A diode is an electrical component that behaves like a one-way valve. It allows current to flow through in the forward direction but does not allow current to flow through it backward. The diode symbol has a triangle that can be viewed as an arrow that denotes the direction of current flow. The diode’s unique qualities make it very useful in many different circuits including AC power rectification (Diode Bridge), Charge Pumps, and Voltage Regulation.
Though ideal diodes act as perfect one-way valves, the reality is slightly different. When using diodes in real circuits, there are two electrical states that must be considered.
Input Voltage Higher Than Output Voltage (Foward Current)
When the voltage on a diodes input is higher than the diode’s output, current will flow through the diode. When current flows forwards through a diode, a fixed voltage is subtracted. This voltage drop is typically about 0.7V. This is called the diode’s forward voltage drop. If the voltage across a diode is lower than its forward voltage drop, current will not flow through the diode.
Output Voltage Higher Than Input Voltage (Reverse Current)
When the voltage on a diodes output is higher than the diode’s input, current will not flow through the diode as long as the voltage differential is less than the diode’s reverse breakdown voltage. If the reverse voltage exceeds the reverse breakdown voltage, current will flow through the diode in reverse. However, the voltage will have dropped an amount equal to the reverse breakdown voltage. In some cases, this property is used beneficially.
Even when the reverse voltage is below the breakdown voltage, a small amount of current will leak through. This is the diode’s leakage current.
The last property that we need to discuss is the diode’s recovery time. This is the amount of time it takes for a diode to “close” when the voltage reverses across the diode. This is especially important for high-speed switching applications.
What are the Different Types of Diodes?
Switching Diodes
Small Current Ratings
Average Forward Voltage Drop (~0.7V)
Fast Recovery Time
Small Leakage Current
Used for High-Speed Switching
Rectifier Diodes
High Current Ratings
Average Forward Voltage Drop (~0.7V)
Average Recovery Time
Average Leakage Current
Used in Power Applications
Schottky Diodes
High Current Ratings
Low Forward Voltage Drop (~0.15 – 0.6V)
Slow Recovery Time
High Leakage Current
Used in Power Applications
Low Power Losses
Zener Diode
Controlled Reverse Breakdown Voltage
Used Backwards to Take Advantage of the Reverse Breakdown Voltage
Light Emitting Diodes (LEDs)
High Forward Voltage Drop (1.8 – 5V)
Generates Light
Used for Indication and Lighting
How Do I Use a Diode?
Diode Symbol
Sizing a Diode
Like any power component, a diode must be sized to handle the power being put through it. The power loss in a diode is proportional to the voltage drop (forward or backward) in the diode and the current flowing through the diode. Power loss in a diode is calculated as shown in this equation below:
\(\large P = V_{drop} \times I\)
Sizing a resistor for an LED
Because diodes, such as LEDs, do not have a resistance, it is very easy to pull too much current through them and burn them out. This is why a resistor should always be used with in series with a LED.
We have an LED with a 0.7V voltage drop and a 20mA current rating. The LED will be connected to a pin on an IC that can drive the LED with 5mA at 5V. To calculate what resistor size we need to first subtract the voltage drop of the LED, and then use Ohms law to calculate resistance.
\(\large R = \frac{V}{I} = \frac{5V – 0.7V}{0.005A} = 860\Omega\)
Because we have to use a real resistor value, we will choose the next higher resistance to ensure that the current is below the specification. This can be done with the help of our resistor calculator. The next higher value in the E24 series is \(\large 910\Omega\).
Note that the resistor can go before or after the LED and have the same effect.
DC Input Reverse Polarity Protection
A diode can be used to protect a device agaist power being applied backwards. This can be particularly useful when designing a product that someone may put the wrong power adapter on. An example circuit with a barrel jack input using a diode for reverse polarity protection can be seen below.
Using a Zener Diode for a Voltage Reference
Because the Reverse Breakdown Voltage of a Zener Diode is controlled, they can be used to create a useful voltage reference. The circuit below will provide a stable voltage reference at Vdd that is equal to the Reverse Breakdown Voltage of the Zener Diode. For the to work, the input voltage must be higher than the Reverse Breakdown Voltage.
Try it Yourself!
In this simulation, there is an AC rectifier that converts Alternating Current (AC) to Direct Current (DC) and an example of using a Zener Diode as a voltage reference.
