Full-Bridge Rectifier Visualization Using LEDs and the Digilent Analog Discovery

This education brief demonstrates the fundamental operation of the full-bridge rectifier by using LEDs and the advanced features of the Digilent Analog Discovery instrument. The conventional 1N4001 rectifiers are replaced with high efficiency red LEDs. The Digilent Analog Discovery serves as both function generator and oscilloscope. Together, the LEDs and advanced functionality of the Digilent instrument allow the student to interact with the power supply to visualize diode current as shown in Figure 1 with the full equipment setup shown in Figure 2.

Video 1 introduces the experiment, with an emphasis on configuring the Digilent Analog Discovery to produce a slow AC signal so that the learner can observe the LEDs. It also shows additional experiments such as the capacitor charging waveform, failed (open) rectifier, full-wave vs half-wave rectification, and the use of the Digilent Analog Discovery differential oscilloscope inputs.

What is a full bridge rectifier?

The full-bridge rectifier is a fundamental building block found in AC to DC power supplies. It consists of four semiconductor diodes arranged in a bridge circuit to convert the AC input signal to pulsating DC. A rudimentary power supply is formed when we add a filter capacitor to filter the pulsating DC thereby providing a steady DC signal.

Figure 1: LEDs may be used to visualize the operation of a full bridge rectifier. At any given time, the current flows through a pair of diodes.

Tech Tip: LEDs are not efficient rectifier as they have a high forward-biased voltage drop. While a diode such as the jellybean 1N4001 may have a 0.7 VDC drop, the typical red LED is much higher at approximately 1.9 VDC. Recall that the power lost in a conducting diode is calculated as the product of forward voltage drop and current. Consequently, diodes with low forward voltage are preferred.

Be mindful of this higher voltage drop especially when viewing the pulsating (unfiltered) full-wave rectified voltage. The waveform will display a flat spot when the diodes pairs are not conducting.

Video 1: Learn more about the LED visualization as well as the Digilent Analog Discovery.

Figure 2: Setup for the full-bridge rectifier test including the Digilent Analog Discovery and the LED-based full bridge rectifier on a breadboard.

Tech Tip: The Digilent Analog Discovery is a family of test instruments. The legacy model was featured in the video. While the latest Analog Discovery is shown in Figure 2. The Professional mixed signal model 2230 could also be used in this experiment. In all cases the instruments use the Digilent WaveForms software.

How does a full-bridge rectifier function

The full-bridge diode consists of four diodes wired as shown in Figure 3. The bridge is typically driven by the secondary winding of a transformer. In this experiment, the transformer has been replaced by the Digilent Analog Discover waveform generator (yellow and black wires).

When the source produced the positive half of the AC waveform diodes D2 and D3 conduct as shown in Figure 1 (left). Later, on the negative half cycle, diodes D1 and D4 conduct.

We can summarize the operation using a mathematical piecewise observation:

  • for positive half cycle diodes pair 2 and 3, V_{OUT} = V_{in} - (2 * V_{Diode})

  • for negative half cycle diode pairs 1 and 4, V_{OUT} = - V_{in} - (2 * V_{Diode})

At no point do the D1 and D3 conduct. Likewise, diodes D2 and D4 are never on at the same time as this would short circuit the source. Normally this shoot through current is not a problem as the 60 Hz (American line frequency) is slow relative to the diode turn on and off times. However, the designer must select diodes that can withstand the reverse bias voltage. In this application the Peak Inverse Voltage (PIV) is equal to the peak voltage of the input. In practice we add a safety factor of about 2 to accommodate input voltage variations, component tolerances, and the noise spikes.

Figure 3: Schematic of the full-bridge rectifier experiment. The multifunction Discovery instrument appears in three locations: once as a signal generator and twice as a differential input oscilloscope.

Tech Tip: The Figure 3 current limiting resistor has been moved into a new location when compared to the original video. This modification allows additional experiments to be performed such as simulating a failed (short) rectifier or an improperly installed rectifier. The current limiting resistor mitigates the potential damage to the Discovery instrument caused by an overload condition.

Tech Tip: The Digilent Analog Discovery features differential oscilloscope inputs. This feature improves the lab as the learner can measure floating signals independent of the circuit grounds. For instance, the Discovery’s black in Figure 3 is circuit ground to which a conventional oscilloscope probe would be connected. With differential oscilloscope inputs, the output of the power supply (blue and blue with white stripe) may be measured without considering the diode drop difference between the circuit ground (black) and supplies return (blue with white stripe).

Visualize the bridge rectifier operation using LEDs

We can visualize the current flow when the traditional bridge rectifier diodes are replaced with LEDs. The secret is to slow down the AC waveform so that the LED operation can be seen. Video 1 and Figure 4 show the operation at 1 Hz. The crisscross LED activation is clearly seen (Figure 1 left vs right).

Figure 4: Display from the Digilent WaveForms interface. The signal generator control is found in the lower panel while the dual channel oscilloscope in the upper. The Oscilloscope displays the input (orange) and output (blue) of the full-bridge rectifier.

Directly related demonstrations

The LED-based full-bridge circuit provides additional learning opportunities. A few ideas were showcased in the video including:

  • Loss of a diode (failed open)

  • Operation with and without the filter capacitor

  • Initial filter capacitor stair-step voltage charging waveform

  • Impact of operating frequency on the ripple voltage

  • Fully charged capacitor (no load)

Several additional demonstrations could be performed provided the 100 Ω resistor is repositioned as shown in Figure 3 and described in the tech tip. This change is necessary to protect the Digilent instrument again unintended short circuit. The new experiments include:

  • Failure of a diode (short circuit)

  • Incorrect placement of a diode

  • Impact of overloading the circuit

  • Short circuit of the load

An added benefit is the reinforcement of basic oscilloscope operating skills. For example, use of the single sweep function is the best way to capture the capacitor’s initial charging voltage waveform.

Parting thoughts

This demonstration is an effective method allowing learners to gain a deep understanding of the bridge rectifier. The combination of LEDs plus the Digilent Analog Discovery allows the student to interact with the circuit in multiple ways including the visual LED circuit operation as well as the oscilloscope display on the Digilent WaveForms software.
The natural next step is to use the technique to explore related circuits such as the voltage doubler and bootstrap circuit featured in a high-side MOSFET driver.

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APDahlen

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About this author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.