Maria Montero

Troubleshooting a diode bridge rectifier

This article will examine the different faults of a diode bridge rectifier to provide information on troubleshooting an AC / DC power supply.

AC / DC power supplies are widely used in different types of electronic equipment. When one fails, how can we determine the cause?

This article will walk you through an example power supply and explain some of the possible reasons why you might experience failure.

An example of AC / DC supply

In order to troubleshoot effectively, you will need to understand your circuit. We will work with an AC / DC supply example, which converts 230 VAC to 5 VDC. Its block diagram is shown in Figure 1 below.

Figure 1. Image courtesy of NUS.

First, let’s first do a brief review of each of these blocks.

Transformer

The transformer converts electricity from the high voltage grid to a lower AC voltage. For example, if we want to generate 12V DC, the transformer can be designed to generate an AC voltage of 22V amplitude as shown in Figure 2.

Figure 2

Rectifier

The rectifier converts the AC voltage to a DC voltage as shown in Figure 3. This is done by reversing the negative part of the AC voltage to generate a positive voltage. The result is a DC voltage because current can now flow in only one direction through a hypothetical load (not shown in figure). However, there are still large variations in voltage and current and it cannot be used as a DC supply to power electronic circuits. Figure 3 indicates a very important property of the rectifier output: since the negative part is reversed into positive values, the rectifier output is a periodic signal with a period that is half the period of the input. Therefore, if the input is a 50 Hz signal, the output frequency will be 100 Hz. This observation can be helpful when troubleshooting an AC / DC power supply.

figure 3

Filter

To get rid of large fluctuations, we apply a low-pass filter to the output of the rectifier. The filter will give waveforms similar to the red curves in Figure 4.

Figure 4

Regulator

Since there are still some ripples, we can apply the filter output to a regulator that uses feedback concepts to further suppress fluctuations and generate the desired DC voltage.

Let’s examine the faults related to the diode bridge rectifier and the low pass filter as shown in Figure 5.

Figure 5

Now that we are familiar with our example, we can begin to discuss some common problems that we may be asked to fix.