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ralf_c: please pardon my lack of knowledge, but why is there a need for a diode in relay circuit?
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No problem: It is not just relays, but any collapsing magnetic field.
Any current flow is accompanied by magnetic fields. The most notable is a circular field perpendicular to the current flow. When you coil a wire the magnetic fields of each winding are added together.
A change in current or direction of current flow results in a change of the accompanying magnetic field.
A change in a magnetic field will induce a voltage potential in a conductor. If the conductor has a complete circuit a current will flow.
When you drop power to a solenoid, relay coil, motor, or simple loop of wire the magnetic field that was already there collapses. As the field collapses it generates a voltage spike in the original control wires. This is similar to an automotive ignition circuit.
Big coils generate big spikes, but little coils can still damage fragile electronics. (You should see the fire in the contacts of a 500 HP electric motor starter! It is so bad that the power company gets interested
)
A DC circuit is particularly bad because the power is always shut off at maximum voltage, current, and magnetic field -and the induced "back EMF" is always maximum. A diode is easy to install in a DC circuit and offers a low impedance path for the induced current to run back into the coil (which is pretty rugged) and dissipate itself.