This link is to a diagram of a transistor operated relay. Signal coming from a sensor with a R2 as an adjustment. The transistor is obviously used as a switch to turn on the relay. What I don’t understand is what is that diode used for? For the life of me I can not figure out why it is even necessary. Can someone explain why it is there and how does it function?
Thank you for any reply.
Hi there,
The diode is serving as a snubber to suppress the voltage spike created in the relay coil when it switches off from going back through the transistor and damaging it 🙂
Maybe a time delay? Relay will be delayed by time that is required to forward bias the diode?
the diode is to snub the inductor ‘kick-back’ when the relay is turned off. The inductance of the relay coil can generate large voltages like an ignition coil in a car. I’m 100% confident in that. I think the pot and resistor is a voltage divider to be adjusted for the 6v or 9v supply and protect the sensor.
@Barron, yet “Frogman” and “videohouse” above are right,
The role of a diode connected in parallel with the coil of a relay is crucial for protecting the electronic circuit in many ways:
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Flyback Protection: When the relay coil is energised, it creates a magnetic field which when the current is switched off, it collapses, generating a high voltage spike (known as back EMF). At that very moment the diode provides a path for this current to circulate, preventing it from damaging other components in the circuit IE the driver transistor (that powers up the coil)
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Current Continuation: The diode also allows the current flowing through the coil to continue circulating (for a brief moment) after the relay has been deactivated by reducing the sudden change in current, which can be harmful too.
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Voltage Spike Mitigation: By clamping the voltage spike to a safe level, (similar to what a Zener diode would do) the diode protects sensitive components downstream from high voltage transients.
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Circuit Stability: the diode also enhances the stability and reliability of the circuit by preventing erratic behaviour caused by voltage spikes.
Now, if you randomly take any number of electronic circuit diagrams that contain relays (EI: large audio amplifiers, timer circuits, certain power supplies etc you name it) you’ll soon realise that a diode is employed in paralel with the relay’s coil for the very same purpose more often than you think. Is basically “the norm”.
@ovi4 Awesome explanation with links. Funny thing is I’m using this in a moisture sensor circuit and got this from a book but they did not explain what that diode was for. Even more funnier is that I had read about back EMF in another book just last week and had forgotten about it. lol In my efforts to learn electronics I bought dozens of books on the subject and had not had the time to read them all. Hopefully over the next year I will get through them and gain a better understanding of the subject. It has been 3 years since I started down this path and I am still a lowly novice. You and Mr. C have been an invaluable resource, Thanks for the help.
@john-barron No problem….any time. Explaining things to other people I’m refreshing my own “brain information storage container” and sometimes adding new info from others who eventually join the conversation (occasionally).