Always wondered this too, never found a good answer

Are the pop bumpers DC and the slingshots AC?

Both DC. Even the same coil.

The diodes on coils are there to prevent a voltage spike created by the collapse of the coil’s magnetic field when the circuit opens. On SS games the circuit is opened/ closed by a transistor, on EMs it is a leaf switch. The spike can destroy the transistor but not the leaf switch. Often, EMs have a capacitor in the circuit to lessen the arc on the leaf switch. So generally, diodes on SS coils, not in EMs.

Don C

Except in this case Bally put diodes on the coils, and no capacitor on the switch.

The diode is important to prevent voltage spikes from resetting the 5V logic components on SS more than it is to protect the transistors.

Additional interesting info: Williams doesn't put diodes on any of their DC coils any time during their run, while Bally seems to have always put diodes on their DC pops, but never on their slings that I can find. Gottlieb put diodes on their pop bumpers, and also used DC flippers with diodes. I don't have any gottlieb schematics with DC slings to compare...

Try taking the diodes off the coils on your SS games and see how long the transistors lasts.

It wont be very long as the back EMF of a collapsing inductive coil can generate in the many thousands of volts (this is why a coil was used on old petrol engines to generate a spark at the spark plugs, with the points acting as the switch). This will be above the maximum voltage of the drive transistors used in pinballs.

I have seem open circuit diodes cause CPU resets, but this is not their primary fucntion.

I am an electronics engineer with 30 years experience and I can see no reason for fitting the diodes. Just finished repairing a Williams Grand Prix where the DC is used to drive both slings and both the pop bumpers with no diodes on any of the coils.

Was this the era of both solid state and em's being produced? It could have been as simple as just stocking one type of coil, with the diode.... although since they drew it on the schematic as well, they must have had some other reason. Maybe to protect the diodes in the bridge rectifier for longevity?

How long does it take for the back EMF to hose the diode on the coil? Must be a LONG time since most survive seemingly forever.

I thought that as well, but it cannot be as it is only on one coil and when the coil without the diode is energized the coil with the diode is out of circuit as its switch is open. Very strange.

The 1N400X range of diodes are very sturdy and have been an industry standard for years. They do fail but considering how many millions of them are used on such a range of equipment for so many years their MTBF must be enormous.

No SS being produced yet for some of these, iirc

I've seen people say the diodes are to protect the rectifier, but that doesn't explain why they aren't on the slings

I've worked on a number of games with missing diodes and the CPU always reboots quickly. Never had the transistor die before I noticed the issue due to the CPU resets.

I have had this when diodes have been open circuit on the flippers, but as these are not transistor controlled on the SS era machines.

Example, maximum collector-emitter voltage for TIP102 is 100V; collapsing coil EMF can generate many hundreds of volts.

https://pinside.com/pinball/forum/topic/what-causes-coilsolenoid-driver-transistors-to-fail

I've seen driver transistors fail and short the very first time they try to activate a coil with a missing/open circuit diode. The coil diodes are there to primarily protect the driver transistors on solid state games from back EMF voltages.

Which again raises the question, why are they on ems