Back To CourseIntroduction to Engineering
14 chapters | 123 lessons
Gerald has taught engineering, math and science and has a doctorate in electrical engineering.
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.
This circuit is drawn with symbols. This is called a schematic. Looking at the schematic from left to right we see:
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 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.
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.
Instead of a schematic with circuit symbols, we can view the same circuit with pictures of the actual components.
The circuit operation:
The transistor is on. Current flows through the coil and the relay closes.
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.
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.
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''.
Let's select a diode by doing some calculations.
We are looking for a diode which has a high enough
''High enough'' is a safety factor or 3 (some people like a safety factor of 10).
In our circuit with the two 1.5V batteries, the source voltage is 3V (battery voltages in series add). Three times 3V is 9V. Thus, a diode with a DC blocking voltage of 9V will work.
Now, for the forward current calculation. The voltage across the transistor when the switch closes is the saturation voltage of the transistor, from 0.05V to 0.2V. The data sheet for a 3V relay specifies a coil resistance of 25Ω. Thus, the maximum current flowing through the coil is (3-.05)/25 = .118 amps = 118 mA (''m'' is 10-3). The general equation for this calculation is
This current will pass through the diode when the magnetic field collapses. Three times 118mA is 354mA.
Thus, we are looking for a diode with a DC blocking voltage of at least 9V and a forward current of a least 354mA.
The series 4000 diodes are commonly used for rectifier and power supply applications. They have a maximum DC blocking voltage of 50V to 1000V and a maximum forward current of 1A; i.e., 1000mA. Thus, these diodes are well within the safety margins for our design.
Another type of diode is the small signal fast switching diode such as the 1N914. This diode has a maximum DC blocking voltage of 75V and a maximum forward current of 300mA. The current rating of 300mA is just under the ''three times'' safety margin of 354mA. Thus, this diode is not as safe as the 4000 series diodes for this application.
A flyback diode is a diode placed in parallel with a coil in switching circuits. The flyback diode reduces the high voltages present when coils are switched off. In this lesson we looked at the schematic of a switching circuit with a relay to show the use of a flyback diode. Two parameters are important in selecting the flyback diode: the DC blocking voltage and the forward current.
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Back To CourseIntroduction to Engineering
14 chapters | 123 lessons
Next LessonTunnel Diode: Characteristics & Applications