How to Use Diodes in Electronic Circuits?

Diodes are one of the very important components used in electronic circuits. These are one which help to safeguard your circuits.

For now, we will ignore what happens inside these diodes (e.g., p-n junction, holes, etc), which makes them work as they do. Let’s keep it for upcoming articles, but making its use in our project circuits is what we need, and that’s what we will be covering in this article.

Table Of Contents

1. Diodes have Polarity!
2. Using Rectifier Diode
3. Conversion and Process of Rectification
FAQ Section

1. Diodes have Polarity!

As we have already discussed about Resistors in our previous articles (Dealing with Resistors: Part 1 and Combining Resistors: Part 2 ), we know that Resistors don’t care in what way you are connecting them in circuits.

BUT for Diodes, it’s not the same! The way you connect the diodes in our circuit does matter. Hence, we have the terminologies of cathode and anode for a Diode.

The figure shows how to identify the cathode and the anode for a diode

2. Using Rectifier Diode

There are many types of diode but the most basic one is the rectifier diode. Diodes mainly behave like one-way valves. One-way valves only allow fluid to flow in one direction. If it flows in one, it can’t come or flow back in the opposite direction.

one way valve is used as an analogy to explain the working of diodes — One-way valve

This is just a common analogy. You can say that the diode offers very little resistance to current in one direction, while it offers a very high resistance in the other direction, making the current difficult to flow in the other direction.

Understanding the flowchart below will help :

The circuits below show the Forward Bias and Reverse Bias Configuration for a Diode:

circuit diagram for forward bias — Fig. Diode in Forward Bias (current flows)

circuit diagram for reverse bias — Fig. Diode in Reversed Bias (NO current flows)

For connecting the Diode in Forward Biased Configuration :

Connect the anode of the diode to a higher voltage and the cathode to a lower voltage

And for Reversed Biased Configuration :

Connect the anode of the diode to the lower voltage and the cathode to the higher voltage

Diodes have a ‘constant’ voltage drop

We know that the voltage drop across a Resistor depends on the current passing through it.

Unlike Resistors, Diodes have a fixed voltage drop that doesn’t change with the amount of current passing through them.
Generally, it is around 0.5V but depends on diode to diode. Checking the datasheet before calculations always helps!

By applying Kirchhoff’s Law in the circuit loop above, we get the voltage drop across Resistor R1 to be 8.5V. From this information, we can calculate the current in the circuit ‘i’ :

3. Conversion and Process of Rectification

Diodes can also help to convert AC Voltage to DC Voltage. Recall that AC has both a positive component and a negative component. If a diode allows one part to go through (say positive), the negative part won’t be allowed.

A picture will help in better understanding :

The voltage versus time graph for an AC source before it passes through a diode — Fig. AC Source (Before passing through Diode)

The voltage versus time graph for an AC source after it passes through a diode — Fig. Voltage Source after passing through Diode (somewhat like DC)

The Voltage source finally does have just a positive part left (unidirectional like DC), but still, it’s not constant. It does have some fluctuations. To smooth this out and obtain a near constant voltage :

Connect a Capacitor in parallel to a Resistor across the voltage source

BUT how does this combination of Resistor and Capacitor help to smooth out the given voltage signal?

Answer :

The capacitor smoothens out the fluctuations by charging and discharging in response to the ‘changing input voltage’.

When the input voltage starts rising, the capacitor charges up to store energy and matches the input voltage very fast
When the input voltage begins to fall, the voltage across the capacitor doesn’t decrease rapidly even if the input voltage falls at a faster rate. The capacitor discharges very slowly, releasing its stored energy.

Hence, this process helps to decrease the fluctuations and hence obtain a near DC-like voltage signal

FAQ Section

What is a diode?

A diode is a two-terminal semiconductor device that allows current to flow primarily in one direction. It consists of an anode and a cathode and is widely used for rectification, switching, and circuit protection.

How does a diode work?

For practical purposes, it conducts when forward-biased (anode positive, cathode negative) and blocks current when reverse-biased. If we go into some depth, it works based on a p–n junction. In forward bias, the applied voltage reduces the junction barrier, allowing current to pass. In reverse bias, the barrier widens and blocks current, letting only a small leakage current flow.

How do I find a diode’s polarity?

The cathode is usually marked with a line or band on one end. In LEDs (light emitting diodes), the shorter lead is the cathode, and the flat edge on the casing also indicates the cathode side. For SMD diodes, a printed bar or stripe marks the cathode.

Where are diodes used?

Diodes are used in power supplies for rectification, in circuits for voltage protection, in logic and switching applications, and in signal processing. LEDs, Zener regulators, and Schottky diodes are some of the specialized forms used for lighting, voltage regulation, and fast switching, respectively.

Refer to the Following Sections of the Articles for more :

Learning about Capacitors (Sections 2 and 3)
Low and High Pass Filter Circuits