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Introduction:
The counter electromotive force (EMF) is an induced voltage generated in the coil that opposes the variation of the electric current that originated it.
In this series of three videos, you will learn how the EMF is generated in the coil of an electromechanical relay, you will be able to observe the EMF in the relay coil when it appears on the digital oscilloscope screen and what methods are used to mitigate its effects.
How the EMF is generated in the coil of an electromechanical relay powered by a direct current source:
1. Turning on the circuit:
The EMF is generated by a variable electric current. When the transistor enters saturation, the direct current of the source increases slowly until it reaches its nominal value in the coil.
This happens because the direct current of the source must first overcome the resistance of the coil and in this time interval, it varies from zero until it reaches the nominal value.
This time interval in which the direct current of the source varies creates an induced electromotive force (EMF) in the relay coil that opposes the growth of the direct current of the source in the coil.
Since the time interval in which this phenomenon occurs is very fast, the EMF disappears from the coil at the exact moment the current reaches its nominal value.
During the time in which the transistor is saturated, keeping the source's direct current invariable in the relay coil, the dominant magnetic field in the coil is that generated by the source current.
This magnetic field has an intensity, strength and direction determined by the source's direct current.
While the source remains in operation, the EMF remains controlled by the action of the magnetic field caused by the electric current.
2. Turning off the circuit:
When the circuit switch is turned off, the transistor cuts off, interrupting the current flowing in the coil. At this moment, the counter electromotive force (CEMF) appears in the coil, with a voltage value that is much higher than the voltage value of the source that generated it.
When it appears in the coil, the EMF promotes:
a) An increase in voltage many times greater than that of the source that generated it;
b) Reversing the direction of the magnetic field; c) Changing the polarity of the coil terminals. The peak voltage value that the EMF reaches can damage sensitive electronic components, such as transistors and integrated circuits, which is why it should be avoided at all costs. 3. How to mitigate the EMF: To ensure the integrity of electronic circuits, components and devices, the following components are used: resistor, diode or suppression capacitor. Since most designers opt for diodes (known as freewheeling), we will use this component to explain how the EMF will be mitigated in the coil, preventing it from remaining in it. The diode is placed in parallel with the coil, so that its cathode is facing the +B of the source and its anode is facing the collector of the transistor. This way, when the switch is turned on, the diode remains in the cut-off while the coil and the transistor are in action. When the switch is turned off, the transistor cuts off, interrupting the current flowing through the coil, creating the EMF. Since the EMF reverses the polarity at the coil terminals when it appears, the diode starts conducting, consuming the EMF, preventing the transistor from being damaged by it.
Hello, I am Professor Valdenilton, Technical Coordinator of the Electronics courses taught by Technology Cursos.
I love transforming people's lives through teaching. My goal is to teach people like you how to repair the electronic equipment they love, helping in every way possible.
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