All inductive devices, operating in d.c. circuitry, which are switched on and off (whether by a contact or by electronic circuitry) should have a diode connected across their coils to catch the inductive fly back.
Coils, inductance and current
A coil (of any sort) is an inductor. Inductors behave to electricity as mass does to a mechanical system. Inductors resist change in current flow, just as masses resists change in physical movement. Stand in front of a moving car and try to stop it: its mass keeps it going.
In the same way, if you suddenly try to stop the current flowing in an inductor – the inductor will resist the change in current. The same way the mass of the car resisted the mechanical stopping, so will the inductance of the coil resist the stopping of the electrical movement – the current flow.
When the car hits an object – the momentum is changed into pressure. In the electrical system, the electrical (current) flow also is turned into electrical pressure – a huge voltage develops in an attempt to keep the current flowing. This voltage will cause an arc across the switch that is interrupting the current. This will eventually burn the switch, but in the short term, it causes electrical noise, which can have all sorts of effects, such as making electronic equipment misbehave – or even fail.
Clearly, what we need is some way of interrupting the current flow slowly – a method of letting the current die away slowly. Fortunately there is indeed an easy way to do this.
Consider the diagram to the right. It shows a coil (maybe a relay coil) and a switch connected in series with a battery. When the switch is closed, current will flow and the coil will energise. When the switch is opened, the current will stop and the coil will de-energise. But life is not that simple!
The drawing below shows how the current and voltage vary with time. At the first red line the switch is closed to energise the relay. At the second, it is opened to de-energise the relay.
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