source, closing the cycle.

How does the current flow through the diodes on the rectifier bridge?

Now, when the voltage switches to the negative semi cycle, you can see that we have a different path from the current! The current arrives at p4 and you need to decide between D3 and D4.

How does the current flow through the diodes on the rectifier bridge?

As the direction via D4 is not allowed, it goes through D3, always remember the arrow.

How does the current flow through the diodes on the rectifier bridge?

Upon reaching p2, there is no way to flow through D1 so the current passes through the resistor to p3.

How does the current flow through the diodes on the rectifier bridge?

Arriving at p3, you can’t go back via D4 because D4 also has a positive on the other side, so the current goes through D2 to p1.

How does the current flow through the diodes on the rectifier bridge?

At p1, as on the other side of D1 it is also positive, the current goes to the negative of the source!

How does the current flow through the diodes on the rectifier bridge?

I don’t know if you noticed, but the current in the resistor was in the same direction, from left to right, both in the positive and negative cycle, see in the image below. In this way, the energy that entered alternating always appears positive and in the same direction at the output of the rectifier bridge.

How does the rectifier bridge work?

What is the direction that the current flows through the diodes in the rectifier bridge?

But what about making the tension really continuous at the output? For this, it is necessary to use a capacitor, placing it in parallel with the resistor. Once this is done, it will charge and fill up with energy every time the voltage rises and the moment the voltage starts to descend it is the capacitor that will send its energy to supply its circuit, leaving the voltage at the continuous output or more as close as possible to this!

If you want to understand even more all of this that we talked about here, we indicate below a video of the World of Electrical. In it you will visualize all this path of the current in a very dynamic way, besides checking several tips and curiosities about the rectifier bridge.

If you want to learn more about capacitors, we’ve also done an article about them here on the site, all of these additional readings are important to further improve your knowledge. If you have any questions about bridge rectifiers, leave them in the comments and we will answer them.