Flyback | Freewheeling Diode

This is another application of diode. After carefully reading this topic you can understand

• What is a freewheeling diode?

• What is inductive flyback? How to prevent it?

• What is the purpose of diode connected across inductive load?

I want to explain the terms flyback or inductive flyback or inductive kickback. The term flyback relates to the sudden voltage spike occur in inductive load, when it's supply current suddenly interrupted. The freewheeling diode is there to suppress unwanted effects. Freewheeling diodes are commonly used in controlled circuits, where SCR controls inductive loads.

First we examine the circuit without freewheeling diode and then with freewheeling diode. Let's get started

Circuit Without Flyback Diode:

Here is a simple circuit, battery is connected across an inductive load.

When switch is in closed position, it means there is a voltage across inductor, which is given below.

v = L ( di/dt )

At constant voltage v the current across inductor L tries to increase at a constant rate di/dt.

Figure 1 Circuit without freewheeling diode

Now if switch is opened, the voltage across inductor becomes zero, while the current flows through the coil. Inductor needs to discharge, the current can not zero instantaneously. The inductor needs a path for discharge. The rate at which current change through inductor depends on inductor time constant, which is τ = L/R.

Sudden change in voltage results in a voltage spike,  or arcing across the switch contacts. This is known as inductive flyback. This voltage spike results in damage of your circuit components.

Figure 2 Circuit with an open switch

Circuit With Flyback Diode:

Figure 3 An inductive load with a freewheeling diode

How do we solve the inductive flyback problem? We use a freewheeling diode, connected across inductive load.

When switch is in closed position, the diode is reverse biased. And the rest of the circuit works normally.

When switch is opened, the inductive flyback is not going to be happened because there is flyback diode. It provides the path for current when inductor is disconnected.

The current flows through the circuit when switch is opened is due to the stored energy of the inductor. The diode becomes forward biased.The output current flows through diode and then flows back in the load itself. In this way the stored energy dissipates through freewheeling diode. That's why it is called freewheeling diode.

Applications:

• It is used to suppress the voltage spikes occur in inductive load. I hope you can easily recognise a freewheeling diode present in any circuit.