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Diode Across a Coil Question - DEC 2023 Model RR Article by Allan Gartner

Dave Hamernik All Messages By This Member #14423 Hi I have a limited knowledge of electric circuit design.? In the December Model RR magazine, Mr. Gartner discusses various circuits and electrical components.? In one circuit, he discusses the requirement of a diode across a coil to safeguard a component (PSXX) within the circuit (see link).? I don't doubt that the diode is needed but I would like to know how the PSXX would be damaged if the diode was not there.? What would cause the damage? I'm looking for more explanation so I can better understand how the circuit works. /g/w4dccqa/files/Diode%20across%20coil.pdf Thank you. -- Dave Portland, OR
Michael Rozeboom All Messages By This Member #14425 开云体育 On 2023-11-15 10:49 p.m., Dave Hamernik wrote: Hi I have a limited knowledge of electric circuit design.? In the December Model RR magazine, Mr. Gartner discusses various circuits and electrical components.? In one circuit, he discusses the requirement of a diode across a coil to safeguard a component (PSXX) within the circuit (see link).? I don't doubt that the diode is needed but I would like to know how the PSXX would be damaged if the diode was not there.? What would cause the damage? I'm looking for more explanation so I can better understand how the circuit works. /g/w4dccqa/files/Diode%20across%20coil.pdf When the current is cut off the coil will create a large reverse voltage spike, which can damage the transistor driving it. The diode provides a route directly between to the two ends of the coil allowing that energy to return to its source without having to pass through other circuit components. -- Michael Rozeboom Team Amiga
Dave Hamernik All Messages By This Member #14426 Thank you. -- Dave Portland, OR
Ken Harstine Member Profile All Messages By This Member #14427 When you shut off power to a coil the coil has stored energy. The energy will cause the voltage to rise at the coil terminals until the energy is dissipated. The reverse diode across the coil will dissipate that energy and insure that no more than a fraction of a volt will be crated by the coil. The high coil voltage can destroy both transistors and switch contacts. A relay consists of a coil that creates a magnetic field that causes the relay contacts to change position. If the relay has polarity marked on it then it may already has a diode internally. Find and read the specifications for the relay. Ken Harstine 413-250-8298
Don Weigt All Messages By This Member #14429 Relay coils, like all inductors, store energy in the magnetic fields they produce when energized. Inductors resist a change in current flow. When the driving solid state switch, or any switch, breaks the circuit, the current flow is stopped abruptly, and that magnetic field collapses, causing a big voltage spike of the opposite polarity. Adding that diode provides a path for the current as the magnetic field decays, making the decay more gradual and limiting the voltage. The resistance of the coil and the diode, and the diode's forward voltage drop, all dissipate the energy stored in the coil as heat. It's far better to "soak up" the energy that way, than to produce a spike of excessive voltage, potentially hundreds or even thousands of volts, that destroys the solid state switch driving the coil. Don W -- Don Weigt Connecticut
Dave Hamernik All Messages By This Member #14437 Thanks all. -- Dave Portland, OR

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