A diode is an essential component in electronics that allow current to flow in only one direction. They are commonly used for rectification (changing AC to DC), voltage regulation, and protection in circuits. This quick reference covers diode symbols, polarity, and two common types: the 1N4148 (signal diode) and 1N4007 (rectifier diode).
Table of Contents
- Common Diodes for Beginners in Electronics
- Diode Schematic Symbol
- Diode Polarity
- Basics of How a Diode Works
- Frequently Asked Questions
- What happens if a diode is connected backward?
- How do I test a diode?
- What is the difference between a Schottky diode and a regular diode?
- What is the difference between a signal diode and a rectifier diode?
- Can a diode be used as a voltage regulator?
- What is the voltage drop across a diode?
- How do I know if a diode is faulty?
- What happens if I exceed the diode’s current rating?
- Can I use any diode in place of another?
- Why does my diode get hot?
- Further Reading
Common Diodes for Beginners in Electronics
Two diodes are shown below that are commonly used in electronic projects. The diode on the top has the part number 1N4148 and the diode below it is a 1N4007.
Typical uses of these two diode parts are shown in the following table.
| Diode Part Number | Type | Typical Use |
|---|---|---|
| 1N4148 | Signal Diode | High-speed switching, low power applications |
| 1N4007 | Rectifier Diode | Power supply rectification, handling higher currents |
An example of a beginner project that uses a 1N4148 diode is the Wailing Siren circuit for beginners.
Diode Schematic Symbol
The schematic symbol for a diode is shown below. Notice the similarity between the diode symbol and the LED symbol. The LED is a special type of diode.
Diode Polarity
A diode has a cathode and anode. In the schematic symbol, the tip of the triangle with the line on top of it is the cathode. The cathode is marked on the body of a diode by a band as shown below.
Basics of How a Diode Works
Current can flow from the anode to the cathode only and never from the cathode to the anode – the diode is like a one way valve.
Forward Biasing a Diode
When a voltage is applied across the diode that is positive (+) on the anode and negative (– or 0V) on the cathode (as shown in the previous image), then the diode is said to be “forward biased” and conducts current, if the voltage across the diode is higher than around 0.6 to 0.7V for a silicon diode. Never connect a diode directly across a power source without a resistor, as it can cause a short circuit. Diodes are used in circuits with other components to reduce the current through them. As a result, they never short out a power source, such as a battery, in a circuit.
Reverse Biasing a Diode
A diode is “reverse biased” when the positive voltage is applied to the cathode and the negative voltage to the anode. In this case the diode does not conduct any current.
Diode Analogy
Think of a diode like a one-way street for electricity. The anode is the entrance, and the cathode is the exit. If you try to drive the wrong way, the diode blocks the current—just like a one-way street sign stops cars.
Frequently Asked Questions
What happens if a diode is connected backward?
If a diode is connected in reverse (cathode to positive, anode to negative), it blocks current, acting like an open circuit. However, if the reverse voltage exceeds its rated limit, the diode may break down and fail.
In hobby electronics, this is rarely a concern since commonly used voltages seldom exceed 24V and are often as low as 9V (such as from a 9V battery). For example, a 1N4148 diode has a maximum reverse voltage rating of 100V, while a 1N4007 can withstand up to 1000V.
How do I test a diode?
Use a multimeter in “diode test” mode. A good diode will show a voltage drop (typically around 0.6V for a silicon diode) when the positive probe is on the anode and the negative probe is on the cathode.
When the probes are reversed, the multimeter should display an open circuit, often indicated as “OL” (over limit), “1”, or “—“, depending on the multimeter model.
For a detailed diode test, see How to Perform a Diode Test Using a Multimeter on the StartingElectronics.com website.
What is the difference between a Schottky diode and a regular diode?
Schottky diodes have a lower voltage drop (~0.2V vs 0.6V for silicon diodes), making them ideal for high-speed switching and efficiency in power circuits.
What is the difference between a signal diode and a rectifier diode?
Signal diodes (like the 1N4148) are designed for low-power, high-speed switching applications, while rectifier diodes (like the 1N4007) are built to handle higher currents and voltages, commonly used in power supply circuits for converting AC to DC.
Can a diode be used as a voltage regulator?
Not directly, but Zener diodes are a special type of diode designed to regulate voltage by maintaining a constant voltage when reverse biased. They are commonly used in power supply circuits for voltage stabilization.
What is the voltage drop across a diode?
For a standard silicon diode, the voltage drop in forward bias is typically 0.6V to 0.7V. Schottky diodes have a lower drop, around 0.2V to 0.4V, making them more efficient in some applications.
How do I know if a diode is faulty?
A faulty diode may:
- Short circuit: Show a low resistance or voltage drop in both directions when tested with a multimeter.
- Open circuit: Show no conduction in either direction (OL or high resistance in both cases).
- Leakage: Allow some current to pass in reverse bias, which is unusual for most standard diodes.
For a detailed diode test, see How to Perform a Diode Test Using a Multimeter on the StartingElectronics.com website.
What happens if I exceed the diode’s current rating?
If a diode is forced to carry more current than its rated limit, it can overheat and fail. In extreme cases, it may crack, burn out, or even short-circuit, potentially damaging other components in the circuit.
Can I use any diode in place of another?
Not always. You need to check specifications like forward voltage drop, maximum current rating, and reverse voltage rating before substituting a diode. Some diodes are optimized for high-speed switching, while others handle high voltages or currents.
Why does my diode get hot?
A diode may overheat if:
- It is passing too much current beyond its rating.
- The forward voltage drop is too high for the application.
- It is not properly heat-sinked (common for power diodes).
Further Reading
For a more in-depth explanation of how diodes work, see Everything You Need to Know About Diodes.