Posted 5/23/2020 08:35 (#8274875 - in reply to #8274092) Subject: RE: Electrical engineering question -- flyback diodes
Southern Minnesota Between Freeborn & Wells
When I first starting working as an Ag Tech, I saw the results of this fairly often. Back in those days it was common to use the TeeJet 144 type solenoid valves on sprayers etc. When energized the coil created a magnetic field which raised a plunger off its seat and allowed spray to flow. When released, the magnetic field collapsed and the plunger was pushed back on its seat due to a spring and the flow was stopped.
If this was connected pretty much directly to a battery this worked quite well. We started using them with certain spray monitors that indicted the spray rate etc. Since both devices needed 12V and ground, it was easier to connect them together and run to the battery. The result of this was that at times, when the solenoids were turned off, the display went dead. What was happening was the collapsing magnetic field induced a spike into the spray controller. Usually the controller came back to life eventually.
The solution was to run the power and ground wires separately apart from each other as much as possible to the battery. This helped the situation as the collapsing magnetic field from the solenoid was then fed more directly to the battery which would absorb it rather than induce it into the power/ground to the spray controller.
A relay could be the same situation although to a lesser degree. The purpose of the diode is to prevent this.