ClassX Video Lessons Youtube Channel The Engineering Mindset Full Bridge Rectifier – How to convert AC into DC power electronics
Working with electricity can be hazardous, so it’s crucial to have the necessary qualifications and skills before engaging in any electrical projects. One of the most widely used methods for converting alternating current (AC) to direct current (DC) is the full-wave bridge rectifier, which employs four diodes.
The full-wave bridge rectifier is set up with the AC supply connected between diodes 1 and 2, and the neutral line between diodes 3 and 4. The DC positive output is taken between diodes 2 and 3, while the negative output is between diodes 1 and 4.
During the positive half of the AC sine wave, current flows through diode 2, passes through the load, continues through diode 4, and returns to the transformer. In the negative half, the current flows through diode 3, passes through the load, continues through diode 1, and returns to the transformer. Although the transformer supplies an AC sine wave, the load experiences a rippled DC waveform because the current flows in a single direction.
To achieve a smoother DC output, filtering is necessary. A common approach is to add an electrolytic capacitor in parallel with the load. This capacitor charges during voltage increases and discharges during decreases, thereby reducing the ripple.
On an oscilloscope, the peaks of each pulse are visible, but the voltage no longer drops to zero; it declines gradually until the next pulse recharges the capacitor. Using larger capacitors or multiple capacitors can further minimize the ripple.
In a practical circuit, an LED turns off immediately when power is interrupted. However, if a capacitor is placed in parallel with the LED, it remains lit during interruptions because the capacitor discharges and powers the LED.
In another example, a lamp is used as the load. Initially, a small 10 microfarad capacitor makes little difference to the waveform. However, a 100 microfarad capacitor prevents the voltage dip from reaching zero. With a 1000 microfarad capacitor, the ripple is minimal, and at 2200 microfarads, the waveform is nearly smooth, suitable for many electronic circuits.
Multiple capacitors can also be used. A 470 microfarad capacitor shows some improvement, but using two capacitors in parallel significantly enhances the waveform.
When using capacitors, it’s important to include a bleeder resistor across the output. This high-value resistor safely discharges the capacitor when the circuit is off, preventing potential hazards. For instance, a 4.7 kilo-ohm resistor can be used to ensure the capacitor discharges quickly when the circuit is turned off.
Without a capacitor, the output voltage is lower than the input due to the voltage drop across the diodes. In a simple full-wave bridge rectifier, if the input is 12 volts AC, the output might be around 10.5 volts DC. Each diode causes a voltage drop of approximately 0.7 volts, resulting in a total drop of about 1.4 to 1.5 volts.
However, adding a capacitor across the output can increase the output voltage above the input voltage. This occurs because the AC input measures the root mean squared (RMS) voltage, not the peak voltage. The peak voltage is 1.41 times higher than the RMS voltage, and capacitors charge to this peak voltage, minus the diode voltage drop.
For circuits with larger loads, a more advanced filter involves placing two capacitors in parallel with a series inductor between them. The first capacitor smooths the ripple, the inductor maintains a constant current, and the second, smaller capacitor smooths out any remaining ripple.
To achieve a constant output voltage, a voltage regulator can be connected to the output. This setup, which includes capacitors on either side of the regulator, ensures a smooth DC output despite variations in the input voltage.
For a practical demonstration, a real circuit connected to a 12-volt AC supply can produce an output of around 5 volts DC. You can learn how to build your own voltage regulator in previous tutorials available on our platform.
For further learning, explore additional resources on electrical and electronics engineering. Stay connected with us on social media and visit our website for more educational content.
Gather the necessary components, including diodes, a transformer, and a load, to construct a full-wave bridge rectifier circuit. Test the circuit with an oscilloscope to observe the AC to DC conversion and the resulting rippled waveform. Document your observations and discuss the results with your peers.
Using the rectifier circuit you built, add different capacitors in parallel with the load to observe how they affect the ripple in the DC output. Record the changes in waveform with each capacitor value and analyze the effectiveness of each configuration in smoothing the output.
Utilize circuit simulation software to model a rectifier circuit with advanced filtering techniques, such as using inductors and multiple capacitors. Experiment with different component values to achieve optimal smoothing for various load conditions. Share your simulation results and insights with the class.
Create a voltage regulator circuit using a voltage regulator IC and capacitors. Connect it to the output of your rectifier circuit to maintain a constant DC voltage. Test the circuit under different input conditions and evaluate its performance in stabilizing the output voltage.
Conduct research on the importance of safety measures, such as using bleeder resistors, in circuits involving capacitors. Prepare a presentation to educate your classmates on potential hazards and best practices for safely discharging capacitors in electronic circuits.
Here’s a sanitized version of the provided YouTube transcript:
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Remember, electricity is dangerous and can be fatal. You should be qualified and competent to carry out any electrical work.
The most common method used is the full-wave bridge rectifier, which uses four diodes. The AC supply is connected between diodes 1 and 2, with the neutral between diodes 3 and 4. The DC positive output is connected between diodes 2 and 3, and the negative between diodes 1 and 4.
In the positive half of the sine wave, the current flows through diode 2, through the load, through diode 4, and then back to the transformer. In the negative half, the current flows through diode 3, through the load, through diode 1, and then back to the transformer. So, the transformer is supplying an AC sine wave, but the load is experiencing a rippled DC waveform because the current flows in just one direction.
In this example circuit, we can see the rectified waveform on the oscilloscope, but this is not a flat DC output, so we need to improve this by adding some filtering. Using a rectifier will result in a ripple in the waveform. To smooth this out, we need to add some filters. The basic method is to simply add an electrolytic capacitor in parallel to the load. The capacitor charges during the increase in voltage and stores the electrons; it then releases these during the decrease, which reduces the ripple.
