"Always wondered" Flyback diodes

[Let'sgoflying!]

I am changing out my three relay diodes and of course will follow the admonishments to install them stripe towards the hot side.
I have wondered however what is the result if they are installed backwards?
Will they burst and spark? Or will they simply fail to perform as designed?
I think I also need, if anyone can provide it, an explanation of how they work. (Beyond "they reduce arcing when the contacts open") I bet Dan could tell me where the energy goes and how the diode does that.

 

[Capt. Geoffrey Thorpe]

I have wondered however what is the result if they are installed backwards?

Magic smoke is released and they fail to function in a very short time.

Diodes block current in one direction and allow it to flow easily in the other. Hooking them up "backwards" (for this application) is kinda like creating a short circuit from the positive to the negative through the diode.

 

[Capt. Geoffrey Thorpe]

I think I also need, if anyone can provide it, an explanation of how they work.

When you shut off the current to some kind of inductor - coil, motor, whatever that is built by winding wire round and round and round - the magnetic field created by the current flow collapses which tries to keep the current flowing - you can get a big voltage spike if the current bangs up against an open contactor - the flyback diode gives that current a place to go without creating any damage.

Think of your magneto - when the points open, the magnetic field in the primary coil collapses which generates the current in the secondary of the coil to spark the spark plugs.

 

[Let'sgoflying!]

I’m good with all the above explanations, looking for more detail on
“flyback diode gives that current a place to go”

Is it “sending it back” to the battery, in the case of the master & starter relays?

 

[Bell206]

Is it “sending it back” to the battery, in the case of the master & starter relays?

It generally directs the current back to the relay coil by keeping the electrons moving when the relay deenergizes thus keeping the voltages low and not burning this up.

 

[geezer]

Terminal 1 and 2 are arbitrary, for point of reference.

With the contact closed, current flows from terminal 1 of the coil to terminal 2 of the coil. The diode blocks current flow from 1 to 2. The contact opens, inductance tries to continue flowing current from 1 to 2, and it flows backwards, 2 to 1, through the diode, until resistance eats all the energy.

This prevents arcing across the contact as it opens, since the flow is through the diode, not across the contact.

 

[Dan Thomas]

Note that they use "current flow", which is backwards to actual electron flow. Benjamin Franklin did that, calling it current, thinking that whatever it was that flowed in conductors went from positive (a surplus of whatever it was) to negative, (a shortage of the stuff). Later research proved that incorrect, and I have always used electron flow instead.

In any case, in both of these diagrams, the electron flow is upward though the coils, from negative to positive. When the switch is opened, the collapsing magnetic field propels the electrons in the same direction they were already going, but we can get a very sudden, sharp surge when it happens: a voltage spike that can damage electronics. The diode is there to just shunt that spike in electron flow back to the other end of the coil.

Note the symbol for the diode: An arrow with a wall blocking it. That is telling you that electrons can't flow in the direction of the arrow. They flow the other way. If one thinks of that arrowhead as a megaphone, the sound comes out of the big end, right? Electrons flow out of the big end of the diode.

Just reverse the direction of the two "current flow" paths and you have the electron flow.

Just to confuse you some more:



Much of my electronics experience was a long time ago, and involved work with vacuum tubes. If one starts thinking "current flow" when working with tubes, you're instantly in trouble. Electron flow is everything there.