What is a Flyback Diode? - Purpose & Calculations

Instructor: Gerald Lemay

Gerald has taught engineering, math and science and has a doctorate in electrical engineering.

In this lesson we explore the flyback diode and how it protects circuit components when they are temporarily exposed to high voltages caused by changing magnetic fields. This lesson also shows the calculation for diode selection.

A Switching Circuit and the Flyback Diode

There are many types of diodes. For example, we have P-N junction diodes, light emitting diodes, and zener diodes. The flyback diode, however, is not a type of diode but rather a diode application. A flyback diode protects circuit components in circuits with changing magnetic fields.

Let's look at one of these circuits and figure out what is going on. This is a switching circuit with a push button switch. The application is to control a relay by pushing a button. A relay is an electrically-controlled switch.


The circuit in symbols
diode_in_circuit


This circuit is drawn with symbols. This is called a schematic. Looking at the schematic from left to right we see:

  • A battery labeled 3V and a resistor labeled R 1kΩ

The positive of the battery is tied to a point called ''net label'' and the negative of the battery is tied to circuit ground. When current flows from the battery through the resistor, the transistor will turn on.

  • The transistor is the symbol just to the right of the resistor.

The transistor has three connections: the emitter (with the arrow) is tied to ground; the base is connected to the resistor; the collector is on top and connects to two other components. When the transistor is on, there is a current path through the transistor from the collector to the emitter.

  • Connected to the collector of the transistor is a coil and a diode.

Winding wire in a loop gives us a coil. When there is current flowing through the coil, the coil becomes a magnet and the relay is closed. Turn off the current and the coil stops being a magnet and the relay is open.

  • The push button switch labeled S1 lets us control the opening and closing of the relay.

Instead of a schematic with circuit symbols, we can view the same circuit with pictures of the actual components.


Circuit in pictures
diode_in_circuit


The circuit operation:

  • press the switch button

The transistor is on. Current flows through the coil and the relay closes.

  • let go of the switch button

The transistor is off. The coil stops being a magnet and the relay opens.

Now we come to the whole point of this lesson! See the diode in parallel with the coil? Why is there a diode in the circuit?

Before we get to the answer, lets explore the workings of the coil in more detail.

The Magnetic Field

Have you ever made an electromagnet or seen one demonstrated? A small one is very easy to make with wire coiled around a nail and a battery.


Making a coil
electromagnet


Touch the wire to the battery and current flows through the wire. Invisible to the eye, a magnetic field surrounds the wire. This field gets intensified by winding the wire around a metal object. Although we can't see the field itself, we can see its effect by picking up metal objects with the coil or making a compass needle spin. The magnetic field stores energy when current flows through the coil. Disconnect the battery and the magnetic field collapses. What happens to the stored energy?

The collapsing magnetic field dumps its energy into the circuit. In a tiny interval of time, a current will flow. What is really cool is this change in current during a change in time produces a voltage. The shorter the time, the larger the voltage. You can imagine a switch opening and in the briefest of moments, the collapsing magnetic field produces a huge voltage across the switch. It takes 30,000V to cause electricity to jump 1 centimeter through air. As the switch opens … BOOM. Lightning!

Well, not quite lightning but at least a spark. What is happening is the voltage between the switch contacts is larger than the breakdown voltage of the air between the contacts. Electric current shoots across the gap and we see a spark.

Transistors also have a breakdown voltage. What happens if we have a transistor in the circuit and the voltage in the spark is greater than the breakdown voltage of the transistor? Pop! Maybe some smoke. Or at least the transistor gets damaged and the circuit no longer functions.

Enter the flyback diode.

When current flows through the coil, the diode acts like an open circuit and it's like the diode isn't even there. However, when the switch opens and the magnetic field collapses and starts producing a large voltage, the diode is forward biased. A forward biased P-N junction diode has 0.7V across it. Thus, the voltage difference across the coil will not get bigger than 0.7V. No huge voltage. No spark. The transistor is spared. Another way to think of the diode is how it protects by providing a path for the current away from the rest of the circuit. The current is ''flying back''.

Selecting a Diode

Let's select a diode by doing some calculations.

We are looking for a diode which has a high enough

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