The Different Applications of Diodes

Diodes are used in many applications for example diodes are employed in reverse-polarity protection, used in voltage regulators, etc. We discuss these applications plus more in this article.

Here are some of the different applications of diodes:

Transient Suppression with Fly-back diodes

Transient Protection

When current flowing through an inductor is suddenly switched off, the collapsing magnetic field will generate a high-voltage spike in the inductor’s coils. This voltage spike or transient may have amplitude of hundred or even thousands of volts. This is specifically common in relay coils. A diode called fly-back diode placed across the relay’s coil can protect the neighboring circuitry by providing a short circuit for the high-voltage spike.

The fly-back diode also protects the relay’s mechanical contacts, which often get roughly shut during an inductive spike.

Nevertheless, the diode is ineffective during turn-on time. Ensure you choice a rectifier diode with sufficient power ratings like IN4001, 1N4002, or equivalent diodes.

Transient Relay Driver with Protection Diodes

This arrangement is better than the one discussed above. Here we have an extra diode placed across the transistor driver in order to protect the transistor from damage due to inductive spikes generated from the relay’s coil when the transistor is turned off. In addition, this arrangement stifles spikes during turn-on time.

This arrangement is sometimes employed in voltage regulator circuits where one diode is wire from the output to the input and another from the ground to the output. This prevents any attached loads from sending damaging spikes back into the integrated circuit (IC) output.

Voltage Dropper

When current passes through a diode there is a voltage drop across it approximately 0.7 V for silicon pn junction and 0.3 V for germanium diodes. By placing diodes in series, the total voltage drop across the combination is the sum of the individual voltage drop across each diode.

Voltage droppers are commonly used in circuits where a fixed small voltage difference between two sections of a circuit is required. Contrary to resistors that can be employed to lower the voltage, the diode arrangement usually doesn’t waste as much power to resistive heating and can supply a stiffer regulated voltage that is less dependent on current variations.

Voltage Regulator

Employing three diodes to create a regulated (steady) voltage output equal to the sum of the threshold voltage of the diodes i.e. 0.7 V + 0.7 V + 0.7 V = 2.1 V. The series resistor is used to set the desired output current (I) and should be less than the value calculated using the following formula but not so low that it exceed the power ratings of itself and the diodes.

The diodes and series resistor must have proper ratings, for the amount of power drawn. You can use IN4148 diodes but for the higher power critical voltage sources, a Zener diode regulator or special IC is employed.

Reverse-polarity Protection

Power polarity reversal or battery reversal can be harmful to portable equipment. Normally a mechanical block can be employed to safeguard against reverse installation though a small mistake in making contacts can be problematic. This can happen in applications that use one or more single-cell AA-alkaline, NiMH, and NiCad batteries. To prevent damage due to reverse-polarity, you must ensure that any flow of reverse current is low enough to avoid causing harm to the circuit or battery. These are two ways this can be accomplished:

• With a series diode
• With a parallel diode

Series Diode

This is the simplest battery-reversal protection. It can be employed with external power connections. The diode allows current from a correctly installed battery to flow to the load, but blocks the current from a backward-installed battery.

The shortcomings with a series diode method are that the diode must handle the full load current, and the forward voltage drop of the diode shortens the equipment’s operating time. Schottky diodes with low thresholds can do better.

Parallel Diode

For applications that require alkaline or other batteries that have high output impedances, you can protect against reverse installation by using a parallel (shunt) diode, while eliminating the diodes voltage drop.

This method protects the load but draws high current from the installed battery. The diode must be properly rated for current and power.

Note, in complex battery-powered systems, special ICs or transistor arrangements are employed to provide the required zero voltage drop protection, while providing other features like reverse polarity protection, voltage level monitoring and thermal shutdown.

Diode Clamps Circuits

Diode clamps are used to clip signal levels or they can shift an AC waveform up or down to create what is termed as a pulsing dc waveform i.e. one that doesn’t cross the 0 V reference. Some of these circuits include:

• Clipper
• Attenuator
• Diode voltage clamp (dc restoration)
• Half-wave and full-wave rectifiers

The Adjustable Waveform Clipper

In this circuit, the maximum output is clipped to a level determined by the resistance of the potentiometer. The goal is to set the negative end of the diode to about 0.7 V lower than the maximum desired output level, to account for the forward voltage drop of the diode. That is what the potentiometer is expected to do. The +ve should be a volt or higher than the peak input voltage.

