Free wheel rectifier

[Kevin Weddle]

I did a little calculation with turning on and off a relay. It doesn't seem like it would always be necessary to add the reverse bias diode. I've seen it used many times, but isn't it just a small precaution? It looks like most of the energy is just burnt up within the coil, and you shouldn't have more than 1 volt in the opposite direction anyway.

[audioguru]

Kevin,
Hold both coil terminals of a relay in one hand.
Apply DC to the coil.
Disconnect the power to the relay's coil and you will get a strong shock from the resulting hundreds of volts as the collapsing magnetic field cuts through the windings and generates the high voltage.

The resistance of the coil doesn't matter because there is no current unless something like a diode conducts or a transistor breaks down.

Most transistors are destroyed by hundreds of volts. The reversed diode arrests the voltage spike so that its voltage is only one diode drop (0.7V) higher than the power supply voltage.

[Cabwood]

An inductor, such as a relay coil, opposes changes in current. That means if an inductor is conducting one amp, and the source of that current is suddenly removed (like a switch opening or transistor switching off), the inductor will still conduct one amp. The question is " through what?"

If there's nothing for the coil to conduct its current through except air, then air will do just fine. In order to pass current through air, the coil will develop hundreds or thousands of volts so that the air ionises and provides a conductive path, causing a spark.

Usually a mechanical switch opening to cut off current flow becomes a smaller air gap than anywhere else in the circuit, and the coil's energy will be dumped into that gap. In the case of a transistor switching off, as the transistor's conductance drops, the coil responds by increasing its voltage in an attempt to maintain current flow - several hundred volts is not uncommon. Being a lower impedance than any local air gap, that poor transistor will suffer the energy dump instead of surrounding air. Few transistors will tolerate this abuse even once.

As Audioguru states, the coil's resistance has little influence. The rule is simple: whatever current the coil was  conducting when switched on, it will continue to conduct immediately after it is switched off, and any conduction path, however resistive, will do.