Maybe this can be moved to the technical library if it's deemed worthy...
I was doing some self-imposed day job work today verifying some new circuit boards I just got in. These were relay boards, where relay as in something with a coil and contacts. Tractors have these all over the place. My 4520 has a glow plug relay. Some people with electrically actuated hydraulics for grapples have switches that run relays that run the grapple.
The win of a relay is that you can take a small switch and power the coil with a small current. The contacts on the relay can then power something that needs a big current. For example, you can put .08 amps through a coil and the contacts can then pass 20 amps through the coil that turns on the hydraulics. Note the .08 amps is also written as 80 mA where the "m" stands for milli or 1/1000. You would say the coil has "80 milli-amps".
So the win here is that the switch on your joystick can be rated for a much lower current (say 1/2 amp) even though you're working something that takes 20 amps. This also means the wiring up into the joystick can be a small stranded wire because it does not have to pass 20 amps.
There's some funny business that goes on with the relay coil. You see, when you energize a coil and it builds up a magnetic field, you are storing energy in the magnetic field. Yes, the magnetic field is pulling in the contacts to make the relay work but there is energy stored in the field. When you let go of the switch that powers the coil, that energy has to go somewhere.
The universe gets really upset if you try to loose energy. If you've got some (like in the coil) it has to go somewhere. Eventually, that energy is turned into heat. In fact, the coil has resistance and that resistance turns some of the energy into heat. Nothing you could feel, since the power driving the coil is a bit less than a watt (.08 Amps x 12 Volts = .96 Watts). So how does the coil get rid of the energy? Well, the next thing about coils and the universe is that the current that flows in the coil likes to keep flowing now mater what.
So the coil will very quickly change the voltage across the terminals in a desperate attempt to keep the current flowing though the coil. Eventually, the voltage gets so large that something arcs (usually the contacts on the switch) and the energy is dissipated in the arc. Of course this is a bit hard on the switch contacts, but they are sort of designed to take this.
So when you have a coil energized and then turn off the switch, the voltage quickly takes off in the opposite direction of what voltage was applied in a frantic effort to keep the current flowing. If you had +12 on the coil (with the other lead at ground or battery minus), that terminal will quickly head off down through ground and want to keep going. You can get -100 or more volts in a few millionths of a second.
Next post, next part of the story... Now stick with me here ...