1N914 diode maybe used or any other equivalent diode.

Adjustable Attenuator

The additional diode in this circuit allows for clipping on both positive and negative swings. A single potentiometer is used though if you want separate positive and negative clipping level control, you can use separate potentiometers.

The +ve should be a volt or higher than the peak input voltage.

Related: Semiconductor Diodes

DC Voltage Clamp (DC restoration)

The diode voltage clamp provides dc restoration of a signal that has been AC-coupled (capacitive coupled) e.g. in a transistor with grounded emitter otherwise an ac-coupled signal would fade away.

Half-wave Rectifier

This circuit is used to transform an AC signal into a pulsing dc by blocking the negative swings.

A filter is usually added to the output to smooth out the pulses and provide a higher average dc voltage. The half-wave rectifier is seldom used in 60 Hz rectification other than for bias supplies. However, it is has a considerable use in high frequency switching power supplies termed to as forward converter and fly-back converter.

Full-wave Center-tap Rectifier

This is basically two combined half-wave rectifiers that transform both halves of an ac wave into a pulsing dc signal. The average voltage is 0.9 V_rms of half of the transformer secondary; this is the maximum that can be obtained with a suitable choke-input filter. The peak output voltage is 1.4 x V_rms of half the transformer secondary; this is the maximum voltage that can be obtained from a capacitor-input filter. The peak inverse voltage (PIV) impressed on each diode is independent of the type of the load at the output. This is because the peak voltage condition occurs when diode A conducts and diode B does not conduct.

As the cathode of diodes A and B reach a positive peak (1.4 V_rms), the anode of diode B is at the negative peak, also at 1.4 V_rms but in the opposite direction. The total peak inverse voltage is therefore 2.8 V_rms.  The frequency of the output pulses is twice that of the half-wave rectifier and thus less filtering is required compared to half-wave rectifier. Because the diodes work alternatively, each diode handles half the load current, the current rating of each rectifier need only to be half the total current drawn from the supply.

Full-wave bridge rectifier

This full-wave rectifier doesn’t require a center-tap transformer. Note, there is at least 1.4 V drop from zero-to-peak input voltage to zero-to-peak output voltage (i.e. there are two 0.7 V drops across a pair of diodes during a half cycle). The average dc output voltage into a resistive load or choke-input filter is 0.9 V_rms of the transformer’s secondary. With a capacitor filter and a light load, the maximum voltage is 1.4 V_rms. The inverse voltage across each diode is 1.4 V_rms. The peak inverse voltage (PIV) of each diode is more than 1.4 V_rms.

Full-wave center-tap rectifier vs. Full-wave bridge rectifieR

Full-wave center-tap rectifier and the full-wave bridge rectifier, have the same rectifier requirements, however the center-tap version has half the number of diodes, as the bridge version. These diodes will require twice the maximum inverse voltage ratings of the bridge diodes i.e. they have a PIV greater than 2.8 V_rms compared to 1.4 V_rms of bridge diodes.

The bridge rectifier makes better use of the transformer’s secondary than the center-tap rectifier since the transformer’s full winding supplies power during both half cycles, while each half of the center-tap circuit’s secondary provides power only during its positive half-cycle. This is typically referred to as the transformer utilization factor, which is unity for the bridge configuration and 0.5 for the full-wave center-tap configuration.

The bridge rectifier is not frequently employed in high-current, low-voltage applications as compared to full-wave center-tap rectifier which is very popular in these types of applications. This is due to the fact that, the two forward-conducting series diodes voltage drops in the bridge introduce a volt or more of additional loss, and thus consume more power (heat loss) than a single diode would within a full-wave center-tap rectifier.

Related: Optoelectronics

Diodes in AM Detectors

Diodes are usually employed in the detection of amplitude modulated signals as illustrated in the simple AM detector below.

The diode is used to rectify out the negative portion of the incoming signal so that it can be in form where it can be changed more in the next stages of the AM detector.

Related: Types of AM Radio Receivers