The oscilloscope will show the peaks of each pulse, but now the voltage doesn’t decrease to zero; it slowly declines until the pulse charges the capacitor again. We can further reduce this by using a larger capacitor or by using multiple capacitors.
In this simple example, you can see the LED turn off as soon as the power is interrupted. However, if I place a capacitor in parallel with the LED, it remains on because the capacitor is discharging and powering the LED during the interruptions.
In this circuit, I have a lamp connected as the load. The oscilloscope shows the rippled waveform. When I add a small 10 microfarad capacitor, we see that it makes very little difference to the waveform. When I use a 100 microfarad capacitor, we see the dip no longer goes down to zero volts. At 1000 microfarads, the ripple is very small, and at 2200 microfarads, it’s nearly completely smooth. This would be fine to use for many electronic circuits.
We could use multiple capacitors as well. Here, we have a 470 microfarad capacitor, which has made some difference, but if I use two capacitors in parallel, we see the waveform is much more improved.
When using a capacitor, we need to place a bleeder resistor across the output. This is a high-value resistor that will drain the capacitor when the circuit is off to keep us safe. Notice with this circuit that when I switch it on, the capacitor charges quickly to over 15 volts, but when I switch it off, the DC output is still at 15 volts because there is no load, so the energy is still stored in the capacitor. This could be very dangerous if the voltage is high.
In this example, I place a 4.7 kilo-ohm resistor across the output. We see the capacitor charges up to 15 volts, and when I switch it off, the capacitor quickly discharges. The electrons flow through the resistor, which discharges the capacitor.
We can also see that without a capacitor, the output voltage is lower than the input voltage because of the voltage drop of the diodes. Here we have a simple full-wave bridge rectifier. On the input, we see there is 12 volts AC; on the output, we have 10.5 volts DC. The voltage on the output is lower because of the diodes. Each diode has a voltage drop of around 0.7 volts.
If we look at this circuit with a diode and an LED, we can measure across the diode to see a voltage drop of around 0.7 volts. The current in our full bridge rectifier must pass through two diodes on the positive half and two diodes on the negative half, so the voltage drop combines and is around 1.4 to 1.5 volts. That is why the output will be reduced.
However, if we were to connect a capacitor across the output, we will see that the output voltage is now higher than the input voltage. How is that possible? That’s because the AC input is measuring the RMS voltage or the root mean squared; this is not the peak voltage. The peak voltage is 1.41 times higher than the RMS voltage. The capacitors are charged up to the peak voltage and then release. There will be a small voltage drop because of the diodes, so the output is less than the peak input but will still be higher than the RMS input.
For example, if we had 12 volts RMS on the input, the peak voltage would be 12 volts multiplied by 1.41, which is 16.9 volts. There will be a 0.7 volt drop here and here, so 16.9 volts subtract 1.4 is 15.5 volts. The capacitors are charged up to this voltage. This is only an approximate answer, though; the amount of ripple and the actual voltage drop of the diodes will cause it to be slightly different in reality. But we can see that the output is higher than the input.
Another common filter is placing two capacitors in parallel with a series inductor between them. This is used for circuits with larger loads. The first capacitor smooths the ripple, the inductor opposes the change in current and tries to keep it constant, and the second capacitor, which is much smaller, will then smooth out the final remaining ripple.
Additionally, we can also connect a voltage regulator to the output. This is very common and allows some variation to the input but will provide a constant output voltage. This again has capacitors on either side of the regulator to ensure a smooth DC output.
Here we can see a real version connected to a 12-volt AC supply, and we see it has an output of around 5 volts DC. You can learn how to build your own voltage regulator in our previous tutorials. Links for this are in the video description down below.
Check out one of the videos on screen now to continue learning about electrical and electronics engineering. This is the end of this video. You can follow us on Facebook, Twitter, Instagram, LinkedIn, and of course, theengineeringmindset.com.
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This version removes any informal language and ensures clarity while retaining the technical content.
Full-wave – A type of rectification that converts the entire input waveform to one of constant polarity (positive or negative) at its output. – The full-wave rectifier is more efficient than the half-wave rectifier because it utilizes both halves of the AC cycle.
Bridge – An electrical circuit configuration used to measure resistance or to rectify AC to DC using four diodes. – The bridge circuit in the power supply ensures that the AC input is converted to a stable DC output.
Rectifier – An electrical device that converts alternating current (AC) to direct current (DC). – The rectifier in the circuit is crucial for powering DC motors from an AC source.
Capacitor – An electronic component that stores and releases electrical energy in a circuit. – The capacitor in the filter circuit helps to smooth out the voltage ripple after rectification.
Voltage – The electrical potential difference between two points in a circuit. – The voltage across the resistor was measured to determine the current flowing through the circuit.
Ripple – The residual periodic variation of the DC voltage within a power supply after rectification. – Minimizing ripple is essential for sensitive electronic equipment to function correctly.
Current – The flow of electric charge in a conductor between two points having a difference in potential. – The current flowing through the circuit was measured using an ammeter.
Transformer – A device that transfers electrical energy between two or more circuits through electromagnetic induction. – The transformer is used to step down the high voltage from the power lines to a safer level for household use.
Output – The electrical power or signal produced by a device or circuit. – The output of the amplifier was connected to the speakers to produce sound.
Diodes – Semiconductor devices that allow current to flow in one direction only, used in rectification and other applications. – The diodes in the rectifier circuit ensure that the current flows in the correct direction to produce DC voltage.
