Diode in action Circuit Clinic 3


What is the function of a diode in the circuit? It  simply act as a switch to pass current only in one direction from Anode to Cathode. So it is essential in all most all circuits. Diodes are semiconductor devices with a p-n junction to allow the one way flow of current. This condition is the forward biasing. If the current flow is reversed, the diode enters into the reverse biased mode and prevents the flow of current. Diodes perform many functions in an electronic circuit like rectification, clamping, protection, voltage multiplication, signal detection etc. Major types of diodes are power diodes, signal diodes, zener diodes, photo diodes, LED etc. Schottky diodes are fast recovery diodes ideal for switching applications.

Diodes can be easily identified by its shape, colour etc.


The leads can be identified using the marking. There is white or black marking at one end of the body of diode. That side is cathode or negative. Typical forward drop  is 1.1 volt in 1N4000 series but in 1N4148, it is 0 .7V


Common uses of diodes

Power diodes are mainly used for rectification and switching applications. 1N 4000 series diodes are most commonly used. These have 1 ampere current capability. The difference in the members of the series is the voltage rating. Easily available type is 1N4007 so you can use that in circuits, if the power supply current is less than 1 ampere. For higher current, use 1N5402 or other high current types depending on the use. Rectifier module is also available which can handle high current.



Diode is essential in power supply circuits to rectify AC to DC. A single diode in the rectifier act as half wave rectifier and rectifies only one cycle of AC while 4 diodes bridge will act as a full wave rectifier. The rectified DC contains some fraction of AC, so we use a filter capacitor to remove the ripples and to produce clean DC.


The figure shows the Half wave rectification. See the direction of current flow. From one secondary winding of the transformer to the output.

full wave

Here 4 diodes form a bridge for full wave rectifier which is more efficient than the half wave rectification. So it is ideal to use full wave Rectifier Bridge in the power supply.


Signal diodes are used in signal detection circuits like radio. These diodes rectify the radio signals available in the atmosphere. A signal diode in conjunction with a capacitor-coil tank circuit functions as a radio signal detector. Typical signal diodes are OA71, OA91 etc.


A power diode is essential to prevent damage of the relay driver transistor or SCR. When inductive loads are connected at the collector of a transistor, and when current flows, high magnetic flux develops in the coil. When the relay switches off this back e.m.f destroys the transistor. So a diode is essential to drain this back e.m.f. In the diagram, D2 performs the function.


This is a Diode pump circuit using two 1N4148 diodes, two capacitors and a 22K resistor. This kind of diode pump is used in circuits sending low AC signals from the front end as in the case of a mic. Diode Pump is a rectifier circuit that makes a varying AC signal output to a DC voltage relative to the peak-to-peak voltage across the AC waveform. By including the Diode Pump between the circuit giving AC output and a Driver circuit, the DC voltage generated can be maintained at the level of the AC signal. That is no voltage drop develops.

Each component has its own role in the circuit.                                                                

C1– This capacitor receives the AC signals from the Audio and transfers the full AC wave form across it. But the midway level is still the same, therefore only the voltage above 0 volt passes through the diode D2.

D1 and D2 – When the waveform from C1 tries to go below 0 volt, D1 takes current from the negative rail. Therefore voltage never goes below 0 volt at the junction of C1 and D1. When the waveform from C1 rises, the rise is from minimum (0 volts) as imposed by D1. Since the waveform is not changing, the total voltage between the upper and lower peak of wave is then pumped into diode D2 and then into C2. But the actual DC voltage from D2 will be slightly lesser than the full peal-to-peak voltage due to the forward voltage drop of D1 and D2.

C2– This capacitor stores the current pumped through D2. It acts as a buffer and prevents the fluctuation of DC as the AC waveform swings. Value of C2 depends on the input current as well as the current to be stored. That is, 1uF is an optimum value but it can be increased to 10 uF depending on the application.

R1 – when the input signal ceases, the stored current from C2 will discharge through R1.Value of R1 is selected to suit the value of C2. A higher value resistor will discharge the capacitor slowly when the input signal ceases.


This is a voltage multiplier using diodes and capacitors. A voltage multiplier is a specialized rectifier circuit producing an output which is theoretically an integer times the AC peak input, for example, 2, 3, or 4 times the AC peak input. Thus, it is possible to get 200 VDC from a 100 Vpeak AC source using a doubler, 400 VDC from a quadrupler.

Wait for the next article How a Zener is used?

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