Surge suppression....

In a solenoid, a magnetic field is electrically induced when energized. When the electricity is turned off, the magnetic field collapses which induces voltage of opposite polarity back on the same wires used to turn the coil on in the first place. This back surge can damage components, so diodes are added as a one-way switch....electricity only flows to the coil...back electricity is blocked.

Early games the diodes were on each coil...Later games, the diodes were moved to the board.

I think Robert nailed this one pretty good. It's all about the back surge. Even though the circuit is not energized, electrical pulses still occur due to the magnetic windings in the coil and the continuity of the circuit...especially if the circuit is "daisy chained" or hooked in series with other devices. The back surge can be prevented by one of the oldest trued and proved methods...the diode. Of special note. A diode only let's electricity flow in one direction. So in a circuit that may be exposed to current from a neighbor (series) circuit, a diode can prevent that surge.

Hey friends..I learned something else new lately after entering the pinball hobby. It seems that bridge rectifiers are nothing more than diodes arranged in a block or pattern! Right? They rectify A/C current to D/C since they prevent current from moving in more than one direction. Since A/C current pulses, or changes direction constantly, a diode(aka: bridge rectifier) puts an end to that and converts it to D/C current moving in one direction only I believe! My friend told me something at work a few weeks ago. He said you cannot charge a car battery, for instance, with A/C. Why? The current will go in the battery in one pulse and then come out of the battery during the next pulse since it is A/C (alternating current). It has to be D/C so it goes into the battery in one direction and then cannot reverse direction (go backwards) and drain any previous charge pulse that entered initially! Makes sense to me!

I am not an electrician here. Perhaps someone who is can confirm or correct what I believe to be true based on beers and friends This is just my post for today after coming home from a hard days work and slamming 5-6 beers I hope anything I posted is true (lottery and I did not buy a ticket lol)...just going from memory here friends

robertmee summed it up. As he stated, make sure that the coil you are talking about isn't missing the diode. It could be on the board instead of at the coil, so it may not be missing. If it is missing the diode and there isn't one for it on the board, you may have found the reason your transistor failed. And the reason why the replacement transistor will also fail.

This video does a very good job of explaining diodes, bridge rectifiers and smoothing caps:

You mean Stern Games? The newest game I have is Totan and all have diodes on the flips.
Just curious when they moved to the boards.

Induced voltage! Argh!

...an xformer's best friend. A pinball CPU's worst nightmare!

Although I don't know for sure, I imagine it is cheaper from a manufacturing perspective to have them placed on the board compared to each coil. You also don't have to stock coils with and without diodes. And during manufacturing and repairs, you just solder in the coil and don't have to worry about which wire goes to which lug.

Edit: Doh, I thought you asked why, not when. So I answered it from a why perspective. Sorry.

_{Post edited by stangbat : Misread the question.}

WPC games (TOTAN) had them on the board....Flipper coils still had diodes as they were controlled by fliptronic boards, not the driver board, but all the other coils I don't believe have diodes.